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   FOUS11 KWBC 081948   
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   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   248 PM EST Mon Dec 8 2025   
      
   Valid 00Z Tue Dec 09 2025 - 00Z Fri Dec 12 2025   
      
      
   ...Pacific Northwest through the Northern High Plains..   
   Days 1-3...   
      
   Prolonged moisture spilling east from a strong atmospheric river   
   (AR) with multiple embedded waves will spread heavy snow into the   
   Intermountain West through late-week.   
      
   The strong AR will surge onshore the Pacific Northwest this   
   evening as a shortwave lifts into British Columbia with   
   impressively confluent and zonal flow pushing into WA/OR. IVT   
   probabilities for 750 kg/m/s exceed 90% during this time near the   
   WA/OR border, aided by a powerful Pacific jet streak of over 150   
   kts moving into British Columbia. This will be the beginning of a   
   long duration and impressive plume of IVT surging from the Pacific   
   Northwest and then spilling across the terrain and diving into the   
   High Plains through late week. While there will be multiple surges   
   of elevated IVT (and so not a continuous period of heavy   
   precipitation), in general the entire forecast period will be   
   unsettled, and IVT exceeding the CFSR climatology is progged via   
   NAEFS for multiple periods Wednesday and Thursday - indicating the   
   impressive available moisture associated with this system.   
      
   As with most ARs, the forcing and moisture will be attributed to   
   potent WAA, and this will drive snow levels to well above normal   
   levels for early December, likely reaching 8000-9000 ft in many   
   areas west of the Continental Divide and south of a wavering   
   stationary front/baroclinic boundary that will not only be a   
   demarcation for colder temperatures, but also provide a surface for   
   a subtle increase in isentropic ascent and fgen across the region.   
   Since snow levels south of this front will be so high, any heavy   
   snow should be confined to only the higher peaks of the northern WA   
   Cascades, far Northern Rockies, and into NW WY D1-D2 where WPC   
   probabilities are continuously high (above 70%) for 6+ inches both   
   days, especially in the northern WA Cascades and higher elevations   
   surrounding Yellowstone NP.   
      
   Then during D3, more significant snowfall becomes probable east of   
   the Northern Rockies in response to Arctic high pressure sinking   
   south out of Saskatchewan, driving a cold front to bank into the   
   terrain from the NE. This will sharpen the baroclinic gradient,   
   intensifying the resultant fgen, and increase isentropic ascent as   
   IVT continues to surge eastward beneath the potent Pacific jet   
   streak arcing to the southeast from British Columbia. While there   
   is considerable spread still in the placement and coverage of   
   precipitation D3, confidence is increasing in an axis of heavy snow   
   spread NW to SE beneath this jet streak and just north of the   
   front across MT and into the Dakotas. Current WPC probabilities for   
   more than 6 inches of snow are as high as 70% in the Northern   
   Rockies near Glacier NP above 7000 ft, with more expansive, but   
   low-end probabilities (10-30%) for 6+ inches extending well into   
   the High Plains of MT.   
      
      
   ...Northern Plains through the Great Lakes and Northeast...   
   Days 1-3...   
      
   Two distinct fast-moving shortwaves will race across the region   
   through late-week bringing several rounds of wintry precipitation.   
      
   The first of these will stem from a potent vorticity maxima and   
   associated shortwave trough which will dig out of Manitoba this   
   evening, reaching Michigan by Tuesday morning, and then crossing   
   New England to exit into Canada by Wednesday morning. This feature   
   will remain of modest amplitude and the corresponding downstream   
   jet streak does not appear well positioned for any enhanced deep   
   layer ascent to cause cyclogenesis. Despite this, some increased   
   isentropic ascent from 280K-290K will cause a swath of moderate to   
   heavy snowfall tonight through Tuesday, with the speed of the   
   system the primary limiting factor to snowfall. Favorable ascent   
   into the DGZ will likely yield briefly heavy snow rates,   
   especially in MN/WI/MI before weakening into New England, with   
   moderate snow accumulations expected. WPC probabilities for even 2"   
   of snow are quite limited for this event, but periods of moderate   
   to briefly heavy snow could create hazardous travel for the monday   
   commutes for parts of WI, MI, and into Upstate NY.   
      
   Immediately in the wake of this first system a second, and likely   
   more amplified, shortwave and accompanying vorticity lobe, will   
   swing southeast out of Alberta/Saskatchewan and dive into the   
   Northern Plains by Tuesday afternoon. This shortwave is progged to   
   deepen during its progression, and while it should maintain a   
   positive tilt through its lifetime, it will gain amplitude and dive   
   farther south than the initial impulse. The resulting impressive   
   height falls will overlap efficiently with the LFQ of an   
   impressive Pacific jet streak following in its wake, producing   
   intense deep layer lift which will help deepen a surface low   
   /Alberta Clipper/. There is continued spread among the various   
   deterministic models (generally a global vs. high-res dichotomy) and   
   their accompanying ensemble spreads, but in general the   
   differences are small leading to high confidence in a track from   
   ND Tuesday aftn to lower Michigan Wednesday morning to northern   
   Maine/Quebec Thursday morning.   
      
   In addition to the robust synoptic lift, ascent will become   
   enhanced through a progressive overlap of 285K-295K moist   
   isentropic ascent (mixing ratios as high as 4g/kg) with 850-600mb   
   fgen. This will be well aligned to the most impressive left-exit   
   diffluence, and directed into the primary snow growth region /DGZ/,   
   suggesting some banded snow with heavier snow rates occurring N/E   
   of the low- level circulation. This is reflected by the WPC   
   prototype snowband tool suggesting at least 1"/hr snowfall rates   
   diving across the region. While the temporal duration of these snow   
   rates,owing to the speed of the system, should cap the snowfall   
   amounts, some secondary banding is possible on the NW side of the   
   low in response to intensifying deformation, especially Tuesday   
   night into Wednesday. This may lead to some locally higher snowfall   
   totals, but at this time, WPC probabilities D1 for 4+ inches of   
   snowfall reach 50-70% from far eastern ND through much of northern   
   MN (south of the Arrowhead, however), with a more pronounced stripe   
   of 50+% probabilities for more than 4 inches on D2 across this same   
   area and extending into WI and lower MI. The transient nature of   
   the system will limit the high-end snowfall, but some areas may   
   receive as much as 8 inches of snowfall.   
      
   Additionally, south of this axis of heaviest snow, a transition   
   zone of mixed precipitation, including freezing rain is expected,   
   leading to at least modest ice accretion. While light freezing rain   
   is expected from far eastern MT through lower MI, the greatest   
   chance (10-30%) for more than 0.1" of icing exists across ND.   
      
   As this storm exits the Great Lakes and moves into Ontario D3, the   
   downstream strong WAA will overrun a very cold antecedent airmass.   
   This will create a broadening plume of wintry precipitation,   
   falling primarily as snow from Upstate NY through northern New   
   England, with some mixing, including periods of freezing rain,   
   expected farther south into the northern Mid-Atlantic and southern   
   New England. Total accumulations of wintry precipitation should be   
   less than areas upstream, but WPC probabilities late D2 into D3 for   
   4+ inches of snow reach 50-90% for portions of western PA and   
   across the southern Adirondacks, Greens, Whites, and terrain of   
   northern ME.   
      
   Finally, behind this system a brief period of favorable NW flow   
   will create lake effect snow (LES) and upslope snow across the   
   eastern Great Lakes and into the Central Appalachians. While some   
   of this snow will fall on top of the synoptic snow noted above   
   (and is included in the snowfall probabilities), and additional 4+   
   inches of snow has a moderate risk of occurring across the Tug Hill   
   Plateau and Chautauqua Ridge D3. Similar probabilities for 4+   
   inches of snow exist in the higher terrain of WV as well where   
   upslope flow and gusty winds could create hazardous impacts.   
      
      
      
   Weiss   
      
      
   $$   
      
   = = =   
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