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   FOUS11 KWBC 090828   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   328 AM EST Tue Dec 9 2025   
      
   Valid 12Z Tue Dec 09 2025 - 12Z 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. With latest   
   WSO and WSSI-P guidance now showing >50% probabilities for snowfall   
   totals surpassing warning criteria and Moderate Impacts, 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 through D3, and   
   into the western Dakotas by D4. Current WPC probabilities for more   
   than 8 inches of snow are as high as 70% in the Northern Rockies   
   near Glacier NP above 7000 ft, with a larger footprint of   
   moderate-to-high chances (50-80%) across the heart of Big Sky   
   Country that includes mountain ranges such as the Little Belt, Big   
   Snowy, and Absaroka.   
      
      
   ...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 this 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 this 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 locally may   
   receive around 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 eastern   
   ND and western MN.   
      
   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 early 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 Chautauqua   
   Ridge from D2 into 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. The Tug Hill Plateau, in   
   particular, has rather high chances (>70%) for snowfall totals   
   surpassing 8" with even low-to-moderate chances (30-50%) for   
   snowfall topping 12 inches.   
      
      
   Weiss/Mullinax   
      
      
      
      
      
   $$   
      
   = = =   
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