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   FOUS11 KWBC 072021   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   321 PM EST Sun Dec 7 2025   
      
   Valid 00Z Mon Dec 08 2025 - 00Z 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. Tonight, 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 and   
   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.   
      
   Tonight, snow levels largely look to start out around 5,000ft. 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. Starting this evening and lasting   
   through early Wednesday evening, 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 Wednesday.   
      
      
   ...Southern Appalachians & Mid-Atlantic...   
   Days 1...   
      
   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 during the day on Monday   
   but taper off by Monday evening. As the 500mb vorticity maximum   
   reaches NC Monday afternoon, a weak 850mb low development off the   
   coast of NC's Outer Banks will allow for easterly low- level winds   
   to increase. This combined with sufficient low-level WAA and   
   moisture advection may create a band of snow that results in a   
   quick hitting 1-3" snow event in southern VA and northern NC 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 and Potomac Highlands of WV. Probabilities for   
   >2" are 40-60% east of the Blue Ridge across southern VA and could   
   lead to slippery road conditions to start the workweek.   
      
      
   ...Northern Plains/Upper Midwest/Great Lakes...   
   Days 2-3...   
      
   By Monday night, as high pressure builds in over the Northeast, the   
   first in a parade of "clipper" systems makes its way toward 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" in over the   
   eastern Michigan U.P. 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". However, the Tug Hill has moderate chances   
   (40-60%) for snowfall totals >4" through early Wednesday morning.   
      
   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 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. WPC probabilities show a swath of   
   moderate chance probabilities (40-70%) for snowfall totals >4"   
   from northeast ND on south and east through the Upper Midwest and   
   the northern half of Michigan's Mitten. Note there are also low-to-   
   moderate chance probabilities (20-40%) for totals >6", which does   
   show up on several deterministic guidance members. There has   
   continued to be some latitudinal spread in guidance with AIFS and   
   AIGFS guidance along with the operational GFS leaning towards a   
   more southern track that gets central MN and southern WI into   
   chances for heavy snow.   
      
   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 to moderate   
   impacts at the very least. This is supported by the WSSI-P which   
   depicts high chances (60-90%) 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/Snell   
      
      
      
      
   $$   
      
   = = =   
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