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   FOUS11 KWBC 200753   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   253 AM EST Sat Dec 20 2025   
      
   Valid 12Z Sat Dec 20 2025 - 12Z Tue Dec 23 2025   
      
      
   ...Great Lakes...   
   Days 1-2...   
   Maximum impact level from WSSI: Minor   
      
   Weather pattern across the Great Lakes quickly shifts away from=20   
   strong cold air advection as a shortwave crosses over the region=20   
   with an associated surface low tracking north of Lake Superior=20   
   towards southern Quebec by Sunday morning. This allows for some=20   
   moderate warm air advection snow across the Upper Great Lakes=20   
   followed by lake effect snow overnight behind a potent cold front.=20   
   This west-northwest flow returns to Lake Ontario and Lake Erie by=20   
   Sunday as well. All in all snowfall amounts from this system don't=20   
   appear that impressive given it's progressive nature. WPC=20   
   probabilities for at least 6 inches of snowfall are highest across=20   
   the eastern U.P. of MI and between 40-70%. Chances for at least 4=20   
   inches of snow downwind of Lake Ontario are low (20-40%) along its=20   
   southern shore and the Tug Hill through Day 2.   
      
      
   ...Cascades, Northern Rockies, & Northern California...=20   
   Days 1-3...   
   Maximum impact level from WSSI: Major to Extreme   
      
   Deep upper troughing in eastern Pacific below the 10th   
   climatological percentile through at least early next week   
   continues to allow for an influx of westerly flow and Pacific   
   moisture into the western U.S. along with increasing IVT within a   
   wavering Atmospheric River. As moisture flux wanes across the   
   northern Rockies on Day 1 the next surge enters the region by the   
   start of Day 2 with snow levels starting around 5000ft across the   
   Sawtooths before steadily increasing throughout the region,=20   
   including the Absarokas, Tetons, and Wind River Range, up to=20   
   around 8000ft within the core of the moisture plume extending all=20   
   the way back to the central Pacific. Moisture will be plentiful as=20   
   PWs increasing to above the 99th climatological percentile over=20   
   northern UT and into WY. The heaviest snowfall is expected=20   
   throughout the ranges of central ID and western WY. WPC=20   
   probabilities for at least 18 inches of snowfall through Tuesday=20   
   morning are high (70-90%) across the Sawtooths of ID and Wind=20   
   River/Teton/Absaroka ranges of WY above 8000ft. Snowfall totals in=20   
   this region could top 2 to 3 feet.   
      
   Farther north and away from the immediate AR plume, persistent   
   westerly flow and lower snow levels are forecast to impact the   
   Pacific Northwest. Snow levels across the Cascades are expected to   
   start the forecast period around 2000ft and only briefly rise to   
   around 3000ft tonight before falling yet again to 2000ft on Monday.   
   This places all major passes at risk of heavy accumulating snowfall   
   and treacherous driving conditions. Snoqualmie (3022ft) and=20   
   Stevens Pass (4061ft) of particular note could see total snow=20   
   accumulations between 1 to 2 feet. WPC probabilities for at least 2   
   feet of snowfall are high (70-90%) across the WA Cascades above=20   
   4000ft, with high chances for at least 12 inches of snow above=20   
   3000ft. Moisture also continues to stream into the northern ID and=20   
   northwest MT ranges, where elevations above 5000ft have high=20   
   chances for at least 8 inches of snowfall over a 72-hr period.   
      
      
   *** Strong Atmospheric River to impact California and the Sierra   
    Nevada through next week ***   
      
   ...Sierra Nevada...   
   Days 1-3...   
   Maximum impact level from WSSI: Major to Extreme   
      
   Multiple AR impulses through Tuesday are expected with snow levels   
   potentially dropping down to around 7500ft as these moisture=20   
   plumes are aimed at the northern/central Sierra Nevada, with the   
   strongest surge forecast on Sunday ahead of a strong shortwave.=20   
   This may impact some of the passes across the crest (e.g., Donner=20   
   Pass at 7239ft) as snow levels dip/oscillate. The AR event will=20   
   continue beyond this forecast period into the medium range. WPC=20   
   probabilities for at least 12 inches of snowfall on days 1-3=20   
   (ending 12Z Tuesday) are between 70-90% and primarily above 8500ft=20   
   elevation. Snow levels will trend a bit lower into Monday and=20   
   Tuesday as WPC probabilities for at least 4 inches of snow are >30%   
   to elevations around 7500ft. This is possibly the start of a very=20   
   active and impactful weather pattern across California, with=20   
   additional high elevation heavy snow likely through the middle to=20   
   end of next week. See our latest Key Messages that cover the event=20   
   through the end of next week, and CPC's Key Messages into week 2.   
      
      
   The probability of significant icing is less than 10 percent.=20   
      
      
   Snell   
      
      
      
   ...Atmospheric River Key Messages are in effect. Please see=20   
   current Key Messages below...   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_1.png__;!!DZ3fjg!68GdDrL0WuNUm2DNvJV3RZ1kO2Bp3LoaocoX2kDXlLn61=   
   3QYlpxY-MzlnpzPb72VtZyArvWymtpFbSYDITP7298IFM8$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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