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   FOUS11 KWBC 070816   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   316 AM EST Sun Dec 7 2025   
      
   Valid 12Z Sun Dec 07 2025 - 12Z Wed Dec 10 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 start out around   
   the 90th climatological percentile throughout the Northwest, before   
   a more impressive plume of PWATs above the 99th climatological   
   percentile stream in on Monday. Even on Tuesday, 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.   
      
   Today, snow levels largely look to start out around 3,000-4,000   
   ft, but rapidly rise across the Pacific Northwest above 5,000ft.   
   As the strong AR and its associated WAA aloft approaches on   
   Monday, these values increase to above 6,000 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 D3 as the AR   
   orients itself farther south as a shortwave trough ejects into the   
   northern Plains. Starting this morning and lasting through early   
   Wednesday morning, 72-hr WPC probabilities for more than 12 inches   
   are high (70-90%) across the WA Cascades above 5,000 ft, as well   
   as parts of the northern Rockies in ID, 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 Tuesday   
   night.   
      
      
   ...Great Lakes & Northeast...   
   Days 1-3...   
      
   This morning, the upper trough responsible for the Midwest storm   
   on Saturday will generate light-to-moderate snowfall across   
   Michigan and much of the Northeast (minus the I-95 corridor   
   megalopolis). Most snowfall totals will range between 1-4",   
   although some areas within the Tug Hill and Adirondacks may manage   
   to see localized snowfall totals eclipse 4" on Sunday. The upper-   
   level disturbance may produce a small area of low pressure off the   
   Maine coast that produces a fast moving band of snow over Downeast   
   Maine Sunday night. WPC probabilities do depict low chances   
   (10-30%) for snowfall totals >4" Monday night.   
      
   By Monday night, as high pressure builds in over the Northeast, an   
   Alberta Clipper makes its way toward the Upper Great Lakes Monday   
   evening with more light-to-moderate snow expected. Guidance is   
   coming into better agreement 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 >4" in over the eastern   
   Michigan U.P. and northwest Michigan. The eastern Michigan U.P.   
   sports the highest chances for >8" of snowfall (30-50%). 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". However, the Tug Hill has moderate chances   
   (40-60%) for snowfall totals >4" through early Wednesday morning.   
      
      
   ...Southern Appalachians & Mid-Atlantic...   
   Days 1-2...   
      
   A quick moving 500mb trough will track from the TN Valley Sunday   
   night to the Southeast Coast by Monday afternoon. 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 Monday morning but taper   
   off by Monday evening. This storm could have a surprise or two up   
   its sleeve as the 500mb vorticity maximum reaches NC Monday   
   afternoon, with some guidance showing some 850mb low development   
   off the coast of NC's Outer Banks. Should easterly low-level winds   
   increase, sufficient low-level WAA and moisture advection could   
   create a band of snow that results in a quick hitting 1-3" snow   
   event from as far north as the Richmond metro area on south along   
   I-95 and I-85 towards the Raleigh/Durham area. WPC probabilities   
   currently show low chances (10-30%) for snowfall totals >4" in   
   portions of the VA/NC Blue Ridge and Potomac Highlands of WV, but   
   it is possible those chances increase should the influences of the   
   500mb trough prove more substantial. This has the potential to   
   cause slick travel conditions for the Monday evening commute in   
   southern VA and northern NC.   
      
      
   ...Northern Plains & Upper Midwest...   
   Day 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 sub-990mb, 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 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. WPC probabilities show a swath of   
   moderate chance probabilities (40-60%) for snowfall totals >4" from   
   northeast ND on south and east through the Upper Midwest and the   
   northern half of Michigan's Mitten. Note there are also low-to-   
   moderate chance probabilities (20-50%) for totals >6", which does   
   show up on several deterministic guidance members.   
      
   While the storm's speedy forward motion should help to reduce the   
   chances for more prolific snowfall totals, hourly snowfall rates of   
   >1"/hr combined with gusty winds will allow for Minor Impacts at   
   the very least. This is supported by the WSSI-P which depicts   
   moderate-to-high chances (50-70%) 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.   
      
      
   Mullinax   
      
      
      
      
   $$   
      
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