Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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614 FXUS63 KDTX 121111 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 611 AM EST Tue Dec 12 2017 .AVIATION... Remaining lake effect snow showers in cold northwest flow will slowly diminish early this morning, leaving sct-bkn MVFR to lower VFR stratus/strato-cu. Wind gusts will reach 30 knots at times throughout the day. Loss of daytime heating/mixing will lead to few- sct clouds and lighter WNW this evening, but clouds thicken again late tonight as next clipper low encroaches rapidly from the west. For DTW...Aside from stray shsns early this morning, generally expect sct-bkn stratus/strato-cu in the 3500-4500 foot range. NW wind gusts to 30 knots can be expected today. Clearing/lighter W flow tonight, but clouds thicken late tonight into Wednesday with snow quite possibly beginning before 18z on Wednesday as the next clipper low bears down on the central Great Lakes. //DTW THRESHOLD PROBABILITIES... * High confidence for bkn ceilings at or below 5000 feet at times today. * Moderate confidence in northwest crosswind thresholds being exceeded today. && .PREV DISCUSSION... Issued at 252 AM EST Tue Dec 12 2017 DISCUSSION... Arctic air surging into to the Great Lakes Region this morning, as northern stream upper level trough axis arrives over the Central Great Lakes, with 500 MB low (510 DAM) tracking across northern Lake Huron today. 00Z Raob out of Pickle Lake Ontario revealed an 850 mb temp of -24 C. Some modification of the airmass will take place as it crosses the northern Great Lakes, but 850 mb temps settling down in the -18 to -20 C range over southeast Michigan seems reasonable, which should hold temperatures in the upper teens to around 20 degrees during the day. The Phasing/rapid deepening of the low tracking into the Eastern Great Lakes has been a bit slower to occur, and think peak wind gusts topping out in the 35 to 40 mph should hold, with the exception of the northern Thumb Region as winds/deeper mixing from Saginaw Bay comes into play, supportive of winds 45+ MPH. With water temperatures over much of Lake Huron still in the mid 40s, still have concerns for heavy snow showers impacting the northern tip of the Thumb region (Grindstone City-Harbor Beach- Richmondville). Once low pressure releases from Georgian Bay, there is a meso-high/pressure rise occuring late in the day, which should help push the lake band(s) over Lake Huron toward the Thumb Region, with snow showers likely scrapping the eastern shoreline of Huron County this evening. Right now, the best support is from the 00z HIRes-ARW, which is indicating almost two tenths of an inch of QPF 00-06Z Wednesday. And unfortunately, it indicates this band will stay uncomfortable close to the shoreline right into Wednesday morning. Expectations at this time is for localized 2 to 4 inches, but there is enough concern to issue winter storm watch for potentially higher amounts. Obviously with Lake Effect, the majority of the county will see little to no accumulation, with highly localized higher amounts right near the shoreline. Outside of the eastern Thumb region, looking for skies to become mostly clear/partly cloudy with the northwest flow. Any streamers from the Lake Superior-Michigan connection likely very narrow and struggling to make it into southeast Michigan. Thus, favoring lows in the single numbers to around ten degrees tonight, and with modest surface wind (5 knots) by Wednesday morning, wind chills of zero to -10 F are expected. A bit of a surprise for Wednesday, although the lead upper wave tracks into the western Ohio Valley as expected, an even stronger PV anomaly races down from northern reaches of Northwest Territories, and it is this feature and moisture flux/lower static stability over Lake Michigan which will help draw the clipper and FGEN band farther north, now progged to be stretched across southern Lower Michigan (along/south of I-69). With what looks to be ideal thermal profiles to work with, 850-700 mb average temp in the negative low/mid teens, coupled with 2-2.5 G/KG of specific humidity, it appears 2 to 5 inches will fall across the south half of the CWA Wednesday afternoon-evening. If the system is a little slower to exit Wednesday night, could even see a little bit more. This solution has unanimous support from NAM/GFS/Canadian and European, but exact placement of highest snow/fgen band varies a little bit. Additonal upper level energy embedded within the northwest flow to bring another chance of snow Friday-Saturday, with some semblence of another clipper tracking through, but the upper level flow trending more confluent leading to a weakening low pressure. The baroclinic zone/low track is also uncertain, with big differences in the solutions by Saturday between Euro/GFS. Very low confidence in the weekend forecast. MARINE... Strong northwest gales will persist today with gusts approaching storm force over the exposed nearshore waters of the Thumb and the open waters of central and southern Lake Huron. Gale warnings will remain in effect into tonight before winds subside by early Wednesday morning. Heavy Freezing Spray Warning will remain in place as well as temperatures fall into the teens and significant wave heights range between 10 to 15 feet. Maximum wave heights will exceed 20 feet. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...Wind Advisory until 4 PM EST this afternoon for MIZ049. Winter Storm Watch from 4 PM EST this afternoon through Wednesday morning for MIZ049. Lake Huron...Heavy Freezing Spray Warning until 4 AM EST Wednesday for LHZ361>363-421-441>443-462>464. Gale Warning until 5 AM EST Wednesday for LHZ362-363-462>464. Gale Warning until 4 PM EST this afternoon for LHZ422. Gale Warning until 4 AM EST Wednesday for LHZ421-441>443. Gale Warning until 11 PM EST this evening for LHZ361. Lake St Clair...Gale Warning until midnight EST tonight for LCZ460. Michigan waters of Lake Erie...Gale Warning until 4 PM EST this afternoon for LEZ444. && $$ AVIATION.....DG DISCUSSION...SF MARINE.......DG You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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