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Scientific Forecaster Discussion
Area forecast discussion National Weather Service Detroit/Pontiac Michigan 418 am EST sun Feb 1 2015 Short term... storm term today and tonight Satellite imagery and National radar mosaic this morning shows the major winter storm system becoming increasingly more organized over the central United States. Overall...the governing dynamics at the hemispheric and synoptic scales has been addressed in earlier discussions and remains valid. A check through iniitializations and first 6 hours of nwp output yields no glaring deviations from reality. Given the consistency of the model trend for slower...wetter...and farther north...the changes this forecast package were to increase forecasted snowfall amounts for all areas...particularly during the 20-05z timeframe. Headline changes are: a Winter Storm Warning is now in effect for the shiawasee/Genesee tier of counties and the issuance of a Winter Weather Advisory for the Saginaw Valley and northern thumb counties. The light snow shield lifted across the in/Ohio borders this morning on cue with precipation onset meeting earlier forecasts. This initial light snowfall is being driven by outstanding isentropic lift in combination with very favorable right entrance region dynamics. In addition...it appears the lead shortwave energy that was earlier associated with the north half of the Desert Southwest circulation arrived on a good trajectory and is at least partially responsible for the impressive Arch of 20-30 dbz echoes that is in place over northern sections of Illinois and Indiana. Right now...difficult to see this moderate to heavy snow band persist as the cyclonic vorticity feature lifts steadily northward because of the dependence on the strong low to midlevel Theta-E gradient/baroclinic zone that is also in the same location. As the vorticity feature lifts northward it will outpace the Richer moisture. As a result...looking at a long duration of light snow to persist through the noon hour/17z. This is supported well by the forecasted soundings with a very stable thermodynamic profile through 25 kft...but note the awesome depth of moisture. It will be a real grind on amounts during the next 9 to 10 hours with 3 to 4 inches expected around Detroit and 2 to 3 inches for the northern suburbs of Detroit up to Saginaw. The high impact portion of the winter storm for southeastern Michigan is expected between roughly 20-05z. It is at this time the phased upper level potential vorticity anomaly and associated 850mb cyclonic circulation will track through northeastern Illinois and northern Indiana. Models forecast a contraction of the main baroclinic zone downstream pivoting and lifting into the vicinity of the far west end of Lake Erie...the rivers and Lake St Clair. Models continue to highlight a well developed and long duration deformation response striping directly over the area from 21-09z. Bulk of the model guidance is tightly clustered with quantitative precipitation forecast over the County Warning Area...while the NAM is on a total island. Have discounted the anomalous NAM values...but did increase quantitative precipitation forecast amounts across the board per trends. Snowfall totals are now expected to range 10 to 14 inches across the far south...7 to 11 inches along the M 59 and I 69 corridors...5 to 7 inches Tri Cities...and 3 to 5 inches far northern Thumb/Bay/Midland. A few more important items to discuss. 1. Very cold surface temperatures in the teens during the height of the event this evening. As a result...salt preperation will likely have little impact on treating surfaces. 2. Low confidence in snowfall ratios. Generally do not like to forecast very high ratios north of 15:1 in winter storm scenarios. The reason is wind will likely lead to Crystal fracture and settling. Could also see a 10:1 bias in accumulation reporting because of the blowing of snow/required estimation. 3. Will reiterate one more time the height of the event is expected this evening and may test some patience due to possible headline fatigue. && Long term... Monday through Friday Rapid departure of the middle level deformation to the east by late Monday morning...with a corresponding increase in upper heights filling eastward into the afternoon. Substantial drying through the column under increasing middle level subsidence and deeper northwest flow indicates a transition toward mostly sunny skies through the day. The resident Arctic air will struggle to respond despite the added insolation...temperatures Monday afternoon reaching the lower and middle teens. Gradient will ease with time as surface ridging builds into the Ohio Valley...but there will be a noted wind chill element /readings below zero/ particularly through the first half of the day. Some increase in high cloud will dampen the temperature drop to some degree Monday night. Strength of the existing air mass will still support lows making a run toward the lower single digits...coldest locales perhaps dipping below zero. Active northern Pacific upper jet will anchor several pieces of shortwave energy into the existing quasi-zonal west-northwest upper flow positioned across the northern Continental U.S.. lead wave remains forecast to pass harmlessly to the south Monday night...with attention focused on a trailing pv filament backed by more favorable left exit region upper jet support. Some increase in large scale ascent will result in a weak surface response over the Midwest by late Tuesday...the attendant cold frontal boundary then lifting into Southeast Michigan by Tuesday night. A modest period of moist isentropic ascent combined with a quick but solid shot of left exit jet dynamics will support an area of snowfall along the northern flank of the surface wave. Some variability yet at this time scale with the placement of the main stripe of middle level dynamics and a corresponding ribbon of accumulating snow...the main feature of interest still a good 36 hours away from proper sampling. Current forecast will hold the line with a general 1 to 2 inch swath centered on the Tuesday night/Wednesday morning period. Stronger shortwave energy of Arctic origin will drive a more pronounced southward plunge of lower heights into the Great Lakes by middle week. This will result in the next Arctic intrusion...the ensuing cold air advection bringing 850 mb temperatures below -20c by Thursday. Low level anticyclonic flow filling in behind the exiting middle level trough suggests a relatively quiet stretch of weather to finish the work week. && Marine... Northeast winds will strengthen today and tonight across the region as a strong winter storm tracks through the Ohio Valley and lower Great Lakes. This increase in the wind field will now bring the potential for a period of gales tonight across southern and Central Lake Huron...including Saginaw Bay. Increasing confidence in both the duration and maginitude of these gusts now support the issuance of a Gale Warning across the aforementioned locations. Gale force gusts will remain too infrequent across the Michigan waters of Lake Erie and will hold with gusts to 30 knots at this time. Winds will diminish on Monday as the storm system exits to the east and high pressure builds into the region. && Aviation...issued 1150 PM EST Sat Jan 31 2015 MVFR ceilings continue to slowly advect southward this evening as low level moisture sinks south and snowfall advects north. Upstream radar observations show a shield of light snow lifting northeast from southwestern Michigan early Sunday morning...the beginning of a long duration winter storm system expected to last through Sunday night. Well organized dynamics at the start suggests IFR visibilities in snow at the start. LIFR conditions are expected during the late afternoon and evening hours during the height of the event. Several inches of accumulating snow are expected with highest totals at the metropolitan taf sites. At dtw...timing of light snow at dtw is between 6z and 8z. Snowfall rates of a quarter inch to a third of an inch per hour is expected at onset. //Dtw threshold probabilities... * high confidence in ceilings below 5000 feet through the taf period. * High confidence in precipitation type of snow. && DTX watches/warnings/advisories... Michigan...Winter Storm Warning until 7 am EST Monday for miz060>063-068>070- 075-076-082-083. Winter Weather Advisory until 7 am EST Monday for miz047>049- 053>055. Lake Huron...heavy freezing spray warning from 7 PM this evening to 10 am EST Monday for lhz362-363-462-463. Gale Warning from 7 PM this evening to 10 am EST Monday for lhz363- 441>443-462>464. Gale Warning from 5 PM this afternoon to 4 am EST Monday for lhz421- 422. Lake St Clair...none. Michigan waters of Lake Erie...none. && $$ Short term...cumulonimbus long term....mr marine.......mr aviation.....Kurimski You can obtain your latest National Weather Service forecasts online at www.Weather.Gov/Detroit (all lower case).
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