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   FOUS11 KWBC 080843   
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   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   343 AM EST Mon Dec 8 2025   
      
   Valid 12Z Mon Dec 08 2025 - 12Z Thu Dec 11 2025   
      
      
   ...Pacific Northwest & Northern Rockies...   
   Days 1-3...   
      
   The Pacific Northwest and Northern Rockies will face a steady   
   barrage of Pacific moisture over the next few days thanks to a   
   multi-day strong atmospheric river that is pinned between a strong   
   subtropical ridge off the coast of CA and a stalled, positively-   
   tilted upper low in the Gulf of AK. Today, PWATs above the 99th   
   climatological percentile will engulf much of the Northwest. Even   
   on Tuesday and into Wednesday, as the subtropical ridge expands a   
   little farther north, a fire-hose of Pacific moisture will continue   
   into the Pacific Northwest that spills into the Northern Rockies   
   as well. Add in steady synoptically-forced ascent aloft from a   
   strong 250mb jet stream and persistent upslope flow into many of   
   these mountains ranges, and the recipe is for heavy snowfall from   
   the Olympics and Cascades to the western WY Rockies above the   
   wavering high snow levels.   
      
   As the strong AR and its associated WAA aloft approaches on   
   Monday, these values increase to above 6,500 ft in the northern WA   
   Cascades and above 7,000 ft from Mt. Rainer and points south.   
   Farther east, snow levels will rise across the northern Rockies as   
   well, topping 6,000ft in many cases before falling on Tuesday as   
   the AR orients itself farther south as a shortwave trough ejects   
   into the northern Plains. Then, snow levels once again rise on   
   Wednesday as levels increase above 6,000 ft from southern   
   Washington to west-central MT. Through Wednesday night, 72-hr WPC   
   probabilities for >18" are high (>70%) across the WA Cascades   
   above 5,000 ft, as well as parts of western MT, and western WY   
   above 6,000-7,000 ft. WPC probabilities even show speckled areas of   
   low-to-moderate chances (20-50%) for localized snowfall totals   
   >30" in the more remote reaches of these regions through Wednesday.   
      
      
   ...Southern Appalachians & Mid-Atlantic...   
   Days 1...   
      
   A 500mb trough tracking over the TN Valley this morning will race   
   east and off the Southeast Coast by this evening. 500mb PVA ahead   
   of the upper trough will be paired with easterly low-level flow   
   that allows for some upslope enhancement along the southern   
   Appalachians. Snow will be at its heaviest during the day on Monday   
   as Atlantic moisture is directed west to the north of a weak 850mb   
   low tracking across northern NC. This combined with sufficient   
   low-level WAA will generate a band of snow that results in a quick   
   hitting 1-4" snow event in southern VA and northern NC, to as far   
   north as the Richmond metro area and south along I-95 and I-85   
   towards the Raleigh/Durham area. WPC probabilities currently show   
   low chances (20-40%) for snowfall totals >4" in portions of the   
   VA/NC Blue Ridge, the Potomac Highlands of WV, and southern VA   
   south of the Richmond metro area. Localized snowfall totals of 5-6"   
   are quite possible, especially in the VA Blue Ridge. Probabilities   
   for >2" are 40-60% east of the Blue Ridge across southern VA and   
   is likely lead to slippery road conditions to start the workweek.   
   Latest WSSI does suggest Minor Impacts for most affected locations,   
   with a narrow Moderate Impacts (hazardous driving conditions) area   
   stretching from Roanoke, VA to the Richmond/Petersburg metro area   
   and as far east as Williamsburg, VA.   
      
      
   ...Great Lakes & Northeast...   
   Days 1-2...   
      
   Tonight, as high pressure builds in over the Northeast, the first   
   in a parade of "clipper" systems tracks over the Upper Great Lakes   
   with more light-to-moderate snow expected. Guidance continues to   
   highlight that strong SWrly flow ahead of the clipper will produce   
   a band of heavy lake effect snow oriented from Lake Michigan and   
   far northwest Michigan aimed northward into the eastern U.P.. WPC   
   probabilities through Tuesday show moderate-to- high chances   
   (50-80%) for snowfall totals >6" over the eastern Michigan U.P.. As   
   the clipper's warm front moves east, sufficient 290K isentropic   
   ascent and 850-700mb WAA will give rise to snow over the eastern   
   Great Lakes and northern New England by Tuesday afternoon.   
   Immediately downwind of Lakes Erie and Ontario, snowfall totals   
   will generally range between 1-4".   
      
      
   ...Northern Plains, Midwest, Great Lakes, & Northeast...   
   Days 2-3...   
      
   By Tuesday morning, a vigorous 500mb shortwave trough with origins   
   from the large Gulf of AK longwave trough will race towards the   
   Northern Plains. Given its origins, this "Alberta Clipper" will be   
   more intense than most clippers. This is because it will have   
   exceptional synoptic-scale forcing aloft in the form of a roaring   
   150kt 250mb jet streak, plus anomalous moisture associated with the   
   strong AR over the Pacific Northwest. By 18Z Tuesday, the   
   clipper's MSLP looks to be in the mid-upper 980mb range, which   
   ranks below the 1st climatological percentile for this time of   
   year. As the storm works its way across the Dakotas Tuesday   
   morning, impressive 290K isentropic lift and corresponding   
   850-700mb FGEN will not only help to sustain this potent clipper,   
   but foster a potent band of snow northeast of the 850mb low track.   
   There is the potential for an icy wintry mix just along and north   
   of the surface warm front as a protruding >0C warm nose forces snow   
   to melt into the form of sleet or freezing rain. But the storm's   
   fast motion should limit the impacts from freezing rain to around   
   minor impacts (ice accretions less than a tenth of an inch most   
   likely).   
      
   By Tuesday night, the storm will remain in the mid-980s for   
   pressure as it races through WI and into MI by Wednesday morning.   
   WAA out ahead of the storm is likely to cause periods of snow to   
   envelope the eastern Great Lakes that include areas such as   
   northwest PA and western NY. From the Northern Plains the the Great   
   Lakes, not only will snow be problematic for traveling, but gusty   
   winds are likely to cause blowing and drifting across roads and   
   cause reduced visibilities. Recent 00Z guidance suite has given   
   some credence to the EC-AIFS and GFSAI being more on the southern   
   track of guidance, but there are still a few members that remain a   
   little farther north. WPC probabilities show a swath of high-chance   
   probabilities (>70%) for snowfall totals >4" from northeast ND on   
   south and east through the Upper Midwest and the northern half of   
   Michigan's Mitten. Note that probabilities have increased to   
   moderate-chance level (40-60%) for totals >6", which is increasing   
   confidence in a travel-disrupting winter storm on Tuesday.   
      
   While the storm's speedy forward motion should help to cap the   
   chances for more substantial snowfall totals, hourly snowfall rates   
   of >1"/hr combined with gusty winds will allow for minor to   
   moderate impacts, some of which will occur during the Monday   
   evening commute. This is supported by the WSSI-P which depicts high   
   chances (>70%) for Minor Impacts on Tuesday from northern ND all   
   the way to northern MI. Residents in the Upper Midwest and Great   
   Lakes should continue to monitor this storm closely, as should   
   residents in the Northeast as probabilistic guidance shows   
   measurable snowfall from this system is possible on Wednesday.   
      
   By Wednesday morning, the storm will race into Ontario with strong   
   850-700mb WAA overrunning a very cold air-mass in the Northeast.   
   Look for periods of snow to occur from the Central Appalachians on   
   north through the northern Appalachians. WPC probabilities show   
   moderate-to-high chances (50-70%) for >6" in the Adirondacks, Tug   
   Hill, the Green and White Mountains. Some valleys in northern New   
   England have moderate chances (40-60%) for >2" of snowfall before   
   any issues with mixing or changing to rain occur. As the storm   
   tracks through the St. Lawrence River Valley, cyclonic flow of the   
   Great Lakes will spark additional LES bands over the typical snow   
   belts, including some upslope ascent into the central Appalachians   
   through early Wednesday morning.   
      
      
   Mullinax/Snell   
      
      
      
      
   $$   
      
   = = =   
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