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Scientific Forecaster Discussion
Area forecast discussion National Weather Service Detroit/Pontiac Michigan 216 PM EDT Friday Oct 31 2014 Aviation... //discussion... Extensive low level moisture will continue to pivot across Southeast Michigan through the evening hours on the backside of exiting low pressure. This will continue to support lower MVFR ceilings within pockets of light precipitation. A cooling thermal profile at the same time will favor an eventual transition toward a rain/snow mix. This process already underway at mbs...with the fnt/ptk corridor in line by 21-22z and dtw likely not until after 00z. Any accumulation would be confined to grassy surfaces. More notably...northerly winds will strengthen through the afternoon...leading to gusts in excess of 30 knots. Strongest winds will exist across the northern taf sites...where some peaks gusts will reach the 35 to 40 knot range /especially mbs with flow directly off Saginaw Bay/. For dtw...northerly winds will strengthen after 21z...peaking this evening with gusts in the 30-32 knot range. Any lingering rain showers may tend to mix with snow after 01-02z. No accumulation expected. //Dtw threshold threats... * high confidence in ceilings below 5000 feet through Saturday. * High confidence in northerly wind gusts to 30 knots 21z-03z. * Medium confidence in snow mixing with rain after 01z. && Previous discussion...issued 1108 am EDT Friday Oct 31 2014 Update... Dynamic middle level wave steadily digging southeast this morning... recent water vapor showing the the center of this system now anchored over Lake Michigan. 1012 mb surface low tied to the lead edge of this height fall gradient now shifting across north- Central Lake Huron...with the attendant surface cold frontal boundary now within a couple of hours from clearing all of Southeast Michigan to the southeast. The ensuing cold air advection in deepening/strengthening Post-frontal northerly flow will ensure temperatures move little off of the current lower 40s readings...taking a steady downward turn by middle-late afternoon into the 30s from northwest to southeast. The existing middle level fgen attendant to the trailing elevated frontal zone maintains support for additional pockets of precipitation to develop through the evening. Some augmentation in coverage is expected as the deformation axis catches up to this forcing and an increasingly favorable trajectory begins to tap into the growing instability across Lake Huron. Going forecast handling the overall anticipated temporal and spatial transition toward a rain/snow mix and then snow reasonably well relative to current observation trends upstream and recent sounding data/thermal profiles. The progressive adjustment toward straight all snow across northern lower does give pause that the advective component may prove more formidable early on in the process toward deteriorating the warmer existing boundary layer and kicking the ptype to snow more quickly. Some minor accumulation remains in play this evening regardless of the timing/duration...confined to grassy surfaces. Main adjustment with this update will be nudge winds upward given recent trends. Wind Advisory has been issued for the M-59 corridor northward from late afternoon through the first part of tonight. A solid increase in the north-northeast gradient will commence immediately behind the surface low in response to the progressive intrusion of high pressure filling into the upper Great Lakes. The overall position will place Southeast Michigan favorably for a solid response...the overall component again augmented by the inbound trajectory coming off Lake Huron. The mixing element will be stunted somewhat by the higher degree of moisture. However given this process will occur within the background of at least modest isentropic descent...a period of gusts into the 45 miles per hour range are plausible particularly along/north of M-59. Potential is there for locales along Lake Huron shoreline to see gusts in excess of 50 miles per hour...a favorable setup for a heightened response to align from Saginaw Bay into Bay County. Previous discussion...issued 336 am EDT Friday Oct 31 2014 Short term...today and tonight Trough extending from northeast Canada into the western Great Lakes early this morning will continue to amplify as a potent pv anomaly embedded within aggressively digs toward the southern United States today. A weak 1012mb surface low is evident within the corridor of strongest height falls over Northern Lake Huron, but with the better dynamics shifting away from the Great Lakes during the forecast period, the low will gradually weaken and become increasingly disorganized as it translates across the Ontario peninsula this afternoon. As of 07z, widespread light showers with embedded moderate banding are observed over western lower. This area of rain is associated with increasing isentropic ascent immediately in advance of the digging shortwave. There will be plenty of continued dynamic support to maintain this activity as the pre-frontal corridor of warm advection shifts across Southeast Michigan this morning. A secondary focus of precipitation will take shape over the area late this morning through about 22z. This activity will mainly be driven by surface-500mb layer fgen associated with the system cold front. Upstream activity over Wisconsin and the Upper Peninsula is already evident this morning, but with model forecasts indicating a noteworthy tightening of the baroclinic zone along with added daytime instability, a solid increase in coverage and intensity is expected, especially for Flint and points southeast, including metropolitan Detroit. In addition, a moderate wind field will be in place around the weakening surface low which should support widespread sustained winds of 15 to 20 knots. Diurnal instability will support gusts increasing to 25 knots. An added component of system relative isentropic descent associated with frontal passage centered within a couple hours either side of 21z will support an enhanced pop of wind that may top out as high as 35 knots. This band of enhanced gustiness will work southeast from the Saginaw Valley around 19z to Detroit metropolitan around 23z. In general, however, marginal boundary layer growth, ongoing precip, unimpressive dynamic support, and a marginal-at-best system relative setup preclude Wind Advisory concerns at this time. The exception is Huron County where winds coming off of Lake Huron will support gusts approaching 40 knots along the shoreline. Similar conditions are not out of the question along the southern/Eastern Shores of inner Saginaw Bay, but potential remains too marginal to warrant a southwestward extension of the Wind Advisory at this time. In the wake of the cold front, 0-1km Theta-E lapse rates over Lake Huron will fall to around -6c, moderate values, but impressive for this time of year. The thermodynamic environment will be similar to that over Lake Superior early this morning and should yield an equally healthy lake response. As a result, categorical probability of precipitation remain in place for the appropriate areas through most of the night. As cold air makes inroads into the area during the afternoon, and particularly toward sunset, a changeover to snow is likely, especially those areas where boundary layer wet bulb temperatures are forecast to fall below 0c. While all areas will be subject to at least a few flakes overnight, locations downstream of Lake Huron may see a few tenths of accumulation in spite of the antecedent wet surface. Warm advection off the lake remains a concern, particularly over far eastern areas, where lows may struggle to fall below 35-37f and ptype will struggle to change over. Conversely, locations west of US-23 will remain removed from the lake plume and the combination of upper 20s lows and a Stout wind field will produce wind chills in the upper teens by Saturday morning. Long term...Saturday through Thursday The core of coldest low level air /850mb temperatures of -7 to -10c/ will rotate across lower Michigan during the day Saturday. Lake effect precipitation is expected to be ongoing across the southern half of Lake Huron Sat morning under northerly flow. Model soundings suggest lake induced equilibrium levels exceeding 1500 feet. Even with some heat flux off the lake...the degree of cold air /1000-850mb thicknesses dropping below 1290m/ suggests precipitation will be mainly in the form of snow. There is some concern that if the deep layer synoptic moisture is a little slower to depart...there could be some accumulations across the eastern thumb. This will be something for later shifts to monitor. Lighter intensity snow showers and/or flurries are certainly possible farther inland. An aggressive push of dry air...backing low level flow and lower inversion bases due to strengthening large scale subsidence will not only end the lake effect but will support a clearing trend from west to east during the afternoon and evening. Sat highs will likely fall short of 40 degrees over much of the area given the degree of cold air. Enough decoupling in the boundary layer with surface high pressure building in from the west Sat night will drop lows well down into the 20s. Although amplified...the large scale pattern will remain progressive with middle level ridging expanding across the Great Lakes by the end of the weekend. This and the departure of the surface high to the east will establish a warm air advection regime across lower Michigan...leading to a modest warming trend heading into early next week. Moist advection will then increase Monday night into Tuesday in advance of a long wave trough...supporting the next chance for precipitation /warm enough for all rain with this system/. The medium range model suite suggest the northern stream will remain progressive through next week...with the next middle level wave and shot at some colder air arriving late in the forecast period. Marine...gale warnings remain in effect for all marine areas today into tonight. A push of Arctic air will continue to funnel into the region behind a cold front associated with a low pressure system forecast to move from Northern Lake Huron this morning into the upper Ohio Valley by evening. High pressure nudging into the western Great Lakes from the plains will sustain a tight pressure gradient along the back side of the departing low. This combined with the highly unstable conditions over the marine areas will support gale force winds developing over Northern Lake Huron this morning and the southern half of the lake by afternoon. Winds and waves will slowly decrease Saturday into Saturday night as high pressure builds in from the west. && DTX watches/warnings/advisories... Michigan...Wind Advisory until midnight EDT tonight for miz047-053-055-060>063- 068>070. Lakeshore Flood Advisory until noon EDT Saturday for miz048-049-054- 055-063. Wind Advisory until midnight EDT tonight for miz048-049-054. Lake Huron...Gale Warning until noon EDT Saturday for lhz363-421-422-441>443- 462>464. Gale Warning until 8 PM EDT this evening for lhz361. Gale Warning until 2 am EDT Saturday for lhz362. Lake St Clair...Gale Warning until 10 am EDT Saturday for lcz460. Michigan waters of Lake Erie...Gale Warning until 10 am EDT Saturday for lez444. && $$ Aviation.....Mr update.......mr short term...jvc long term....SC marine.......SC You can obtain your latest National Weather Service forecasts online at www.Weather.Gov/Detroit (all lower case).
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