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   FOUS11 KWBC 030755   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   255 AM EST Wed Dec 3 2025   
      
   Valid 12Z Wed Dec 03 2025 - 12Z Sat Dec 06 2025   
      
      
   ...Southern/Central Rockies...   
   Days 1-2...   
      
   Shortwave which emerged from British Columbia Monday will continue   
   to track southeast across the Four Corners today while amplifying.   
   The guidance has trended a bit farther south and more amplified   
   with the evolution today, which should result in heavier snowfall   
   especially across the San Juans and Sangre de Cristos on D1.   
   Forcing for ascent downstream of this trough (in response to height   
   falls and modest divergence) will be enhanced by a strengthening   
   jet streak, especially tonight into Thursday morning, and periods   
   of impressive upslope flow as cold N/NE winds develop in advance of   
   an Canadian high pressure dropping into the Plains. Moisture within   
   the region is progged to be around to slightly below normal as   
   reflected by NAEFS ensembles, but the impressive overlapping ascent   
   will still maximize favorable to support heavy snow, especially   
   above 4000-5000 ft, with snow levels falling in conjunction with   
   waning snowfall intensity. WPC probabilities, especially across the   
   San Juans and Sangre de Cristos, have increased tonight, and now   
   feature a high risk (>70%) for more than 12 inches in the higher   
   terrain of these ranges. Into the lower terrain, WPC probabilities   
   indicate a moderate threat (30-50%) for at least 4" along the I-25   
   urban corridor, with locally more than 6" in the higher terrain of   
   the Raton Mesa and southern Palmer Divide.   
      
      
   ...Great Lakes & Northeast...   
   Days 1-3...   
      
   Progressive flow across the Northeast will manifest in response to   
   a large gyre centered near Hudson Bay, which will shed periodic   
   shortwaves from the Great Lakes through the Northeast leading to   
   occasional enhanced ascent and lowered heights throughout. In   
   response to this evolution, the period will remain active through   
   LES and arctic fronts, and multiple rounds of snowfall, sometimes   
   heavy, are expected.   
      
   The first event will be associated with an arctic cold front   
   dragging southeast beneath a shortwave aloft. This front will   
   likely move rapidly across the Great Lakes today, and then across   
   the Northeast Thursday. Along and ahead of this front, the   
   environment continues to suggest an increasing threat for snow   
   squalls, especially across PA/NY and New England on Thursday. The   
   NAM SnSq parameter continues to be more aggressive than the GFS,   
   but the antecedent low-level environment ahead of the front appears   
   favorable for squall development. With 0-2km fgen maximizing along   
   the front into elevated RH and SBCAPE as high as 300 J/kg,   
   widespread linear squalls could result. Upstream into MI the threat   
   is not as high on D1, and although isolated squalls could occur,   
   the greater risk will be D2 in the Northeast. While total snowfall   
   will be minimal in any squall, rapidly changing conditions due to   
   heavy snow and gusty winds will create periods of dangerous travel   
   Thursday aftn. These squalls could become widespread, and after   
   coordination with the northeast WFOs, it is possible that with   
   future model cycles some enhanced messaging may be needed to   
   highlight the threat.   
      
   Behind this front, as well as behind a subsequent shortwave on   
   Friday, increased CAA will spread across the Great Lakes leading to   
   periods of lake effect snow (LES). The progressive flow will   
   support shortwave ridging between the two impulses, so the duration   
   of any LES with each shortwave/front should be somewhat limited.   
   However, 850mb temps falling to -15C to -20C Wednesday   
   night/Thursday, and then -10C to -15C late Friday, will create   
   steep lapse rates and deepening inversion depths to support bands   
   of heavy LES. The heaviest LES snowfall accumulations within the   
   LES bands are expected in the typical NW snow belts across the U.P.   
   of MI, the NW L.P. near Traverse City, and then along the   
   Chautauqua Ridge and into the Tug Hill Plateau. In these belts,   
   3-day snowfall of 6-12 inches is possible, highest across the U.P.   
      
      
   ...Mid-South to the Mid-Atlantic...   
   Day 3...   
      
   Uncertainty abounds D3 with respect to a strung-out shortwave   
   exiting the Central Plains and becoming embedded within the more   
   zonal flow to the east. Although this shortwave is expected to be   
   of modest amplitude and fast moving, the accompanying upper jet is   
   progged to be powerful at over 150kts across the Mid-Atlantic,   
   leading to surface low pressure development across the Southeast.   
   This low will then race E/NE, producing a swath of at least light   
   wintry precipitation from the Tennessee Valley to the Mid-Atlantic   
   states.   
      
   There has been at least a modest northward shift/trend in the MSLP   
   fields from the various global ensembles the past few runs, and   
   while the ECMWF operational remains the most aggressive/north, that   
   solution cannot be ruled out. In fact, the trends in the 250mb jet   
   streak and from evaluation of the clusters (for which the primary   
   variance is the amplitude of the trough across the east) could   
   suggest a more subtle northward track of this low which would shift   
   more wintry precipitation into the area. It is still too early to   
   iron out these details, but at least a low-end risk for a few   
   inches of snow is possible as reflected by WPC probabilities which   
   reach 10-30% for 1+ inches of snow across the Central Appalachians   
   and into eastern VA/MD. This is notably lower than the recent NBM   
   probabilities, and the trends continue to increase, so it is worth   
   watching the evolution of this event during the next few days.   
      
   South of the potential moderate snow, an axis of light freezing   
   rain is also possible where a p-type transition occurs. While the   
   location of the heaviest icing is also uncertain, current WPC   
   probabilities indicate a 10-30% chance for at least 0.1" of ice   
   across northern NC and into southern VA.   
      
      
   ...Northern to Central Rockies...   
   Days 2-3...   
      
   The next chance for heavy snowfall totals in the northern Rockies   
   will develop as a low amplitude shortwave slides off of the top of   
   the Pacific ridge into the region late Thursday into Friday,   
   coincident with the LFQ of an impressive upper jet streak swinging   
   across the Northern Pacific. This will result in favorable mid to   
   upper level forcing along with a notable influx of Pacific   
   moisture, sufficient for some heavy totals , especially above   
   around 5000 ft as snow levels rise coincident with this moisture   
   plume moving into the area. Heavy snowfall accumulations are likely   
   D2 from the WA Cascades through the terrain of NW WY near   
   Yellowstone NP as reflected by WPC probabilities that suggest a   
   high risk (>70%) for 4+ inches, with impacts to many of the area   
   passes becoming likely.   
      
   Probabilities become even more impressive D3 across the higher   
   terrain from Glacier NP through Yellowstone NP including portions   
   of the Sawtooth/Salmon River ranges, Absarokas, terrain of NW WY,   
   and into the Park Range of CO. Here, WPC probabilities D3 reach   
   above 70% for 8+ inches, and D3 snowfall could be impressive above   
   generally 6000 ft where locally 1-2 feet is possible.   
      
   Weiss   
      
      
      
   $$   
      
   = = =   
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