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   FOUS11 KWBC 121847   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   147 PM EST Fri Dec 12 2025   
      
   Valid 00Z Sat Dec 13 2025 - 00Z Tue Dec 16 2025   
      
      
   ...Montana...   
   Day 1...   
      
   The ongoing strong upper jet streak angled NW to SE will continue   
   tonight and into early Saturday, while westerly mid-level flow   
   isentropically ascends the baroclinic gradient left from a cold   
   front banked against the Continental Divide. This will result in=20   
   persistent moderate snowfall with rates 0.5"/hr to 1"/hr at times=20   
   across central MT. The heaviest snowfall should taper off quickly=20   
   late tonight with snow ending by Saturday morning in response to=20   
   the strongest ascent pivoting east away from the area. The=20   
   continuing snowfall should remain fluffy with high SLRs thanks to a   
   very cold column, so additional accumulations exceeding 4 inches=20   
   are likely (>70% chance) in central MT, with local additional=20   
   snowfall as much as 8 inches possible (10% chance) in a few areas.   
      
      
   ...Northern Plains through the Ohio Valley into the Central=20   
   Appalachians...=20   
   Days 1-2...   
      
   A closed 500mb low dropping along the Ontario/Great Lakes border   
   will be the catalyst for another fast moving clipper-type low=20   
   racing from the Northern Plains to the Ohio Valley D1, and then=20   
   exiting the Mid-Atlantic coast D2. The primary driver of this=20   
   clipper is expected to be a secondary impulse/vorticity lobe=20   
   swinging through the pinched flow, to combine with the persistent=20   
   and intense upper jet streak aloft driving deep layer ascent. While   
   the temporal duration of any lift will be modest due to the=20   
   progressive nature of the system, the overlap of height falls with=20   
   intensifying fgen (both through WAA and the result of the upper jet   
   streak position) will be sufficient to drive robust omega through=20   
   the snow growth zone. With strong lift occurring into a DGZ that is   
   deepening in the cold airmass, periods of heavy snow are likely,=20   
   especially within a narrow translating band from eastern SD through   
   IN on D1, and then continuing into OH D2 before running into the=20   
   terrain of WV and wringing out the remaining moisture via upslope=20   
   enhancement before Saturday night.   
      
   WPC probabilities for snow exceeding 4" reaches 10-30% from eastern   
   SD through central IN, but contains a local maxima above 50% in   
   central IL/IN where the best overlap of ascent into the deepening   
   DGZ occurs. This could support snowfall rates above 1"/hr at times   
   leading to local maxima above 6" (10% chance). During D2 the   
   heaviest snowfall axis shifts eastward, with a 50-90% chance of   
   more than 4 inches occurring across southern OH and into the   
   terrain from the Laurel Highlands of PA through central WV. The   
   greatest snowfall, which may reach 10" in local spots, is expected   
   in the higher terrain of WV.   
      
      
   ...Mid-Atlantic through Southern New England...=20   
   Day 2...   
      
   Extremely challenging forecast for Sunday as a system develops east   
   of the Mid-Atlantic Sunday morning and then skirts off into the   
   Atlantic Ocean. The primary mechanism for this system will be the   
   upstream shortwave racing across the Midwest. As this feature moves   
   east, it will amplify rapidly into a sharpening trough, but still   
   maintain a positive tilt. Downstream of this trough, especially 06Z   
   to 18Z Sunday, rapid height falls will occur through the Mid-   
   Atlantic states and into southern New England, concurrent with a   
   rapid amplification of a jet streak which peak above 150 kts in New   
   England while arcing more poleward. This will place the favorable   
   RRQ for diffluent ascent directly beneath the strongest height   
   falls, and the accompanying (albeit modest) fgen due to WAA will   
   merge with the resulting fgen from the jet streak to drive a period   
   of rapidly intensifying deep layer ascent. The guidance continues   
   to suggest that the strongest lift will occur directly into the   
   DGZ, so despite marginal thermals at onset (especially across the   
   Mid-Atlantic) precipitation will become increasingly all-snow, and   
   may be heavy at times as banded snow develops and pivots northeast.=20   
      
   This event should last no more than 12 hours in any given location,   
   and the exact placement, timing, and intensity of the jet streak   
   will create the important differences in snowfall placement and   
   amounts. While guidance continues to feature some differences,   
   there has been a reduction in spread this aftn which is reflected   
   by increasing probabilities for accumulating snowfall, especially   
   along the I-95 corridor from Washington, D.C. to NYC. At this time,   
   WPC probabilities suggest a 30-70% chance of at least 2 inches from   
   Washington to NY and onto Cape Cod, with the greatest potential for   
   4+ inches occurring from near Philadelphia through central Long   
   Island. While this will occur early Sunday morning, at least=20   
   briefly heavy snowfall rates will make for hazardous travel.   
      
      
   ...Great Lakes...=20   
   Days 1-3...   
      
   Persistent troughing across the east will maintain cold=20   
   conditions, with multiple shortwaves and accompanying cold front   
   driving renewed CAA for lake effect snow (LES) with modest synoptic   
   snow also accompanying each impulse. These impulses are most likely   
   to cross the region on Saturday, with another impulse on Monday.   
   While the accompanying synoptic/WAA snow is likely to produce only   
   modest accumulations, the post-frontal CAA will cause 850mb temps=20   
   to drop to as low as -20C to -25C Sat/Sun, with more tempered cold=20   
   of around -10C Monday. While the heaviest LES is expected   
   Saturday/Sunday as inversion depths over the lakes climb towards   
   10,000 ft, significant LES is also likely Monday despite the weaker   
   overall ascent/lake-induced instability.   
      
   WPC probabilities D1 and D2 are elevated, and feature a moderate   
   risk (30-70% chance) of exceeding 6 inches both days across the   
   southern shore of Lake Superior in the U.P. of MI, as well as   
   portions of NW L.P. near Traverse City, and across the Tug Hill   
   Plateau east of Lake Ontario. Slightly lower probabilities exist   
   along the Chautauqua Ridge. Snow totals of 1-2 feet are possible in   
   these areas as well. Then during D3 the flow becomes more westerly   
   focusing the heaviest LES across the Chautauqua Ridge and Tug Hill   
   Plateau where WPC probabilities indicate a high chance (>70%) for   
   additional snowfall of 6+ inches.   
      
   Weiss   
      
      
   ...Extreme Cold Key Messages are in effect. Please see current Key   
   Messages below...   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_1.png__;!!DZ3fjg!-R0uB7_t2x6rqwmMIRkN23YJYqdVbS_8eeqYB5LOVq_UD=   
   5EiK3VDt10MQf31TpTfsOgP5R_0a9gVXeYjJXS3kl7W9zQ$=20   
      
      
   $$   
      
   =3D =3D =3D   
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