NCHurricane2009's Blog

2012 Atlantic Hurricane Season Birdseye Discussion #20A (Special Update)

By: NCHurricane2009, 11:18 PM GMT on May 30, 2012

...MAY 30 2012...7:19 PM EDT...
As expected...Beryl becomes non-tropical (extratropical) while pulling NE away from the mid-Atlantic region of the United States. With no other Atlantic tropical activity in the near term...this is my last post until hurricane season starts June 1 2012.

During the first week of the season (June 1 to 7)...I will be on vacation with some limited computer access. So it is likely that my posts will not be daily during that week. Feel free to let me know how I am doing with these new type of discussions...there are the past 20 of them you can browse through by visiting the April and May 2012 sections of http://www.wunderground.com/blog/NCHurricane2009/ archive

Updated: 11:26 PM GMT on May 30, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #20

By: NCHurricane2009, 1:25 AM GMT on May 30, 2012

...MAY 29 2012...9:28 PM EDT...
Threat of Beryl shifting into large section of Carolinas...SE Virginia...and SE Maryland south of the Delaware border. See special feature section for further details.

This is the twentieth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1934Z-released HPC analysis.

In light blue is upper air anlaysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicating surface lows, Hs indicating surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL DEPRESSION BERYL...
Beryl will move NE from here on out while in southwesterly flow ahead of frontal system in paragraph P1. No change to the track forecast from the previous discussion...as the NHC forecast came into agreement with mine yesterday and the NHC has ever-since kept the track forecast the same. Albeit...Beryl is a bit behind in schedule from yesterday's track forecast...so the time points have been moved back a bit along the forecast track. Latest forecast for Beryl in Figure 2 below. NHC and computer models continue insisting that she will begin re-strengthening while taking advantage of frontal upper trough divergence associated with the system in paragraph P1. The status of Beryl will then become questionable as such a re-strengthening method is a non-tropical method. However...what supports Beryl being tropical during some of this duration is that very recently...some of the upper ridging in paragraph P2 is now directly overhead of Beryl. This upper ridging will allow Beryl to resemble a warm core structure in the upper-levels...where the upper ridge can ventilate the system like a tropical system. However...I expect Beryl to be fully non-tropical (extratropical) by Thursday.

Impact swath drawn in Figure 2 is based on recent radar structure shown in Figure 1. Notice now that the heaviest rains are now on the NE side of the center...something that we have been expecting thanks to shear from the incoming frontal upper trough of paragraph P1. The radar presentation in Figure 1 has forced me to expand the impact swath to now include north-central NC...SE Virginia...and Maryland (S of Delaware border). It should be noted that my impact swath yesterday did not include those areas. Rain and t-storms are occuring to the west of and outside my impact swath...in a band associated with the incoming frontal system in paragraph P1. In fact...this band is seen towards the upper-left side of the radar image in Figure 1. It should be noted my impact swath is focused exclusively on the spiral of Beryl...and not this band of t-storms (not associated with Beryl).


Figure 1: Radar image of Beryl taken at 7:15 PM EDT on May 29, 2012.


Figure 2: My current best-guess forecast for Tropical Depression Beryl.

...MID-LATITUDES DISCUSSION...
P1...Frontal system in the central US is pushing into the eastern US at the expected pace...and therefore the NE track forecast for cyclone Beryl is essentially maintained from the previous discussion (see special feature section for details). Lowest surface pressure of this frontal system currently is 1002 mb over the W Great Lakes. Upper convergence on the west side of the system's upper trough supports some weak surface ridging tailing the system...such as the 1014 mb centers seen to the left side of the above charts...and the 1021 mb center in NE Montana.

P2...Central US upper ridge has broken into two fragments...one over Texas and northern Mexico...the second moving into the eastern US. This upper ridge continues to be supported by warm air advection ahead of frontal system in paragraph P1. Eastern convergence of this upper ridge system now supports the long-lived dry air in the Gulf of Mexico...Gulf of Mexico surface ridging...a 1021 mb ridge over the E coast of Canada...and another 1021 mb ridge center over Bermuda.

P3...See paragraph P8 of previous discussion #19 for how a few weather systems created a longwave upper trough regime across the entire high seas of the Atlantic. This longwave upper trough has an upper low vortex at its north end...and this entire longwave upper trough is cut-off to the south of the upper anticyclone in paragraph P4. At the surface...dominant feature is frontal system that was located in the high seas W of Greenland 24 hrs ago. It contained a 1005 mb center S of Greenalnd that has rapidly deepend to 988 mb as shown in the above charts. The rapid deepening was supported by split flow divergence between the upper low vortex (mentioned earlier in this paragraph) and upper anticyclone (mentioned in paragraph P4). Expect this 988 mb cyclone to whirl to a position beneath said upper low vortex...which will create a deep-layered low. On the south side of this emerging deep-layered low...a new 1004 mb frontal depression (marked in the above charts) is taking advantage of accelerational upper divergence and could intensify rapidly while moving ENE around the south side of the emerging deep-layered low.

P4...Full-fledged and anomalous upper anticyclone west of the British Isles continues. Days ago...its eastern convergence supported Atlantic surface subtropical ridging that has since weakened (due to the surface ridge separating from the upper convergence). Currently this weaker surface ridge consists of 1021 to 1019 mb centers SE of the weather system mentioned in paragraph P3.

...TROPICAL BELT DISCUSSION...
P5...Upper vorticity very near Tropical Depression Beryl has degenerated into a weak shortwave upper trough over the SE US. Upper convergence west of this shortwave (and east of the upper ridge in paragraph P2) supports long-lived sinking dry air across the Gulf of Mexico. As this shortwave upper trough gets absorbed by frontal upper trough mentioned in paragraph P1...responsibility of the Gulf dry air will shift to the upper ridge in paragraph P2. As discussed yesterday...upper divergence on the NE side of this vorticity had created a weak mid-level low pressure spin seen in satellite animations well E of Beryl that was heading east toward Bermuda. The mid-level low is not as defined from a satellite perspective today while it crossed Bermuda. Instead...it seems to live on as a shortwave upper trough over Bermuda at this hour. I believe this shortwave upper trough was caused when the mid-level low had advected in some of the upper vorticity very near Beryl. I expect this shortwave upper trough over Bermuda to move east and assimilate into the upper-levels of the weather system in paragraph P3.

P6...Long-lived Caribbean upper ridging persists. What's left of the embedded upper trough in this upper ridge is currently an upper vortex over the Yucatan and SE Mexico. See paragraph P10 of discussion #19 on how a tropical disturbance developed beneath this upper ridge. The surface trough of the disturbance was SW of Jamaica at 1200Z TAFB yesterday...and now is over the Yucatan at 1800Z TAFB tonight...as it seems the surface trough was steered west by Gulf of Mexico surface ridging mentioned in paragraph P2. Because the surface trough is now aligned with the upper vortex over the Yucatan...the system is analyzed as a deep-layered trough over the Yucatan in the above charts. The disturbance has not done what it needed to do for tropical cyclone devlopment...which was to maitain intense T-storm clouds to inflame the Caribbean upper ridging directly overhead of itself (to reduce vertical shear and enhance outflow). Now...the best of the caribbean upper ridging is found east of of the disturbance...and so the disturbance is in unfavorable southerly vertical shear E of the Yucatan deep-layered trough and W of the Caribbean upper ridge. Coincidentally...the southerly upper flow is also divergent...and so t-storms continue in the western Caribbean sea.

P7...Tropical wave that has been kept stationary (midway between Cape Verde Islands and Lesser Antilles) is finally moving westward in TAFB analyses...and will be reaching the Lesser Antilles soon. Currently...the tropical wave is in a hostile westerly vertical shear environment on the south side of weather system in paragraph P3.

P8...Upper ridge over the E tropical Atlantic continues.

Updated: 1:34 AM GMT on May 30, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #19

By: NCHurricane2009, 8:52 PM GMT on May 28, 2012

...MAY 28 2012...4:55 PM EDT...
Beryl inland near the Florida-Georgia border and has weakened to a tropical depression. Watching new disturbance in the western Caribbean Sea. See special feature section and paragraph P10 in tropical belt discussion for further details.

This is the nineteenth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1321Z-released HPC analysis.

In light blue is upper air anlaysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicating surface lows, Hs indicating surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery.Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL DEPRESSION BERYL...
Not a whole lot to say about Beryl today...since it pretty much followed both the track and intensity forecast I presented in the previous discussion. For the NE recurvature forecast...I was a bit left of yesterday's NHC's track based on the timing of the frontal system mentioned in paragraph P1 below. The frontal system's timing remains on cue...and today's NHC forecast track has "dead-nuts" lined up with the track I presented yesterday...so my updated track forecast in Figure 1 below is a continuation of the previous. Beryl weakened a tad faster than I forecasted...so I am now expecting a 30 mph remnant low by 11 PM tonight instead of by 11 AM Tuesday. I still have some re-strengthening after 11 AM Tuesday as the NHC and computer models insist that the frontal upper trough recurving her NE will supply some supportive upper divergence...so the status of Beryl is questionable during the NE recurvature as such a re-strengthening method is a non-tropical (extratropical) method.

Diameter of blue-dashed impact swath in Figure 1 is based on the current diameter of the spiral rain shield seen on radar. The impact swath diameter I drew in during the NE-recurvature today is larger than the drawn-in diameter during NE-recurvature yesterday. This is because I am a bit surprised how far out the spiral rain bands from Beryl are reaching on current radar...but the gusty winds can only be near the center of Beryl...or in any severe T-storms that Beryl produces. I expect this rain shield will eventually get distorted to a bias E of center as the incoming frontal upper trough shears Beryl. However...with a NE track also expected...the impact swath remains symmetric about the forecast track.


Figure 1: My current best-guess forecast for Tropical Depression Beryl.

...MID-LATITUDES DISCUSSION...
P1...Frontal system from the western US is ejecting eastward into the central US at the expected pace...and therefore the NE recurvature forecast for cyclone Beryl is maintained from the previous discussion (see special feature section for details). Lowest surface pressure of this frontal system currently is 1000 mb in SE Manitoba. Upper convergence on the west side of the system's upper trough supports a 1020 mb surface ridge in N Utah.

P2...Central US upper ridge continues to be supported by warm air advection ahead of frontal system in paragraph P1. This upper ridge is now quiet expansive...and extends eastward into the NW Atlantic. Eastern convergence from this upper ridge supports a plethora of surface ridging...including a 1015 mb center in SW Louisiana...1019 mb center in N Virginia..and 1023 mb center near Bermuda.

P3...A frontal system has entered the high seas W of Greenland as expected 24 hrs ago...but further east progression is being halted by an anamalous full-fledged upper anticyclone mentioned in paragraph P6. Its upper trough supports a 1005 mb center S of Greenland with eastern divergence...and supports a 1027 mb center over S Hudson Bay with western convergence. We were discussing a southern fracture of this upper trough 24 hrs ago...which continues SE into the open Atlantic at a position E of Bermuda.

P4...Upper vortex NW of the Azores continues to support a less-than-1012 mb surface frontal cyclone...and is cut-off to the south of anamalous upper anticyclone in paragraph P6. The cold front attached to the frontal cyclone is decaying rapidly...and leaves behind a surface trough just NE of a 1023 mb ridge center as shown in the above charts.

P5...Upper vortex located well NE of the Lesser Antilles 24 hrs ago has moved eastward to a position NW of the Cape Verde Islands...where it has weakened to an upper trough...and where its eastern divergence is producing a comma shaped cloud mass.

P6...Full-fledged upper anticyclone west of the British Isles continues to be associated with warm air advection ahead of frontal system in paragraph P3. Days ago...its eastern convergence supported Atlantic surface subtropical ridging that has since weakened (due to the surface ridge separating from the upper convergence). Currently this weaker surface ridge consists of 1023 mb centers SE of the less-than-1012 mb cyclone in paragraph P4.

P7...Deep-layered non-tropical low continues spinning on satellite to the SW of the British Isles as marked in the upper-right corner of the above charts...trapped on the south side of the upper anticyclone mentioned in paragraph P6.

P8...The weather systems in paragraphs P3...P4...P5...and P7 are beginning to create a longwave upper trough regime that spreads across the entire high seas of the Atlantic. Accelerational divergence on the east side of this emerging regime appears to have created a 1011 mb surface depression over Spain as marked in the above charts and 1200Z TAFB.

...TROPICAL BELT DISCUSSION...
P9...Upper vorticity remains trapped offshore of SE US and E Gulf of Mexico...and is located very near Tropical Depression Beryl. Upper convergence west of this vorticity (and east of the upper ridge in paragraph P2) supports long-lived sinking dry air across the Gulf of Mexico. Upper divergence between the NE side of this vorticity and mainstream upper westerlies has created a weak mid-level low pressure spin seen in satellite animations E of Beryl and west of Bermuda that was getting some attention from bloggers yesterday. This mid-level low is turning eastward toward Bermuda (while steered by the north side of 1023 mb low-level ridging seen in the above charts)..and shows no signs of developing under an unfavorable northwesterly shear environment caused by the upper ridge system in paragraph P2.

P10...Long-lived Caribbean upper ridging persists...and its eastern convergence supports persistent dry air in the eastern half of the Caribbean and waters E of the Lesser Antilles. This upper ridge had an embedded central Caribbean upper trough (as mentioned in discussion #18 paragraph P8)..and divergence E of this upper trough was moistening the air in the western Caribbean 24 hrs ago. Also mentioned in previous discussion # 18 (paragraph P9) was a dissipating tropical wave that had entered the Caribbean...and it appears the remnants of the tropical wave interacted with the divergence E of the central Caribbean upper trough to produce a tremendous increase in T-storms last night...and now we have a new disturbance to watch in the Caribbean Sea. This t-storm mass locally inflamed the Caribbean upper ridging with latent heat release such that the central Caribbean upper trough has split into an upper low vortex over the Yucatan and upper trough SE of Jamaica. 1200Z TAFB has just added a surface trough SW of Jamaica associated with this new disturbance...but the heaviest rains/cloudiness appear to be headed for the Cayman Islands and eventually Cuba. Current prognosis is that if the system wants to develop into a tropical cyclone...the system needs to keep firing t-storm activity to further inflame Caribbean upper ridging...which would reduce southwesterly shear and enhance its outflow.

P11...Tropical wave has been kept essentially stationary (midway between Cape Verde Islands and Lesser Antilles) in the last four days of TAFB analyses. A t-storm mass E of the tropical wave is present along the ITCZ...but the mass will remain sheared-off from the tropical wave by the emerging longwave upper trough regime to its north (discussed in paragraph P8 above). I beleive the formation of this T-storm mass was caused by convergence E of the tropical wave and enhanced poleward upper outflow from the longwave upper trough regime in paragraph P8.

P12...Upper ridge over the E tropical Atlantic continues.

Updated: 9:46 PM GMT on May 28, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #18

By: NCHurricane2009, 7:42 PM GMT on May 27, 2012

...MAY 27 2012...3:42 PM EDT...
Beryl is now fully-tropical...is stronger...and its center is heading toward NE Florida. Weather conditions will deteriorate through the rest of this afternoon and evening in SE Georgia and NE Florida. See Special features section for further details.

This is the eighteenth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

Any feedback on how to improve these discussions can be left in the comments section below. In particular...any suggestions on how to improve the standard two "birsdeye view" charts below will be much appreciated. Are you able to understand the markings in the charts? Are you able to relate what is said in the discussions to these charts?

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0729Z-released HPC analysis.

In light blue is upper air anlaysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicating surface lows, Hs indicating surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery.Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM BERYL...
My track forecast positions in the previous discussion were too far NE of the NHC...computer models...and what has actually happened. Therefore...my updated track forecast (in Figure 1 below) more closely follows the NHC track this time around. The NHC track would make the center move ashore at Jacksonville, FL (30N latitude)...all while radar imagery suggests a center currently just south of 30N and visible imagery suggwsts a center just north of 30N. With the radar versus satellite conflicting a bit like this...I will agree with the NHC track (landfall at Jacksonville, FL)....which is down the middle of the sat/radar conflict. Concerning the recurvature to the NE later in the forecast period...I have been following how the frontal system (in paragraph P1) has been ejecting eastward with the HPC surface maps:

(a) I concluded an 18-hourly motion about the width of Colorado.
(b) That suggests the front in E Kansas 18 hours from now (5 AM Mon)
(c) That suggests the front in E Missouri/W Tennessee 36 hours from now (11 PM Mon)...where it can just start to bend Beryl northward as it erodes the blocking ridge (1019 and 1022 mb centers of paragraph P2) to Beryl's north
(d) That suggests the front over the eastern US 54 hours from now (5 PM Tue)...sending Beryl northeastward by that time.

The 8 PM Mon NHC position suggests a NW track by the time the front is in aforementioned position (c). But I think given the current strength of the blocking ridge to Beryl's north...I think a more WNW track is appropriate by that time...so my NE recurving track of Beryl is a bit left of the NHC's as shown in Figure 1 below.

Intensity-wise...the forecast in my previous discussion was much better than my track forecast. As if right on cue with my previous forecast...Beryl gained fully tropical status today (on Sunday) and strengthened to 65 mph winds. The ominous thing however is Beryl has gained 65 mph winds ahead of when my previous forecast said it would. Beryl's cloudiness was best described as a doughnut in the last several hours (as can be typical with a subtropical cyclone beneath a cold core upper low)...featuring a large-radius of clear skies at the center surrounded by a ring of convective T-storm clouds. This ring of T-storms has been intensifying...gaining some warm core anticyclonic outflow at its top (hence why Beryl is now fully tropical)...and finally has contracted such that the large-radius of clear skies is a smaller radius (i.e. now looks more like a hurricane's eye). Therefore...there is a remote chance of a minimal hurricane before landfall tonight..but my newest forecast in Figure 1 is more conservative at 70 mph max central winds (just under hurricane status).

After landfall...my intensity forecast weakens Beryl to a 35 mph wind tropical depression by 11 PM Monday...and then a remnant low by 11 AM Tuesday with 30 mph winds. The NHC and computer models insist that Beryl will then take advantage of upper trough divergence associated with the frontal system recurving her NE...so my max wind speed forecast suggests some re-strengthening after 11 AM Tuesday. But the status of Beryl will be questionable by that point...because it is more likely to be non-tropical (extratropical) as it re-strengthens...as such a re-strengthening method is a non-tropical method.

Finally...all impacts expected are summarized in Figure 1. Tonight...the most severe impacts are likely to be in the Jacksonville metro area in NE Florida with damaging winds and heavy rain. In fact...winds have been rising hour-by-hour in advance of the storm center in Jacksonville...based on official NWS (National Weather Service) station reports. The blue-dashed swath of impacts in Figure 1 is first symmetrical about the center using the current wind radii from the NHC 11 AM advisory. I then shrink the swath diameter thru landfall and weakening. Notice by 11 AM Tue...the swath has a rightward bias with respect to forecast central position...the rightward bias caused by westerly vertical shear associated with the incoming frontal upper trough. This rightward biased structure is expected from 11 AM Tue and onwards...but with a NE track at that time as well...the swath ends up being symmetrical about my forecast track.


Figure 1: My current best-guess forecast for Tropical Storm Beryl.

...MID-LATITUDES DISCUSSION...
P1...In the past 36 hours...frontal system over the western US has been largely stationary but is beginning to eject eastward. It had a surface 989 mb center in W Utah 36 hrs ago that is now 996 mb over W Nebraska. The timing of how fast this frontal system and its supporting upper trough moves east is crucial...as this will affect when exactly Tropical Storm Beryl recurves to the NE (see special feature section for details).

P2...Upper ridge was over the eastern US 36 hrs ago...once supported by warm air advection ahead of the frontal system in paragraph P3 below...is now supported by warm air advection ahead of the system in paragraph P1 and has shifted westward into the central US where the warm air advection is maximal. Eastern convergence of this upper ridge supports surface ridge centers of 1019 mb in E Tennesee and 1022 mb E of North Carolina.

P3...A frontal system was rapidly ejected NE from the central US into SE Canada 36 hrs ago...and now this front is ejecting eastward into the Atlantic high seas W of Greenland. Lowest surface pressure of this frontal system is 988 mb and about to make landfall in SW Greenland. Convergence on the west side of this front's upper trough supports a surface 1029 mb center over S Hudson Bay. The southern end of this upper trough appears to be fracturing southeastward (due to the strength of the upper ridge mentioned in paragraph P2)...with divergence east of this fracture supporting SE-moving cloudiness E of Bermuda that shows some light cyclonic turning in satellite animations.

P4...In the upper-levels...an upper vortex S of Greenland 36 hrs ago was expected to merge with a southern mid-ocean upper trough (see last sentence of discussion #17 paragraph P5). After this merger took place...this large mid-ocean upper trough is splitting...with the upper vortex S of Greenland now located NW of the Azores and another upper vortex located far NE of the Lesser Antilles. The northern upper vortex was supporting a 1000 mb frontal cyclone...which has since whirled beneath the upper vortex such that a 1012 mb deep-layered low is NW of the Azores as marked in the above charts.

P5...Full-fledged upper anticyclone near the British Isles has shifted west while now being associated with warm air advection ahead of frontal system in paragraph P3. When this upper anticyclone was just merely an upper ridge axis day ago...its eastern convergence supported Atlantic surface subtropical ridging that has since weakened since the surface ridge had split from this upper convergence. Currently this weaker surface ridge consists of 1024 and 1022 mb centers N of the Canary Islands while located SE of the 1012 mb deep-layered cyclone in paragraph P4.

P6...Thirty-six hours ago...an upper vortex NE of the Azores created a new frontal extratropical (non-tropical) cyclone NW of Spain while cut-off to the south of the upper anticyclone in paragraph P5. This new extratropical cyclone has since cyclonically orbited to a position beneath the upper vortex...and we now have a deep-layered non-tropical low spinning on satellite to the SW of the British Isles as marked in the upper-right corner of the above charts.

...TROPICAL BELT DISCUSSION...
P7...Upper vorticity remains trapped offshore of SE US and E Gulf of Mexico...and is located very near Tropical Storm Beryl. Upper convergence west of this vorticity and east of the upper ridge in paragraph P2 supports the long-lived sinking dry air across the Gulf of Mexico.

P8...Long-lived Caribbean upper ridging remains split...with a central Caribbean upper trough between the split that has merged with the upper vorticity very near Tropical Storm Beryl. The eastern split is a full-fledged upper anticyclone SE of the Dominican Republic...supported by warm air advection ahead of the deep-layered cyclone that is Tropical Storm Beryl. The western split is a leftover upper ridge axis over SE Mexico and Guatemala. Residual dry air dominates E half of the Caribbean and tropical waters E of the Lesser Antilles....once created by convergence on the SE side of this Caribbean upper ridging. This residual dry air has been kicked out of the western Caribbean...replaced by moistening air created by upper divergence E of the central Caribbean upper trough and W of the upper anticyclone SE of the Dominican Republic.

P9...Pair of tropical waves has changed in the last 36 hours. The western of the two had entered the Caribbean Sea...and was in a favorable low shear environment on the south side of the Caribbean upper ridging in paragraph P8...but suppressed by the residual dry air also mentioned in paragraph P8. This western tropical wave has been removed from 0600Z TAFB as if it dissipated...but I kept it in my charts above based on the 0600Z TAFB isobars. Even if this western tropical wave still exists...it is entering an unfavorable southwesterly vertical shear environment caused by central Caribbean upper trough also mentioned in paragraph P8. Meanwhile...the eastern of the two tropical waves has been kept stationary (midway between Cape Verde Islands and Lesser Antilles) in the last three days of TAFB analyses...as if its transitioning to a surface trough supported by divergence SE of an upper vortex. This upper vortex is located well NE of the Lesser Antilles (paragraph P4).

P10...Upper ridge over the E tropical Atlantic continues...and the air in the eastern half of the tropical Atlantic has seen a great increase in moisture content thanks to divergence beneath this upper ridge and divergence east of the mid-oceanic upper trough (now two upper vortices) in paragraph P4.

Updated: 9:14 PM GMT on May 27, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #17

By: NCHurricane2009, 5:20 AM GMT on May 26, 2012

...MAY 26 2012...1:21 AM EDT...
Subtropical Storm Beryl forms south of Cape Hatteras, NC and east of Charleston, SC! I think we will need to take a look back at records to see if there ever was an Atlantic season with two named cyclones before June 1. Residents from the Carolinas to NE Florida should keep tabs on this developing situation. See special features section below for further details.

This is the seventeenth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

Any feedback on how to improve these discussions can be left in the comments section below. In particular...any suggestions on how to improve the standard two "birsdeye view" charts below will be much appreciated. Are you able to understand the markings in the charts? Are you able to relate what is said in the discussions to these charts?

One improvement being attempted this evening is the implementation of paragraph (P) numbers in general discussions. Paragraphs will cross-reference each other to make the discussion easier during complex weather patterns.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1930Z-released HPC analysis.

In light blue is upper air anlaysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicating surface lows, Hs indicating surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery.Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...SUBTROPICAL STORM BERYL...
Some of my predictions (from discussion #16) that came true this late evening on Invest 94-L were:
(a) Upper voriticty to the SW amplified...which hooked the track more northward...reduced the shear....and divergence on the NE side of this upper voriticty and adjacent upper ridging strengthened 94-L to Subtropical Storm Beryl right on cue.
(b) The northward track has slowed due to a blocking pattern consisting of what is now surface ridging in the Gulf of Mexico (mentioned in paragraph P6 below)...and what is now a 1033 mb ridge just south of Newfoundland (mentioned in paragraph P1).

The rest of the predictions I made in discussion #16 fell apart...mainly track-wise. Beryl is much further northeast in position than I thought it would be...but this is because I made a forecast track on a center that had not yet become established.

But the bigger flaw in discussion #16's forecast was expecting a frontal system from the NW (now covered in paragraph P1 below) to break through the aforementioned blocking pattern and drag the system in an offshore clockwise loop turn thru 0000Z May 28. I should have listened more to the models...which were correctly playing down the steering influence of this frontal system. However...I think there is some very light steering influence this evening from this frontal system...as the recent infrared imagery still shows a northward track for Beryl in Figure 1 below. Utlimately...1016 mb ridge behind this front and over Iowa (paragraph P1) will re-enforce the blocking pattern to the northwest of Beryl...forcing Beryl to turn west and then southwest while steered by the block.

I have adjusted my predicted track from the NHC's as shown in Figure 2 below...accounting for the initial northward motion announced in the first NHC public advisory and supported by the two satellite frames in Figure 1. Intensity-wise...the most recent AVN-color-scheme infrared satellite shows a decrease in the orange-shaded deep T-storm clouds...all as Beryl ingests dry air from the Gulf of Mexico. Source of this dry air covered in paragraph P6. I expect Beryl will pulse on and off in central T-storm activity as it slowly mixes out the dry air with vertical motion...the vertical motion supported by instability driven between the warm ocean surface and cold upper vorticity. Therefore...I don't expect full vertical warm core formation (with anticyclonic upper outflow for strengthening) until Sunday (in other words I don't expect a fully tropical Beryl till Sunday). I expect Beryl to then begin weakening as it approaches the Georgia coast (from the shelf of cooler waters W of Gulf stream). Weakening is then exacerbated by westerly vertical shear induced by the frontal system in paragraph P2...the same frontal system that will recurve and accelerate Beryl NE from Georgia thru the Carolina coast late in the forecast period. Land interaction late in the forecast also would contribute to weakening.

The impact forecast in Figure 2 is based on if Beryl follows my track and intensity forecast. Notice that the drawn-in (blue-dashed line) heavy-impact swath has a rightward bias in relation to my forecast track late in the period...an effect I expect from westerly vertical shear forecast by that time.


Figure 1: Latest infrared satellite imagery trend of Subtropical Storm Beryl


Figure 2: My current best-guess forecast for Subtropical Storm Beryl. I don't expect a fully tropical Beryl till Sunday.

...MID-LATITUDES DISCUSSION...
P1...Main energy of frontal system from central US is moving NE into Canada tonight. Lowest surface pressure of frontal system is marked by a 989 mb cyclone approaching the south tip of Hudson Bay. Upper trough of frontal system is just west of this cyclone while supporting the cyclone with upper divergence. Upper convergence west of this upper trough supports a surface ridge to the SW...with a 1016 mb center over Iowa. Warm air advection ahead of this frontal system supports an upper ridge that has sprouted NE across the eastern US from the Texas/Arkansas area. The NE US upper anticyclone ahead of this frontal system has moved into Atlantic Canada and NW Atlantic as an upper ridge wave. Upper convergence E of this upper ridge wave supports a surface ridge center S of Newfoundland which has strengthend from 1028 to 1033 mb in the last 24 hrs...and another ridge center of 1026 mb just NE of Bermuda.

P2...Tail end of frontal system in paragraph P1 features a rapidly developing 989 mb cyclone over W Utah. Upper-level pressures over the west US are falling as this 989 mb cyclone advects in cold air from the NW such that an upper trough is forming. This 989 mb cyclone and attendant upper trough will be the next frontal system hot on the heels of the system in paragraph P1...and is expected to eventually recurve Subtropical Storm Beryl currently off of the US east coast. See special features section for details on this.

P3...Frontal system currently over the high seas S of Greenland has become cut-off from the mid-latitude westerlies...thanks to the Atlantic Canada/NW Atlantic upper ridge wave covered at the end of paragraph P1. In the upper-levels...the cut-off is well defined with a small upper vortex. The associated 1000 mb frontal surface cyclone is in the process of whirling cyclonically beneath this new cut-off upper vortex.

P4...Upper ridge axis in north-central Atlantic 24 hrs ago...supported by warm air advection ahead of frontal system in paragraph P3...is now a full-fledged upper anticyclone near the British Isles (based on animation of latest infrared imagery). Eastern upper convergence of this system used to support surface Atlantic subtropical ridging 24 hrs ago...which are now 1026 and 1028 mb centers in the above charts. With these surface ridge centers no longer beneath upper convergence...expect these centers to lose dominance while the 1033 and 1026 mb centers become the dominant.

P5...Massive upper trough in the E Atlantic has become strongly cut-off to the south of British Isles upper anticyclone featured in paragraph P4. In addition...this massive upper trough has split...now an upper vortex NE of the Azores...and a southern upper trough in the mid-ocean. 24 hrs ago...this massive upper trough supported a 1011 mb depression in Spain and surface trough NE of the Azores. Based on 1200Z TAFB and visible satellite animation before sunset...these surface features have evolved into a new frontal extratropical (non-tropical) cyclone NW of Spain in the above charts. Expect this new extratropical cyclone to cyclonically orbit about the upper vortex NE of the Azores. Expect the southern mid-ocean upper trough to merge with the upper vortex mentioned in paragraph P3.

...TROPICAL BELT DISCUSSION...
P6...Upper vorticity remains trapped over the SE US and E Gulf of Mexico ...and has triggering Tropical Storm Beryl tonight as explained in the special features section. Upper convergence west of this vorticity continues to support the long-lived dry air across the Gulf of Mexico...as well as relatively new Gulf of Mexico surface ridging which now connects NE all the way to the 1033 mb center in paragraph P1.

P7...Long-lived Caribbean upper ridging has split into two areas tonight...with an inverted upper trough forming between the split and south of Jamaica. The eastern split is a full-fledged upper anticyclone over the Dominican Republic tonight...supported by warm air advection ahead of subtropical cyclone covered in the special feature section. The western split is a leftover upper ridge axis over SE Mexico and Guatemala. Residual (but strong) dry air dominates the SE half of the Caribbean and tropical waters E of the Lesser Antilles....once created by convergence on the SE side of this Caribbean upper ridging.

P8...A pair of tropical waves are in the tropical belt of the Atlantic...one that has entered the Caribbean from the Lesser Antilles...the other following behind hundreds of miles to the east. Both waves are suppressed by dry air mentioned at the end of paragraph P7 above. Upper easterlies (aligned with low-level easterlies) on the south side of paragraph P7's upper ridging means favorable low shear. If the western of the two tropical waves overcomes the dry air and starts to develop T-storm activity in this low shear environment...this may lead to yet another area of interest. Absolutely no signs of that at this time.

P9...Upper ridge over the E tropical Atlantic is subsiding as a major mid-ocean upper trough is taking shape to the NW of it. This mid-ocean upper trough will be a merger between two current upper features as explained by the last sentence of paragraph P3.

Updated: 5:22 AM GMT on May 26, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #16

By: NCHurricane2009, 6:17 AM GMT on May 25, 2012

...MAY 25 2012...
A subtropical cyclone is possible this upcoming Memorial Day weekend. Residents in the Carolinas...SE Georgia...and NE Florida should keep tabs on this developing situation through the weekend. See special features section for further details.

This is the sixteenth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

Any feedback on how to improve these discussions can be left in the comments section below. In particular...any suggestions on how to improve the standard two "birsdeye view" charts below will be much appreciated. Are you able to understand the markings in the charts? Are you able to relate what is said in the discussions to these charts?

One improvement being attempted this early morning is the implementation of paragraph (P) numbers in general discussions. Paragraphs will cross-reference each other to make the discussion easier during complex weather patterns.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1930Z-released HPC analysis.

In light blue is upper air anlaysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicating surface lows, Hs indicating surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery.Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...SUBTROPICAL DISTURBANCE INVEST 94-L...
As special update #15A concluded...this disturbance has been days in the making. A more brief statement on how this disturbance emerged is found in paragraph P6 of tropical belt discussion below. The surface low of the disturbance has moved NE out of the W Caribbean...across the Florida Straits...and now into the Atlantic waters between SE Florida and the west Bahamas. As it did so...there has been some increased definition to the spin of the surface low in the low-level clouds. The surface low of this disturbance was 1008 mb as of 1800Z TAFB. The NE motion of the surface low means that the rains have ended in the W Caribbean...and now are concentrated over portions of Cuba and the Bahamas.

Special update #15A concluded two challenges for 94-L...dry air in the Gulf of Mexico that would wrap into the circulation (paragraph P3 below mentions source of this dry air)...and westerly vertical shear from the frontal system also covered in paragraph P3 below. Westerly vertical shear has been maintained by trapped upper vorticity (over E Gulf of Mexico/SE US) left behind by the frontal system covered in paragraph P3. Because the National Hurricane Center and computer models are more "gung-ho" about development...I am upgrading my forecast for 94-L...summarized in Figure 1 below.

To say the least...atmospheric pattern is complicated...and so is the forecast. The central US frontal system in paragraph P2 is putting out plenty of warm air advection ahead of itself...which is amplifying a Texas-Arkansas upper ridge...and in turn this will cause equal amplfication of the trapped upper vorticity SW of 94-L. As the upper vorticity SW of 94-L amplifies...this will tend to (a) bend the track more directly northward as shown in Figure 1 and (b) lead to subtropical cyclone formation by late Friday/early Saturday (0000Z May 26). Subtropical cyclone formation will be possible by this window as the amplified upper vorticity allows for relaxation in westerly vertical shear...but more importantly there will be plenty of upper divergence to work with between the NE side of the amplified upper vorticity and adjacent upper ridging. I do not expect full-on tropical cyclone formation at this time because 94-L has deeply-swallowed the dry air from the Gulf of Mexico...which will cause a struggle for central convective T-storms and associated vertical warm core formation.

Note how I briefly slowed-down the northward speed of 94-L by 0000Z May 26...the slow-down I believe will be created by a blocking pattern consisting of (a) surface ridging in the Gulf of Mexico mentioned in paragraph P3...(b) upper anticyclone mentioned in paragraph P2...and (c) 1028 mb ridge center mentioned at the end of paragraph P3.

By 0000Z May 26...I expect the frontal system in paragraph P2 to be marching across the Ohio valley...to the NW of 94-L. From this position...I expect the frontal system to bend the track of 94-L eastward after 0000Z May 26. I expect upper westerlies from the frontal upper trough to shear whatever subtropical cyclone emerges out of 94-L....the center of the sheared cyclone passing just south of Cape Lookout, NC by 0000Z May 27 (late Saturday/early Sunday).

Afterwards...I predict 94-L (the subtropical cyclone) to make a clockwise loop turn and begin accelerating westward toward the Carolinas by late Sunday/early Monday (0000Z May 28). This westward acceleration is caused by the NE side of the trapped upper vorticity and south side of surface ridging that builds behind the frontal system of paragraph P2. I state the system as "recovering" by this point as it escapes the westerly shear imposed by frontal system of paragraph P2.

Numerical models such as the GFS (Figure 2 below) have a different take on how 94-L's track concludes. This is because they give less credence to paragraph P2's frontal system...and I think the models are under-estimating how much this frontal system will steer 94-L. The end result is that my forecast 0000Z May 27 and 0000Z May 28 positions are to the northeast of the numerical models. The GFS for example brings the system into S Georgia/N Florida by 0000Z May 28...while I am accelerating the system westward toward the Carolinas by 0000Z May 28.

Residents in the Carolinas...SE Georgia...and NE Florida should keep tabs on Invest 94-L through the Memorial Day weekend.


Figure 1: My current best-guess forecast for the evolution of disturbance Invest 94-L during Memorial Day weekend.


Figure 2: GFS model output for Invest 94-L during its 1800Z May 24, 2012 run.

...MID-LATITUDES DISCUSSION...
P1...Frontal system covered previously in discussion #15 (paragraph P2) has undergone complex evolution. The cold front it was driving southward from Canada in discussion #15 has since expanded southward into the central US (see paragraph P2 below). The remainder of this frontal system had zoomed east...absorbing the upper vorticity near Alberto along the way (see paragraph P3 below for details).

P2...Frontal system currently in the central US originated from southern Canada as paragraph P1 above mentions. As this front pushed south and ushered in colder air...this created a new upper trough currently present in the upper-left corner of the above birdseye charts. Meanwhile...warm air advection ahead of this frontal system has created upper ridging...with an upper ridge axis from Texas to Arkansas...and an upper anticyclone presently over the NE US. The lowest pressure at this surface with this frontal system was 995 mb over SW Iowa as of 1930Z HPC. The north end of this frontal system was anchored by a single cyclone over S Canada 24 hrs ago...which has since split into 997 mb and 996 mb centers as shown in the above birdseye charts.

P3...Frontal system currently over the high seas S of Greenland (and whose tail end reaches into the NE US) had a complicated evolution over the last days. Its upper trough had absorbed the upper vorticity near what was once TS Alberto. It subsequently disconnected from this upper vorticity while the upper vorticity became trapped by building upper ridging covered in paragraph P2 above. This trapped upper vorticity is currently over the eastern US and E Gulf of Mexico and could aid in subtropical cyclone formation beginning 24 hrs from now (see special features section above for details). Meanwhile...northwesterly flow on this upper vorticity's west flank still heavily converges with westerly flow on the north side of Caribbean upper ridging...which firstly has begun to create surface ridging in the Gulf of Mexico...and secondly has done a great job in re-enforcing sinking dry air over the Gulf of Mexico over the last several days. Finally...divergence on the east side of the upper trough S of Greenland supports a broad surface frontal cyclone with lowest pressure of 1000 mb...and convergence on the west side of this upper trough supports a 1028 mb ridge S of Newfoundland.

P4...Upper ridge axis in north-central Atlantic supported by warm air advection ahead of frontal system discussed in paragraph P3. Eastern convergence of this upper ridge supports a surface 1027 mb subtropical ridge center.

P5...In paragraph P4 of discussion #15...a large upper vortex was marching ESE while located S of Greenland...and another upper vortex was NE of the Lesser Antilles in paragraph P8 of discussion #15. Both of these upper vortices have since merged into a singular massive upper trough in the E Atlantic...from N of the Azores...to the tropical waters midway between the Lesser Antilles and Cape Verde Islands. Over the last 48 hours...divergence on the NE side of this E Atlantic upper trough has been supporting a new surface trough NE of the Azores...and a depression over Spain (1011 mb in the above charts).

...TROPICAL BELT DISCUSSION...
P6...Upper vorticity remains trapped over the SE US and now into the E Gulf of Mexico (see paragraph P3 above for details). Southwesterly upper flow has been directionally divergent between this upper vorticity and Caribbean upper ridging over the last several days...which ultimately has led to the development of disturbance Invest 94-L and its massive area of clouds (see special feature section above for details). The convective cloudiness of 94-L has maintained a good chunk of the Caribbean upper ridging via latent heat release. Convergent northerly flow on the south side of this upper ridging is leading to sinking dry air across the Caribbean's SE half and into the tropical waters east of the Lesser Antilles.

P7...A pair of tropical waves are in the tropical belt of the Atlantic...one approaching the Lesser Antilles...the other following behind hundreds of miles to the east. Both waves are suppressed by dry air mentioned at the end of paragraph P6 above...although the southern Lesser Antilles is seeing some convective cloudiness from the western of the two tropical waves.

P8...Upper ridge over the E tropical Atlantic continues as it has for the last days. In water vapor animation over the last days...noticed some moistening of the air beneath this upper ridge...perhaps as the upper ridge's outflow is allowing for some rising of air below.

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2012 Atlantic Hurricane Season Birdseye Discussion #15A (Special Update)

By: NCHurricane2009, 4:13 AM GMT on May 23, 2012

...MAY 23 2012...
Early tropical activity persists in the Atlantic basin! The Naval Reasearch Laboratory (NRL) of the United States Navy has upgraded an area of disturbed weather in the western Caribbean Sea to Invest 94-L.

This disturbance has been several days in the making as a browse through previous discussions shows:
Second special feature section of discussion #12
Paragraph P6 of discussion #13
Paragraphs P7 and P8 of discussion #14
Paragraph P7 of discussion #15

The prognosis for this disturbance had been that it would not develop due to exposure under westerly vertical shear on the north side of Caribbean upper ridging. The upper ridge was expected to remain suppressed to the south (and maintain the shearing) due to (a) the upper vorticity near Tropical Storm Alberto and (b) the next upstream frontal upper trough behind this upper vorticity.

Shortly after full discussion #15 was written...the lowest surface pressures in the western Caribbean were able to collocate with the Caribbean upper ridge axis...and the system became a little better organized with T-storm banding features E of the low surface pressures. T-storm activity west of the low surface pressures remains absent due to the persistence of aforementioned westerly vertical shear.

The upper vorticity near what was once Alberto has been absorbed by the next upstream frontal upper trough...as this frontal system moves into the eastern US. The first challenge this system faces is surviving the impulse of westerly vertical shear on the south side of this frontal upper trough.

The second challenge is adjacent dry air that has been persisting in the Gulf of Mexico and northern Florida over the last several days (for example see discussion #15...paragraph P5 for the source of this dry air). If the low surface pressures spin up into a cyclone...the cyclonic circulation could easily ingest this dry air source.

Prognosis this early morning is to watch this system carefully...and see how well it fares against the above two challenges. If I see signs of tropical cyclone formation...I will once again be resuming pre-season full birdseye discussions. If not...this will be the last until June 1 2012. Regardless...the presence of this disturbance will continue rains in the western Caribbean (Honduras...Nicaragua...Cayman Islands...Jamaica...Cuba) and Bahamas over the next days. Some of these areas have been saturated by the persistent rainfall with this sytem...so flooding is another possibility.

Updated: 4:15 AM GMT on May 23, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #15

By: NCHurricane2009, 3:04 AM GMT on May 22, 2012

...MAY 21 2012...
Alberto moving northeast away from the US east shore...and could lose its tropical cyclone status in the next 24 hours. If so...this will be the last discussion until June 1 2012...or unless yet another round of early-season tropical activity occurs in the Atlantic.

This is the fifteenth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

Any feedback on how to improve these discussions can be left in the comments section below. In particular...any suggestions on how to improve the standard two "birsdeye view" charts below will be much appreciated. Are you able to understand the markings in the charts? Are you able to relate what is said in the discussions to these charts?

One improvement being attempted this evening is the implementation of paragraph (P) numbers in general discussions. Paragraphs will cross-reference each other to make the discussion easier during complex weather patterns.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1924Z-released HPC analysis.

In light blue is upper air anlaysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicating surface lows, Hs indicating surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery.Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ALBERTO...
Alberto...hmmmm....not much to elaborate on since it has been well-behaved with the previous forecast in discussion #14A. He moved SE...ESE...then NE while orbiting cyclonically about the adjacent upper low as expected...and will continue NE as the frontal system in paragraph P2 (mid-latitudes discussion below) approaches from the west. Alberto has just weakened to a tropical depression due to the southwesterly vertical shear developing across the system (as evidenced by the exposed center and weak convective clouds biased to the NE of that center). Southwesterly shear was initialized by the adjacent upper low now to Alberto's NW...and will be maintained by incoming frontal upper trough in paragraph P2. Updated track and intensity forecast in Figure 1 below.


Figure 1: My best guess forecast for Tropical Depression Alberto as of May 21, 2012 late evening. Trackwise...being to the right of the NHC's track last night was a good prediction...as indeed it did go to the right of the NHC's best guess track. In fact my rightward bias was not right enough. Recent ENE motion suggests the rightward bias to the NHC track will continue...and this is what I forecasted above.

...MID-LATITUDES DISCUSSION...
P1...In the upper-left of the above charts...next upper ridge in the mid-latitudes has entered the western US. Upper convergence on its east side supports central US surface ridge centers...with a max of 1025 mb in NW MIssouri.

P2...Upper trough from western US has arrived to the central US Divergent southwesterly flow ahead of this upper trough continues supporting a complex surface frontal system. The 1006 mb frontal depression over SW Hudson Bay 24 hrs ago is now the dominant low pressure system of this front...and has zoomed to the NE corner of Hudson Bay such that it its not in scope of the above charts. This dominant frontal low is also driving another cold front pushing southward across Canada...marked in the upper-left in the above charts with a weak 1012 mb low.

P3...Upper ridge in NE US and E Canada will be pushing offshore into the far north Atlantic in the next hours. It has temporarily merged with the Caribbean upper ridge mentioned later in paragraphs P6 and P7. It continues to be supported by warm air advection ahead of the frontal system in paragraph P2. Convergence on the east side of this upper ridge supports a greater-than-1028 mb surface ridge that has moved offshore from E Canada.

P4...Upper low vortex south of the S tip of Greenland is tracking ESE...the southward component of its track likely caused by cold air advection behind its associated surface cyclone. This surface cyclone was weakening in the last 48 hours while beneath the non-divergent center of the upper low...and had reached 995 mb in its weakening phase 24 hrs ago. Animation of North American HPC surface analyses suggests this surface cyclone regenerated beneath NE upper divergence of the upper low...which has allowed it to re-strengthen to 990 mb. It is now again in a weakening phase beneath the upper low...rising back to 991 mb per latest HPC.

P5...Upper vorticity near TS Alberto seemed to perform a "fujiwhara-like dance" with Alberto per last night's special update #14A...which caused this upper vortex to make landfall in SC last night while tracking NW. This upper low is now well-inland in the SE US. The associated 1014 mb surface low that made landfall on the mid-Atlantic shore yesterday is now in the Ohio Valley at 1015 mb. Expect the upper trough associated with the system in paragraph P2 to absorb the upper low within the next 24 hours. Expect the surface frontal system in paragraph P2 to absorb the 1015 mb low in the next 24 hours. On a final note...northwesterly flow on the SW side of this upper low is converging heavily with westerly flow on the N side of Caribbean upper ridging (paragraphs P6 and P7)...continuing to maintain the dry air across the Gulf of Mexico and north Florida.

P6...Caribbean upper ridging persists...and its NE lobe has merged with the upper ridge mentioned in paragraph P3. Disturbed weather associated with this upper ridge is covered more extensively in the tropical belt discussion...paragraph P7. 24 hrs ago...a portion of the upper ridge's NE extension broke off into the NE atlantic....the break-off caused by locally intense warm air advection ahead of the cyclone in paragraph P4. The 1034 mb surface ridge center W of the Azores (24 hrs ago) now has dual centers...a 1030 mb western center supported by convergence E of the Caribbean upper ridge's NE extension...and an eastern 1026 mb center supported by convergence E of the NE Atlantic upper ridge.

...TROPICAL BELT DISCUSSION...
P7...Upper vorticity near Alberto is now inland over the SE US (see paragraph P5 for details). Southwesterly upper flow has been directionally divergent between this upper vorticity and Caribbean upper ridging mentioned in paragraph P6...leading to T-storm activity in the W Caribbean...Cuba...Bahamas...and waters NE of the Bahamas. See paragraph P8 of discussion #14 on how I connected a few surface trough features into a "single surface trough" in the region (although technically it hasn't been analyzed as a single entity in TAFB). The 1009 mb W Caribbean disturbance at the west end of this "singular surface trough" has weakened slightly to 1010 mb. At the NE end of this "singular surface trough" is a new 1012 mb low west of Bermuda. Expect the weather in the W Caribbean...Cuba...Bahamas...and waters NE of Bahamas to remain unsettled due to the convergence of this "singular surface trough" and directional upper divergence between the Caribbean upper ridge and upper vorticity near Alberto. Because the upper vorticity near Alberto will be replaced by the incoming upper trough in paragraph P2...this will prolong this unsettled weather. Latent heat release from the clouds of this unsettled weather is also now supporting a good chunk of the Caribbean upper ridge and its NE extension.

P8...Upper low vortex persists NE of the Lesser Antilles. Animation of water vapor imagery suggests lift created by divergence at the boundary of the upper low (and perhaps also created by instability associated with its cold upper air temps) has eroded a lot of the dry air it had. Weak surface trough that it spawned yesterday is no longer in TAFB analyses.

P9...Upper ridging continues in the E tropical Atlantic. A tropical wave to the SW of the Cape Verde Islands 24 hrs ago is now midway between the Cape Verdes and Lesser Antilles (at the W end of this upper ridge)...and will soon enter a hostile shear environment created by the upper low in paragraph P8. An increase in clouds and t-storms is possible as this tropical wave interacts with the SE peripherial divergence of the upper low. Another tropical wave has moved offshore from Africa...beneath the E side of this upper ridge.

Updated: 4:43 AM GMT on May 22, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #14A (Special Update)

By: NCHurricane2009, 4:45 AM GMT on May 21, 2012

...MAY 21 2012...
Just after stroke of midnight...Alberto shows a definite and more offshore course. I am writing this special update to change my forecast from discussion #14 accordingly.

It has been interesting to watch Alberto move in relation to an adjacent upper low...which first was west of Alberto (over Georgia) in discussion #13...south of Alberto in discussion #14...and now northeast of Alberto in this special update (#14A). Recently...Alberto's track has shifted from SW...to south...to now SE. Putting Alberto's motion together with the upper low's motion...both seem to be cyclonically orbiting about each other in a fujiwhara-like interaction (normally fujiwhara interactions are associated with two tropical cyclones cyclonically orbiting each other).

I now expect Alberto to continue cyclonically orbiting about the adjacent upper low...accelerating to the SE...E...then NE. The incoming frontal upper trough from the west will then absorb this upper low and accelerate Alberto NE for the remainder of its lifespan. My updated forecast track (below) is just to the right of the NHC's...accounting for the very recent acceleration to the SE toward 30N latitude. In terms of how far north Alberto will get...I expect it to be behind in timing compared to the NHC's forecast track due to the recent SE motion. So I took all 8 O'clock positions and made them around 11 O'clock.

Intensity-wise...Alberto keeps on firing small bursts of T-storms near the center...perhaps due to instability caused by the temp difference between the warm ocean surface and adjacent cold core upper low. So I keep it at 40 mph (minimal tropical storm) for the first few forecast points...then I weaken it to 35 mph (tropical depression or remnant low) when I think southwesterly vertical shear (from the incoming frontal upper trough) and cooler waters will start killing off Alberto.

This more offshore forecast (compared to the close-shore forecast in discussion #14) means that the threat of direct impacts (i.e. gusty showers) no longer applies to all land areas. The narrow blue-swath of gusty shower impacts in the graphic below is based on the current small diameter of convective T-storms near the center...and then I lean this swath toward the right-of-center later in the forecast as gusty winds are always to the right in an accelerating cyclone.

Updated: 6:19 AM GMT on May 21, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #14

By: NCHurricane2009, 8:45 PM GMT on May 20, 2012

...MAY 20 2012...
Alberto moves closer to the South Carolina and Georgia shores more than expected...but is also in unfavorable conditions and should weaken. See special feature section for further details.

This is the fourteenth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

Any feedback on how to improve these discussions can be left in the comments section below. In particular...any suggestions on how to improve the standard two "birsdeye view" charts below will be much appreciated. Are you able to understand the markings in the charts? Are you able to relate what is said in the discussions to these charts?

One improvement being attempted this afternoon is the implementation of paragraph (P) numbers in general discussions. Paragraphs will cross-reference each other to make the discussion easier during complex weather patterns.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1330Z-released HPC analysis.

In light blue is upper air anlaysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicating surface lows, Hs indicating surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery.Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ALBERTO...
Current synoptic situation of Alberto is summarized in Figure 1 below. The tropical storm moved more southwestward than shown by any forecast yesterday...and is now closer to southern SC and Georgia shores more than expected. General discussion paragraph P4 explains that the upper vorticity around Alberto has consolidated as a single upper low just south of Alberto. And with low-level ridging persisting to the northwest....one can see (with the blue and red arrows in Figure 1) the northeasterly flow that has been pushing Alberto southwestward since its birth. Alberto has also weakened...one big reason being that its anticyclonic outflow upper ridge (vital for maintaining Alberto's low surface pressure) has been absorbed by an upper ridge in the NE US (see paragraph P2). Alberto now looks more like a subtropical storm on satellite while beneath the cold core upper low centered to its south. This upper low has been wafting in dry air from the Gulf of Mexico into Alberto's circulation (source of dry air covered in paragraphs P2 and P4). In addition...this upper low perhaps is contributing to easterly shear which is keeping whatever remains of Alberto's T-storms biased to the west of center.


Figure 1: Alberto's current synoptic environment

Track-wise...my forecast yesterday was further southeast (i.e. more offshore) than the NHC. While being further south was perhaps more accurate...the more offshore aspect was not. I was expecting the 1006 mb cyclone NE of Alberto (now 1014 mb in the atmospheric features birdseye chart) to impart a drag that would keep Alberto more offshore than it has been...but it appers this cyclone is doing ditto when it comes to steering Alberto. My new track forecast (Figure 2 below) extrapolates the 3-hourly SW track seen in Figure 1 for the next several hours..then slows the track and bends it more west as the northwestern low-level ridge (in Figure 1) begins eroding in advance of the frontal system in paragraph P1. Southwesterly flow ahead of the frontal system will then begin the long-awaited acceleration towards the NE later in the forecast period. All-in-all...I am now left of the NHC forecast instead of to the right (as I was yesterday).

Intensity-wise...It was good yesterday to not predict additional strengthening (reasons cited were Alberto's anityclonic outflow getting squeezed out and adjacent dry air). However...it would have been better to suggest weakening as we have seen in the last day. Additional unfavorable factors (easterly shear...dry air advection covered in Figure 1 and the 1st paragraph of this special feature section) suggest additional slow weakening...and so that is what is shown in Figure 2 below. Note that a recent burst of T-storms covering the center prevents me from showing very immediate weakening (burst perhaps caused by some instability between the cold temp of adjacent upper low and warm Gulf stream surface waters).

Impact-wise...I was expecting in the previous forecast that the tangible weather (i.e. some rainfall) of Alberto would spread onshore in NE SC and SE NC. The rainfall never did reach SE NC...instead has been tracking SW with Alberto along the SC shore...and now the heaviest rains are moving southward across Georgia's coast and even into the NE corner of Florida using the latest radar animations. Using my track forecast...I don't expect these rains (which may contain some gusty winds) to go further south than NE Florida. See the drawn-in blue-dashed swaths and blue-text statements in Figure 2 for further details on Alberto's forecast impacts.


Figure 2: My best guess forecast for Tropical Storm Alberto as of May 20, 2012 afternoon. Note how the swath of impacts (blue-dashed swath) is leaned to the right-of-track late in the forecast...as an accelerating storm has its worst impacts to the right-of-center...and as Alberto is expected to be under southwesterly shear from the frontal upper trough steering it NE.

...MID-LATITUDES DISCUSSION...
P1...In the above birdseye charts...an upper trough continues pushing eastward from the western US. Divergent southwesterly flow ahead of this upper trough continues supporting a disorganized frontal cyclone with fairly low pressures...right now the lowest pressure being 1009 mb in SE Minnesota. Southerly low-level flow ahead of this 1009 mb cyclone is in competition with northerly low-level flow behind a 995 mb cyclone mentioned below in paragraph P3...resulting in a southern Canada front dividing the air mass contrasts between the cyclones. This front had an east-west orientation during the past 48 hours...but is now being lifted north ahead of the upper trough and features an intensifying 1006 mb depression over SW Hudson Bay.

P2...Upper ridge that dominated central North America has shifted eastward into NE US and E Canada in advance of weather system in paragraph P1. It continues to be supported by warm air advection ahead of 1009 mb cyclone in Paragraph P1. This upper ridge has an anticyclonic center over W New York whose southern lobe has absorbed the remainder of Alberto's warm core upper anticyclone. Convergence on the east side of the upper ridge supports surface ridge centers of 1025 mb moving offshore from NE US...and 1028 mb that has moved east from Hudson Bay into the Quebec/Newfoundland border. 24 hrs ago...northwesterly flow on the east side of this upper ridge converged heavily with westerly flow on the north side of Caribbean upper ridging mentioned below in Paragraphs P5 and P7...which created a lot of dry air in the Gulf of Mexico/N Florida. This dry air is no longer associated with this upper ridge...but now with the upper voriticity near Alberto as mentioned at the end of paragraph P4.

P3...Upper low vortex that moved from Labrador in the direction of S Greenland has passed south of Greenland's S tip. 993 mb surface cyclone (24 hrs ago) that was weakening with a lack of divergence beneath this upper low has weakened further to 995 mb.

P4...The cut-off upper low vortices 24 hrs ago....one over Georgia...and the other E of Virginia...have merged into a singular elongated upper low centered S of Alberto...extedning W into the Gulf of Mexico and NE into the W Atlantic (offshore of US). A 1006 mb cyclone 24 hrs ago was supported by divergence supplied from the upper vortex that was E of Virginia. This 1006 mb cyclone has now weakened to 1014 mb while beneath the less divergent axis of the now singular elongated upper low. This 1014 mb low is also tracking slowly west under the guidance of a 1025 mb ridge mentioned in paragraph P2...and could make landfall along the mid-Atlantic shore...but with weak (if any) weather. A final note about this elongated upper low is that it is now responsible for what has been dry air in the Gulf of Mexico/N Florida. Northwesterly flow on its SW side today is heavily converging with westerly flow on the north side of Caribbean upper ridging (paragraphs P5 and P7)...leading to sinking motion below for the persistence of this dry air.

P5...Caribbean upper ridging persists...and continues to extend NE into the open Atlantic for several-hundred miles. A portion of this NE extension has broken off and is a seperate upper ridge over the Azores. This break-off is caused by locally intense warm air advection ahead of the cyclone in paragraph P3. The remainder of this NE extension is now sustained by latent heat release of extensive T-storm activity in the W Caribbean...Cuba...and Bahamas as discussed in detail at the end of paragraph P8. The 1034 mb surface ridge center W of the Azores used to be supported by conergence on the E side of this NE upper ridge extension. Technically it is not beneath any such upper convergence at the moment...so this 1034 mb surface ridge may weaken in the next several hours.

P6...The upper trough from the E Azores to Europe 24 hrs ago is undergoing a split due to the strength of the NE upper ridge extension covered in previous paragraph P4. One split is an east-west upper trough S of the Azores...and the second split is an upper trough fragment venturing into W Europe that will soon leave the scope of the above charts. A surface trough over the Iberian peninsula was shown at 1200Z TAFB...probably supported by divergence on the E side of the W Europe upper trough fragment.

...TROPICAL BELT DISCUSSION...
P7...As mentioned earlier in paragraph P5...upper ridging continues across the Caribbean..and this upper ridge has been extended NE into the Atlantic for several hundred miles. The embedded weak upper low S of Jamaica has dissipated. Low surface pressures in the western Caribbean (with a 1009 mb center) used to be maintained by the outflow of this upper ridge...but is now being maintained differently as explained in paragraph P8.

P8...The upper vorticity near TS Alberto has consolidated into a single center just south of Alberto (see paragraph P4 for details). Southwesterly upper flow is directionally divergent between this upper voriticty and Caribbean upper ridging mentioned in paragraphs P5...P7...and P8...leading to T-storm activity in the W Caribbean...Cuba...Bahamas...and waters NE of the Bahamas. A surface trough formed just N of the Bahamas yesterday under this directional divergence...and the 1200Z TAFB has just added another surface trough in the W Bahamas. A 1009 mb W Caribbean disturbance used to be supported by the outflow of the Caribbean upper ridging...but is now supported by this directional upper divergence. One could almost connect this 1009 mb disturbance...the W Bahamas surface trough...and the surface trough N of the Bahamas into a "singular surface trough." Expect the weather in the W Caribbean...Cuba...Bahamas...and waters NE of Bahamas to remain unsettled due to the convergence of this "singular surface trough" and directional upper divergence between the Caribbean upper ridge and upper vorticity near Alberto. Because the upper vorticity near Alberto will be replaced by the west US upper trough near paragraph P1...this will prolong this unsettled weather. Latent heat release from the clouds of this unsettled weather is also now supporting a good chunk of the Caribbean upper ridge's NE extension.

P9...New upper low vortex persists NE of the Lesser Antilles (its formation covered by paragraph P5 in discussion #13). It has begun mixing out some of the extensive dry air with divergence on its NE side. The lift created by this NE divergence is creating a new batch of T-storm clouds seen NE of the Lesser Antilles in satelllite imagery. This upper low's divergence has also created a new surface trough midway between the Cape Verdes and Lesser Antilles...added at 1200Z TAFB today. Expect this new surface trough to move west under the guidance of strong 1034 mb ridge mentioned in paragraph P5. Under such a west track...the surface trough will leave the upper low's divergence that spawned it...so it should weaken once it moves west.

P10...Upper ridging continues in the E tropical Atlantic. Upper outflow of this ridge coupled with surface convergence from a tropical wave (now SW of the Cape Verde Islands) has generated some lift to moisten away some of the dry Saharan air that was wafted into this region during the last 48 hours. However...the tropical wave's development is not expected as it will encounter hositle southweserly shear thanks to the upper low in paragraph P9.

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2012 Atlantic Hurricane Season Birdseye Discussion #13

By: NCHurricane2009, 1:55 AM GMT on May 20, 2012

...MAY 19 2012...
Special feature off of the Carolina coast becomes Tropical Storm Alberto. Meanwhile...I am no longer considering the western Caribbean disturbance a special feature due to unfavorable upper winds.

This is the thirteenth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

Any feedback on how to improve these discussions can be left in the comments section below. In particular...any suggestions on how to improve the standard two "birsdeye view" charts below will be much appreciated. Are you able to understand the markings in the charts? Are you able to relate what is said in the discussions to these charts?

One improvement being attempted tonight is the implementation of paragraph (P) numbers in general discussions. Paragraphs will cross-reference each other to make the discussion easier during complex weather patterns.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1336Z-released HPC analysis.

In light blue is upper air anlaysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicating surface lows, Hs indicating surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery.Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM ALBERTO...
The rapid genesis of this tropical storm off of the Carolina coast is covered by discussions #12 and #12A written in the last 24 hours. My track and intensity forecast (versus the National Hurricane Center's) this evening is shown in Figure 1 below.

Track-wise...my best guess leans to the right (or more offshore) when compared the hurricane center's projected track...as I expect the 1006 mb cyclone to the northeast (in the above charts and in paragraph P3 below) to keep the system tugged on a more southward track than the hurricane center shows in the short-term. The eventual acceleration towards the NE is caused by the 999 mb frontal cyclone covered in paragraph P1. This frontal cyclone is expected to erode the low-level ridging to the NW as it nears Alberto...the southwesterly flow from the frontal cyclone causing that NE acceleration.

Intensity-wise...let me first say that I bet no one in the meteorological community was expecting a 60 mph wind tropical storm to develop so fast when this was such a disorganized disturbacne 24 hours ago! What continues to favor Alberto's strengthening is warm water Gulf Stream waters and its warm core anticyclonic outflow it has established...extending northward into E North Carolina and Virginia as remarked later in paragraph P3. I am reluctant however to suggest any further strengthening as Alberto's upper anticyclone (its source of vital upper outflow) is being squeezed tightly between an upper low vortex over Georgia and another E of Virginia (these upper vortices covered in paragraphs P3 and P7). Another negative factor preventing strengthening is how dangerously close Alberto is to a Gulf of Mexico/N Florida dry air mass covered at the end of paragraph P2. My forecast intensity points are covered in Figure 1 below...with a maintainence of 60 mph max winds at the center...and then a weakening to 50 mph max winds as Alberto exits the warm swath of Gulf stream waters.

Impact-wise...surf and rip currents for the next few days as Alberto parallels the east US shore. Based on my best-guess forecast track and current wind radii from the National Hurricane Center...I drew out how the swath of Alberto heavy weather (high winds/heavy rain) should evolve. First it is generally symmetric about the center...and then notice how I later lean this swath towards the east of center as it accelerates to the NE. This is because the stronger winds are always to the right of center during an acceleration...and because I expect Alberto to get some upper-level southwesterly shear induced by the frontal cyclone that will steer it northeastward...the shear also acting to bias weather east of the center. All-in-all...this small diameter swath of strong Alberto weather remains offshore using my current best-guess track. Radar imagery and a recent northwestern flare up in convective clouds on satellite imagery suggests some rain will spread onshore in NE SC and SE NC...with this rain potentially spreading into the NC Outer Banks when Alberto accelerates towards the NE later in the week.


Figure 1: My best guess forecast for Tropical Storm Alberto as of May 19, 2012 evening. NOTE: This chart has an error...all 11 PM positions should actually be 5 AM. "11 PM Mon" should be "5 AM Tue"...and "11 PM Tue" should read "5 AM Wed"

...MID-LATITUDES DISCUSSION...
P1...In the above birdseye charts...an upper trough has moved into view from the western US. Divergent southwesterly flow ahead of this trough supporting a disorganized frontal cyclone with fairly low pressures...right now the lowest pressure being 999 mb in north Texas/western Oklahoma. Southerly low-level flow ahead of this 999 mb cyclone is in competition with northerly low-level flow behind a 993 mb cyclone mentioned below in paragraph P3...resulting in an east-west frontal boundary dividing air mass contrasts between the two cyclones. This east-west front runs just north of the US/Canada border featuring a 1004 mb depression on the Ontario/Manitoba border and 1019 mb depression in S Quebec.

P2...Upper ridge dominates the central North America...supported by warm air advection ahead of 999 mb cyclone in Paragraph P1. The shortwave upper trough that was embedded in the ridge has merged with the upper vorticity over SC that triggered Tropical Storm Alberto. Convergence on the east side of the upper ridge supports surface ridge centers of 1024 and 1023 mb over the NE US...and a 1028 mb center over Hudson Bay. Northwesterly flow on the east side of this upper ridge converges heavily with westerly flow on the north side of Caribbean upper ridging mentioned below in Paragraphs P4 and P6. This heavy upper-level convergence is leading to a strengthening in the dry air shield over the Gulf of Mexico and northern Florida.

P3...Upper trough/surface frontal system in the western Atlantic has undergone complex evolution in last 24 hours. An upper low vortex over Labrador has moved offshore in a direction toward S Greenland...and what was a 984 mb cyclone (24 hrs ago) supported by this upper low vortex has weakened to 993 mb while beneath a less divergent environment under the upper low vortex. The cut-off upper low vortex over South Carolina 24 hrs ago supported two 1012 mb frontal lows...the northeastern of which has deepened to 1006 mb in the above charts...and the southwestern of which is now Tropical Storm Alberto (see special features section above for further details). The newly-developed vertical warm core of Alberto is supporting an upper ridge extending northward thru E North Carolina and Virigina...and this upper ridge has forked the South Carolina upper low into one upper low over Georgia and another upper low east of Virginia. The upper low east of Virginia is supporting the 1006 mb surface low with divergence on its NE periphery.

P4...Caribbean upper ridging persists...and continues to extend NE into the open Atlantic for several-hundred miles by warm air advection ahead of the frontal system in paragraph P3. Convergence on the east side of this upper ridge supports a strong surface 1035 mb ridge center W of the Azores...and this surface ridge coveres much of the open Atlantic with low-level anticyclonic flow. The divergent anticyclonic flow at low-levels and eastern covergence of the upper ridge supports a tongue of dry, sinking air extending southward into the waters E of the Lesser Antilles.

P5...A Large upper trough was positioned from the Lesser Antilles to the Azores 24 hrs ago has shifted east and is undergoing a split due to upper ridging mentioned in paragraph P4. The southern split is a new upper low vortex covered below in paragraph P8. The northern split as an upper trough from the E Azores into Europe. Divergence on the east side of this northern split supports a cold front in E Spain (where the remnant trough of last week's 92-L was finally absorbed/assimilated) and another surface trough just west of Portugal.

...TROPICAL BELT DISCUSSION...
P6...As mentioned earlier in paragraph P4...upper ridging continues across the Caribbean..and this upper ridge has been extended NE into the Atlantic for several hundred miles. The embedded weak upper trough S of Jamaica is now a weak upper low. Low surface pressures in the western Caribbean (with a 1009 mb center) are being maintained by the outflow of this Caribbean upper ridging. The disturbance associated with this 1009 mb center is no longer a special feature...currently under westerly vertical shear on the north side of the Caribbean upper ridge. Shear is expected to continue with the upper ridge remaining suppressed to the south by (a) the upper vorticity that has triggered Tropical Storm Alberto...and (b) the western US upper trough mentioned in paragraph P1 that will pinch in from the northwest. Gulf of Mexico dry air mentioned in paragraph P2 is also beginning to affect the 1009 mb disturbance.

P7...The upper vorticity that triggered TS Alberto has a center over Georgia and another center E of Virginia as mentioned towards the end of paragraph P3. Southwesterly upper flow is directionally divergent between this upper voriticty and Caribbean upper ridging mentioned in paragraph P4 and P6...leading to heavy storm activity NE of the Bahamas and the formation of a weak surface trough.

P8...A new upper low vortex is NE of the Lesser Antilles...a southern split of the upper trough covered in Paragraph P5. It shows up impressively in water vapor imagery as a spinning mass of dry air...this dry air source covered at the end of paragraph P4.

P9...Upper ridging continues in the E tropical Atlantic. This upper ridge promotes favorable low shear with easterlies on its south side aligned with easterlies generated by the 1035 mb center mentioned in paragraph P4. Animation of water vapor imagery suggests strong dry air pulses from Saharan Africa being wafted westward by this devloping deep-layered easterly flow....albeit the tropical wave SE of the Cape Verde Islands has moistened the area a little bit. Desipte the favorable low shear in this deep-layered easterly flow...the unfavorable dryness will prevent this tropical wave from developing. This tropical wave will also later encouter hositle southweserly shear thanks to new upper low in paragraph P8.

Updated: 2:44 PM GMT on May 20, 2012

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2012 Atlantic Hurricane Season Birdseye Discussion #12A (Special Update)

By: NCHurricane2009, 5:46 PM GMT on May 19, 2012

...MAY 19 2012...
This partial discussion is issued as a special update concerning the special feature off of the Carolina coast mentioned in full discussion #12. This special feature appears to be forming into a tropical storm...and I am forecasting a 100% chance that it will do so. In lieu of this forecast...I am waiting to write the next full birdseye discussion when the National Hurricane Center upgrades this to a tropical storm. If it is not upgraded...I will be writing the next full birdseye discussion late this evening.

See full discussion #12 for how this disturbance off of the Carolinas originated. Figure 1 below serves as an update to Figure 1 from discussion #12. The system has rid itself of the two unfavorable conditions mentioned in the previous discussion...warding off the dry air in its SE half by firing persistent heavy T-storms near the center...and having the SW LLC (low-level circulation) tighten up and consolidate so that it is now seperate from the northeastern LLC. Its current structure and favorable conditions (anticyclonic outflow to its north shown in Figure 1 and warm Gulf stream waters) suggest an imminent chance it will become a tropical storm...and so this is what I am predicting at this time (1:46 PM EDT). The National Hurricane Center put the odds at 50% late this morning...and the Navy NRL upgraded the system to Invest 93L.

Radar from Wilmington, North Carolina (Figure 2 below) suggests the southwestern LLC (Invest 93L) is moving slowly toward the west at the moment. It also suggests a very small tropical storm developing with the compact spiral rain shield entirely offshore. There is also deep-layered cyclonic flow towards the northeast with the NE LLC evolving into a mature extratropical (non-tropical) low. And finally...previous discussion #12 mentioned the system was "hugged by low-level ridging to its west...north...and east." With all this initial information...where will the system go and what impacts will it give?

I forecast the current slow west and southwest motion to continue while influenced by the deep-layered LLC to the northeast and low-level ridging to the northwest. This will keep the compact rain shield offshore and limit coastal impacts (Carolinas...Georgia...and N Florida) under some increased surf but more importantly rip currents. Once the northeastern LLC accelerates away to the NE...I expect the storm to stall and meander erratically like a pinball inside a pinball machine...bouncing between the low-level riging surrounding the system to the west...north...and east. Then late on May 21...I expect an acceleration towards the NNE as the next frontal system from the western US erodes the ridging to the NW. Exactly where the center is after its erratic motion depends on the coastal impacts during its NNE acceleration. Given its compact sturcture...a more offshore NNE track will continue to limit impacts to surf and rip currents. A NNE track closer to shore will add heavy rainfall and gusty northerly winds to the list of impacts. Interests along the Carolina and New England coasts should monitor this system carefully through the weekend into early next work week.


Figure 1: Updated view of coastal Carolina storm's structure. This is updated from last night's Figure 1 in discussion #12


Figure 2: Radar image of organizing storm off of the Carolina coast. Note the well-defined LLC (low-level circulation) at the center of spiral rain bands.

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2012 Atlantic Hurricane Season Birdseye Discussion #12

By: NCHurricane2009, 5:09 AM GMT on May 19, 2012

...MAY 18 2012...
Yet again...early activity pops up in the Atlantic Basin before hurricane season starts June 1...but this time with two areas of interest. See both special feature sections below for further details. There are no imminent signs of tropical development in either area at this time. If this changes...special updates or additional full birdseye discussions will be written. Otherwise...this will be the final discussion until the season starts June 1.

This is the twelvth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

Any feedback on how to improve these discussions can be left in the comments section below. In particular...any suggestions on how to improve the standard two "birsdeye view" charts below will be much appreciated. Are you able to understand the markings in the charts? Are you able to relate what is said in the discussions to these charts?

One improvement being attempted tonight is the implementation of paragraph (P) numbers in general discussions. Paragraphs will cross-reference each other to make the discussion easier during complex weather patterns.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1928Z-released HPC analysis.

In light blue is upper air anlaysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicating surface lows, Hs indicating surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery.Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...SUBTROPICAL DISTURBANCE OFFSHORE OF CAROLINAS
Mentioned in previous discussion #11 (five days ago) was a central US upper trough supporting a 1016 mb frontal low over Mississippi. The supporting upper trough collapsed...causing the front to decay into a surface trough across the Gulf of Mexico...Yucatan Peninsula...and over the SE US by May 16. On this day...a quickly amplifying upper trough from Canada was digging toward the SE US portion of the surface trough...which re-invigorated this portion into a cold front by May 17.

Amplification of a central US upper ridge (paragraph P2 of mid-latitudes discussion below) caused the south end of the upper trough from Canada to cut-off into an upper low over Georgia on May 17...which has moved eastward into South Carolina on May 18.

It is the development of this cut-off upper low that has triggered this special surface feature...which is located along the re-invigorated cold front. On May 17...divergence along the NE boundary of the upper low triggered a broad surface low along this cold front. As this broad surface low spun up...it triggered some heavy shower/T-storm activity in the eastern Carolinas...which has since abated to being only on the immediate coast of North Carolina as this broad surface low has tightened.

The present structure of this broad surface frontal low is shown in Figure 1. It features twin LLCs (low-level circulations)...marked as 1012 mb in the above atmospheric features birdseye chart. The satellite appearance of the northeastern LLc is less interesting while looking like a classical frontal boundary low. It is the southwestern LLC off of the South Carolina coast that is gaining attention.

The NW half of this southwestern LLC has been producing persistent convective clouds (perhaps helped by the cold upper air temps of the cut-off upper low which helps to de-stabilize things). Very recently...the cut-off upper low has weakened to an upper trough due to adjacent warming caused by these convective clouds. Newly-developed upper-level anticyclonic fanning (as shown in Figure 1) suggests this warming is allowing for a more tropical structure with the southwestern LLC. Meanwhile...the SE half of the southwestern LLC is suppressed by a dry air shield (paragraph P2 of mid-latitudes discussion below).

Bottom line is that favorable conditions include the recent development of upper-level anticyclonic outflow (or fanning) and warm Gulf stream waters. Unfavorable conditions include the ingestion of the dry air shield and the elongated surface structure of this system with twin LLCs instead of a singular LLC. Its hard to say at this time if the favorable or unfavorable conditions will start/stop further tropical development. At the very least...a special update or another full birdseye discussion will be issued when this system shows a definitive trend toward developing or not developing.


Figure 1: Structure of subtropical disturbance off of Carolina coast as of the late evening of May 18

Where will this system go? Right now...nowhere with the system hugged by low-level ridging to its west...north...and east as shown by the surface analysis in the atmospheric features chart above. The GFS model suggests that what is now a west US 996 mb cyclonic system (paragraph P1 in mid-latitudes discussion below) will erode the ridging to the NW and accelerate the system parallel to shore towards the NNE...the acceleration beginning late on May 21. Between now and May 21...the system should be stationary. The effects this system brings to the coasts of mid-Atlantic US...NE US...and eventually Atlantic Canada depends on how close to shore the system develops a consolidated LLC. Extreme case scenario in my mind would be weak tropical storm gusts...some rain...and light surf/rip currents on the west side...that is if the system acquires tropical characteristics and strengthens a bit in the first place.

...SPECIAL FEATURE...wEAK WESTERN CARIBBEAN TROPICAL DISTURBANCE...
Mentioned in previous discussion #11 (five days ago) was a central US upper trough supporting a 1016 mb frontal low over Mississippi. The supporting upper trough collapsed...causing the front to decay into a surface trough across the Gulf of Mexico...Yucatan Peninsula...and over the SE US by May 16.

The Yucatan portion of this surface trough has been recently enhanced by the outflow of Caribbean Sea upper ridging (mentioned in paragraphs P4 and P6 below). This enhancement is to the degree that surface pressures have dropped to 1009 mb as of 1800Z TAFB...and a LLC (low-level circulation) has appeared as shown in Figure 2. There is also a curved band of T-storms east of this LLC...but this band is well removed from the center due to westerly vertical shear present on the north side of the Caribbean upper ridging.

With warm waters and a moist air mass seen in the thermodynamics chart above...the only inhibiting factor against tropical cyclone formation at this moment is exposure to the shear on the north side of the Caribbean upper ridging. The GFS model suggests this shear pattern will continue for the next days...with the upper ridge remaining suppressed to the south due to the remaining upper vorticity associated with the Carolina coast special feature....followed by upper vorticity (an upper trough) swinging in from the northwest associated with what is now a 996 mb cyclonic west US system mentioned in paragraph P1 below. Therefore...I do not expect this system to develop into a tropical cyclone.


Figure 2: Structure of tropical disturbance in the western Caribbean Sea during the afternoon of May 18.

...MID-LATITUDES DISCUSSION...
P1...Moving into the western US is an upper trough (not marked in the above charts)...with divergent southwesterly flow ahead of this trough supporting a surface 996 mb cyclone over E Wyoming. Southerly low-level flow ahead of the 996 mb cyclone is in competition with northerly low-level flow behind a 984 mb cyclone mentioned below in paragraph P3...resulting in an east-west frontal boundary dividing air mass contrasts between the two cyclones extending eastward from the 996 mb cyclone. This east-west front runs just north of the US/Canada border featuring a 1002 mb depression in south-central Manitoba and 1014 mb depression near Nova Scotia.

P2...Upper ridge dominates the central US...supported by warm air advection ahead of 996 mb cyclone in Paragraph P1. Embedded in the upper ridge is a shortwave upper trough just west of Missouri in the above charts. Divergence east of this embedded shortwave supports a weak surface trough over Kentucky/the Ohio Valley. Convergence on the east side of the upper ridge supports surface ridge centers of 1024 mb near New York and 1025 mb just offshore of Maine. Upper convergence/surface divergence from the ridge system covered in this paragraph has created a dry air shield seen propagating eastward across the southern tier of the US over the last days. This dry air shield is strongest over the northern Gulf of Mexico and Florida in the above thermodynamics chart.

P3...Upper trough/surface frontal system dominates the eastern edge of North America and western edge of the Atlantic tonight. 996 mb cyclone in paragraph P1 had caused warm air advection across the central US...which has pumped up the ridge system in paragraph P2...and in turn this ridge system has caused the frontal system covered in this paragraph to split into two main areas. The first is an upper low vortex over Labrador whose divergence has supported a rapidly intensifying 984 mb cyclone just offshore. The second is a cut-off upper low vortex over E South Carolina supporting a pair of 1012 mb frontal lows that are beginning to gain tropical characteristics as covered in the first special features section above.

P4...Dome of upper ridging has spread eastward into the Caribbean Sea over the last few days...an eastward extension of an upheaval in tropical activity in the eastern Pacific. This caribbean upper ridging has been spread NE for several-hundred miles by warm air advection ahead of the frontal system in paragraph P3. Convergence on the east side of this upper ridge supports a strong surface 1034 mb ridge center NW of the Azores...and this surface ridge coveres much of the open Atlantic with low-level anticyclonic flow. The divergent anticyclonic flow at low-levels and eastern covergence of the upper ridge supports a tongue of dry, sinking air extending southward into the Lesser Antilles.

P5...Large upper trough dominates the open Atlantic from the Lesser Antilles to the Azores. This upper trough formed in the last five days from a consolidation of three features mentioned in previous discussion #11...first the parent upper low vortex of last week's subtropical disturbance 92-L...the second was an upper trough over the Lesser Antilles...and the third was the upper trough of a frontal system moving toward 92-L. The remnants of 92-L believe it or not are still out there...with the surface low finally being downgraded to a surface trough just recently at 1800Z TAFB. The remnant surface trough of 92-L was located just north of the Canary Islands and is supported by divergence on the NE side of the upper trough.

...TROPICAL BELT DISCUSSION...
P6...As mentioned earlier in paragraph P4...dome of upper ridging has spread eastward into the Caribbean Sea over the last few days...and this upper ridge has been extended NE into the Atlantic for several hundred miles. There is an embedded weak upper trough south of Jamaica located in relatively lower pressures between this NE extension and upper ridge centers over Central America. Low surface pressures in the western Caribbean are being maintained by the outflow of this Caribbean upper ridging...see second special features section above for further details...

P7...It is possible a weak tropical wave is over the Lesser Antilles tonight. This tropical wave was dropped from TAFB at 0600Z May 18 with a statement of its landfall in South America. However, a small batch of clouds is present just NE of the Lesser Antilles...suggesting that the tropical wave is producing surface convergence in this area. It is also possible that the tropical wave is indeed dissipated and this batch of clouds is due to directional upper divergence with the northerly upper flow branching between the upper trough mentioned in Paragraph P5 and Caribbean upper ridging in Paragraph P6. If the tropical wave is not resurrected soon in TAFB analyses...I will drop it from my analyses as well.

P8...To the southeast of the upper trough in Paragraph P5...upper ridging has flared up in the eastern tropical Atlantic...perhaps due to relatively higher upper air pressures as the upper trough in paragraph P5 is gradually weakening. This eastern tropical Atlantic upper ridge is promoting favorable low shear with easterlies on its south side aligned with eastelies generated by the 1034 mb center mentioned in paragraph P4. Animation of water vapor imagery suggests strong dry air pulses from Saharan Africa being wafted westward by this devloping deep-layered easterly flow. So desipte the favorable low shear in this deep-layered easterly flow...the unfavorable dryness will prevent a tropical wave to the SE of the Cape Verde Islands from developing.

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2012 Atlantic Hurricane Season Birdseye Discussion #11

By: NCHurricane2009, 6:35 PM GMT on May 13, 2012

...MAY 13 2012...
Subtropical low south of the Azores not expected to be upgraded to a named storm or numbered depression. This is the last birdseye discussion until June 1 2012 unless yet another early tropical disturbance develops before then.

This is the eleventh birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

Any feedback on how to improve these discussions can be left in the comments section below...and some of these ideas may be implemented in time before hurricane season officially starts June 1. In particular...any suggestions on how to improve the standard two "birsdeye view" charts below will be much appreciated. Are you able to understand the markings in the charts? Are you able to relate what is said in the discussions to these charts?

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0729Z-released HPC analysis.

In light blue is upper air anlaysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicating surface lows, Hs indicating surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery.

Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...SUBTROPICAL LOW SOUTH OF THE AZORES...INVEST 92-L...
The formation of this subtropical surface low is best explained in the special feature section of yesterday's discussion #10. This surface low has since moved north and then west around the northeastern semicircle of its parent upper low...which took it from waters 20 to 21 deg C twenty-four hours ago...to now waters of 19 to 20 deg C. Its central pressure has risen from 1009 to 1010 mb in the last day. This is because it has no upper divergence to work with as in a non-tropical system while beneath the non-divergent upper low center. Its convective clouds have also weakened such that no warm core upper-level outflow is present to strengthen it as in a tropical cyclone.

The loss in convective clouds may be associated with its travel over slightly cooler waters...and also the fact that the cut-off upper low (like all cut-off upper features) is gradually weakening. With the cold core upper low above it weakening (warming) and with cooler surface waters...the surface to upper air temp contrasts necessary for instability and convective clouds is diminishing...and it is very unlikely the National Hurricane Center will be upgrading this to a named storm or numbered depression. As noted in yesterday's discussion #10...the system had the characteristics of a subtropical storm (or even a tropical storm with cirrus outflow clouds)...so it is possible that the National Hurricane Center considers this an unnamed storm in the post-season analysis.

This surface subtropical low remains trapped in the southeastern "armpit" of a deep-layered ridge (with surface center of greater-than-1032 mb in the above charts). Moreover...the subtropical surface low is also trapped directly beneath its parent upper low. Both of these facts mean the subtropical low should meander for the next several hours while making cyclonic loops beneath its upper low. Although these cyclonic loops may take it intermittenly southward over warmer waters...honestly we are only talking about water temp fluctuations of +/- 1 deg C within the cyclonic loops...not enough to compensate for the fact that the upper low above continues to weaken (warm). Therefore...I am not expecting much re-development of convective clouds.

The next feature that could affect its steering is the upper trough/surface frontal system off of the eastern US (in the western Atlantic)...but this next system is highly amplified and somewhat cut-off from the jet stream as noted in today's west-to-east discussion below. Therefore it may take a while for this next upper trough/surface front to work its way eastward. If/when this next frontal system erodes the deep-layered ridge to the NW depends on if/when the subtropical low begins acceleration to the east-northeast towards Europe. It may very well dissipate before this next frontal system gets a chance to steer it toward Europe.

...WEST-TO-EAST DISCUSSION...
Large upper trough over the central US has fractured into two upper trough due to the strength of the adjacent upper ridge to its east. The northern fracture is over Canada's Hudson Bay whose eastern divergence supports a cold front (with 1016 mb low) and whose western convergence supports a 1026 mb ridge over Wisconsin. The southern fracture remains over the central US...and has picked up yesterday's weak 1013 mb frontal low over SE Texas and sent it northeastward into the SE US...and this frontal low is presently 1016 mb over Mississippi in the above charts. Divergence on the central US upper trough's SE end also supports a weak surface trough in the Bay of Campeche (in the SW Gulf of Mexico). Convergence on its west side of the central US upper trough has strengthened the 1028 mb W Nesbraska ridge to 1029 mb.

Southerly warm air advection ahead of the aforementioned 1016 mb frontal low continues to support an amplified upper ridge starting from Honduras/Guatemala...across the E Gulf of Mexico...and into the eastern US. Eastern convergence of the upper ridge supports a surface ridge of 1027 mb near Cape Hatteras, NC. Eastern convergence of this upper ridge and surface divergence from the 1027 mb center supports sinking, dry air in the western Atlantic. However...this dry air wedge has shrunk in size as upper westerlies from the upper ridge axis have wafted moist air eastward from the 1016 mb frontal low and into the wedge of dry air.

Upper trough/surface frontal system contines in the western Atlantic off of the eastern US. Its northern fracture SW of Greenland 24 hours ago has raced off to the SE of Greenland...and is now supporting a strengthening surface cyclone (1011 mb in yesterday's charts) all headed toward northern Europe. Behind this northern fracture...a shortwave upper ridge has built in SW of Greenland and cut-off the remainder of this upper trough in the western Atlantic. The 1009 mb cyclone associated with this cut-off west Atlantic upper trough (near N Maine 24 hrs ago) has weakened to a 1019 mb triplet of centers as shown in the above charts. Cold front continues to extend south of this now-disorganized cyclone. Two distinct features mentioned yesterday remain ahead of this cold front's south end:

The first feature was a weak surface trough near the east Bahamas created T-storm clouds over the east Bahamas and Hispaniola (Haiti and the D.R.) yesterday. This surface trough has dissipated and is no longer in the above charts.

The second feature was a mass of T-storm clouds N of the Lesser Antilles which was upgraded to a surface trough at 1200Z TAFB yesterday (and this surface trough is still in the above charts). Latent heat release with these T-storm clouds had caused upper-level warming such that an outflow enhancing upper ridge formed overhead...and I was watching this system for possible development into a tropical disturbance. This is no longer the case as the T-storm clouds have basically fizzled. It appears this system coughed on the dry air wedge to its west...this dry air wedge mentioned three paragraphs ago. The dissipation of T-storm activity has caused the overhead upper ridge to diminish...and upper winds are no longer favorable for any sort of tropical development.

The weak upper trough over the Lesser Antilles (mentioned in yesterday's special feature section and yesterday's west-to-east discussion) has actually amplified due to the formation of the upper ridge associated with the surface trough/T-storm mass that was located N of the Lesser Antilles. It has amplified to the degree there is a weak upper low at its north end presently marked in the above charts. Far to the west of the Lesser Antilles upper trough is a convective T-storm mass that has been over Colombia and Panama in the far south Caribbean...which is indirectly related to the upper trough itself. Northerly upper-level flow over Colombia and Panama heavily split...some splitting off eastward into the Lesser Antilles upper trough...some splitting off westward around the flow of the upper ridge mentioned four paragraphs ago. The highly divergent split upper flow coupled with surface ITCZ convergence is causing these T-storms over the Panama/Colombia area.

Deep-layered ridge SE of Newfoundland and NW of the Azores continues due to warm air advection ahead of the disorganized cyclone (with triple 1019 mb centers) mentioned four paragraphs ago. This deep-layered ridge is also mentioned in today's special feature section.

Finally...surface trough along NW Africa continues...and reaches into southern Spain in today's TAFB analyses. This surface trough continues to be supported by divergence along the eastern boundary of the parent upper low above Invest 92-L.

Hurricane

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2012 Atlantic Hurricane Season Birdseye Discussion #10

By: NCHurricane2009, 6:52 PM GMT on May 12, 2012

...MAY 12 2012...
Surface low pressure center with characteristics of a subtropical storm is south of the Azores. It is now a subjective call on whether or not the National Hurricane Center will declare this system. See special features section below for further details.

This is the tenth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0727Z-released HPC analysis.

In light blue is upper air anlaysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicating surface lows, Hs indicating surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery.

Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...SUBTROPICAL LOW SOUTH OF THE AZORES...INVEST 92-L...
For much of the first half of May...a semi-permanent upper trough spanned the Caribbean Sea northeastward into the Atlantic for several hundred miles. The above charts today show another upper trough/surface frontal system off of the eastern United States coast...with warm air advection ahead of this system promoting a deep-layered ridge presently northwest of the Azores. The development of this deep-layered ridge has cut-off the semi-permnanent upper trough into a few features:

The first feature is a remnant weak upper trough over the Lesser Antilles (in the above charts) whose eastern divergence is promiting weak cloudiness east of the Lesser Antilles.

The second feature is far more significant...a deep-layered low pressure south of the Azores that has managed to gain tropical characteristics over cooler waters around 20 to 21 deg C. The thermodynamics for such mild-water tropical development are no stranger to this part of the world (e.g. Hurricane Vince 2005 and Tropical Storm Grace 2009). In these cases...a cut-off upper low produces a surface frontal low with its boundary upper divergence...and the surface frontal boundary low eventually whirls into a position beneath its cut-off upper low (hence the formation of a deep-layered low with low wind shear). The cold temps of the upper low allow for instability despite the mild surface water temps...and sustained convective thunderstorms develop at the surface low. The surface low...which develops a low-level warm core due to its occluded front...sees the warm core grow vertically due to the latent heat release of the thunderstorms...and a subtropical depression/subtropical storm forms.

These processes have taken place with the 1009 mb surface low in the above charts south of the Azores. The Naval Reasearch Laboratory (NRL) took notice and upgraded the system to Invest 92-L this morning. As breaking news...the National Hurricane Center issued a Special Tropical Weather OUtlook while I was writing this discussion (at 2 PM EDT)...giving this a 40% chance of become a subtropical cyclone in the next 48 hours.

My personal opinion is more aggressive...as I believe this is already a subtropical storm. This is because the surface low's circular convective canopy (albeit a small diameter one) has been seperate from all (non-tropical) frontal boundary clouds...and moreover this ball of convective clouds has been sustaining itself for several hours. Features of this convective ball of clouds are shown in Figure 1.

Figure 1 shows a faint banding-type eye...but more impressively signs of anticyclonic cirrus outflow in the northern semicircle shown by the light blue arrows. This could be a sign that the surface low's vertical warm core has grown enough in height to promote high pressure upper-level anticyclonic outflow as a fully tropical system has.

This surface subtropical low is trapped in the southeastern "armpit" of a deep-layered ridge (with surface center of greater-than-1032 mb in the above charts). Moreover...the subtropical surface low is also trapped directly beneath its parent upper low. Both of these facts mean the subtropical low should meander for the next several hours. The next feature that could affect its steering is the upper trough/surface frontal system off of the eastern US...but this next system is highly amplified and somewhat cut-off from the jet stream while forming a cut-off upper low just N of Maine. If/when this next frontal system erodes the deep-layered ridge to the NW depends on if/when the subtropical low begins acceleration to the east-northeast towards Europe. If it ever does begin moving toward Europe...the water temps are much colder...so whatever tropical characteristics the subtropical low has now will be long gone by that time.


Figure 1: Features shown by the clouds of subtropical low Invest 92-L

...WEST-TO-EAST DISCUSSION...
The upper-levels in this part of the world today are in a west-to-east ridge-trough-ridge pattern...and so today's general discussion will sweep from west-to-east.

A large upper trough over the central US supports a very long surface cold front Hudson Bay in Canada (near a 1010 mb low)...through a 1017 mb frontal low in Wisconsin...to a 1005 mb low at the California/Arizona border. This upper trough has recently absorbed the upper-levels of a cut-off system that had been producing severe weather over Texas over the last days. What's left of this cut-off system at the surface is a 1013 mb weak frontal low in SE Texas. Convergence behind (west) of the upper trough axis supports a 1029 mb surface ridge over W Nebraska.

Southerly advection ahead of the large central US frontal system has pumped in warm air...resulting in a prominent and amplified upper ridge axis starting from Guatemala...across the Gulf of Mexico...across the eastern US...and into eastern Canada. Eastern convergence of the upper ridge supports a surface ridge of 1026 mb over the Virginias...and another 1025 mb center on the Carolina coast. Eastern convergence of this upper ridge and surface divergence from the 1025 and 1026 mb centers supports sinking, dry air in the western Atlantic...the driest in the basin.

Upper trough/surface frontal system is in the western Atlantic off of the eastern US. The amplification of the aforementioned upper ridge to its west has caused the upper trough itself to also get more amplified...resulting in an upper low along the upper trough axis just N of Maine. The remainder of the upper trough has fractured away to a position just SW of Greenland...with divergence east of this fracture supporting a 1011 mb surface frontal low just SE of Greenland. The upper low just N of Maine has caused a 1009 mb low to whirl cyclonically. A cold front extending from this 1009 mb low reaches the waters just N of the Bahamas. It is here where there are two features ahead of the cold front.

The first feature is a weak surface trough and band of T-storm clouds near and over the eastern Bahamas...both of which are supported by the frontal upper trough's divergence.

The second feature is more interesting...a mass of T-storm clouds east of the Bahamas and N of the Lesser Antilles. Very recently...1200Z TAFB added a surface trough below these clouds (not shown in the above charts). This more impressive T-storm mass was initialized by eastern divergence ahead of the frontal upper trough. Latent heat release with these T-storm clouds has caused upper-level warming such that an outflow enhancing upper ridge is forming. If this upper ridge continues to amplify...it could promote yet another tropical disturbance in this area in addition to today's special feature Invest 92-L.

The remaining features in the eastern half of the Atlantic are associated with what was once a semi-permanent upper trough and now is a weak upper trough over the Lesser Antilles and 92-L south of the Azores. These are covered in the special feature section above. However...what is not mentioned in the special feature section is that the upper low above 92-L is supporting a surface trough off on the NW African coast with its eastern boundary divergence.

Deep-layered ridge SE of Newfoundland and NW of the Azores has formed due to warm air advection ahead of aforementioned 1009 mb cyclone near N Maine. This deep-layered ridge is also noted in the special features section.

Updated: 8:10 PM GMT on May 12, 2012

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