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   FOUS11 KWBC 202018   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   318 PM EST Sat Dec 20 2025   
      
   Valid 00Z Sun Dec 21 2025 - 00Z Wed Dec 24 2025   
      
      
   ...Great Lakes...   
   Days 1-2... Maximum impact level from WSSI: Minor   
      
   Clipper system moving through southern Ontario will bring=20   
   generally light snow to the Great Lakes via warm air advection=20   
   first, followed by light to moderate lake effect snow over the U.P.   
   of Michigan and into the Tug Hill Plateau post-FROPA. Residence=20   
   time will be on the shorter side, limiting total accumulations=20   
   Sunday and early Monday. WPC probabilities for at least 6 inches of   
   snow are highest across the eastern U.P. of MI and between 40-70%.   
   Chances for at least 4 inches of snow downwind of Lake Ontario are   
   moderate (40-70%) along its southern shore and the Tug Hill=20   
   through Day 2.   
      
      
   ...Northeast...   
   Day 3... Probability of at least Minor Impacts: >50%   
      
   Jet stream over the northern Plains to the Upper Midwest will help   
   carry a surface low pressure through southern Canada and a front=20   
   through the Great Lakes. Broad WAA ahead of the system over the=20   
   still cold Northeast will bring generally light snow to much of the   
   Northeast and northern Mid-Atlantic away from the immediate coast.   
   Some light freezing rain is possible on the southern edge of the=20   
   precipitation (central PA and perhaps into parts of WV) though the=20   
   highest QPF will lie over Northern/Upstate/Central NYS at the nose=20   
   of a 130kt jet through 00Z/Wed. WPC probabilities for at least 4=20   
   inches of snow are >50% in the Tug Hill (embedded probabilities=20   
   >70%) as well as into the Green Mountains.   
      
      
   ...Cascades, Northern Rockies, & Northern California...=20   
   Days 1-3... Maximum impact level from WSSI: Major to Extreme   
      
   Strong west-to-east 160+kt jet into Oregon will lift northward=20   
   tonight in response to digging heights along 140W. This will=20   
   maintain light to moderate precipitation over the Cascades and=20   
   downstream to the Northern Rockies on the cooler side of the=20   
   Atmospheric River that is positioned over Northern California. Snow   
   levels will remain on the lower side north of the moisture plume,=20   
   but continued upslope into the Cascades and Rockies will still=20   
   yield modest snow totals for these regions, especially days 1-2.=20   
   The incoming shortwave tomorrow night will briefly raise snow=20   
   levels in the WA Cascades from around 2000 to 3000ft, but then fall   
   again with some CAA behind the system. This will continue to=20   
   impact the passes in the region (e.g., Snoqualmie and Stevens)=20   
   where significant snow is likely. This may be focused around two=20   
   waves -- the initial one tonight/tomorrow with another Monday night   
   into Tuesday. WPC probabilities for at least 12 inches of snow for   
   the three-day period are >50% above 3000ft or so. Farther east,=20   
   snow levels around 4000-6000ft over the northern Rockies will rise=20   
   later tonight as heights rise and moisture increases. SW flow and=20   
   sufficient moisture will support lighter but still appreciable snow   
   compared to the Cascade over northern Idaho into northwestern=20   
   Montana, generally above 5000ft.=20   
      
   The strongest moisture flux will be in the core of the AR where PW   
   and IVT anomalies lie near/above the 99th percentile, generally in   
   an arc from the southern OR Cascades into the central/southern=20   
   Idaho ranges and into Wyoming. There, WSW to W flow will maximize=20   
   accumulations over parts of the Bitterroots and especially into the   
   Tetons, Absarokas, and Wind River Range. WPC probabilities for at=20   
   least 24 inches of snow over the three-day period are high (>70%)=20   
   over western Wyoming. High mountain peaks/ridges above=20   
   9000-10,000ft may see several feet of snow.=20   
      
      
   *** Strong Atmospheric River to impact California and the Sierra=20   
    Nevada through next week ***   
      
   ...Sierra Nevada...   
   Days 1-3... Maximum impact level from WSSI: Major to Extreme   
      
   A long-lived Atmospheric River event will contain waves of=20   
   precipitation as embedded systems move inland from the Pacific over   
   the next several days (and beyond). An area of surface low=20   
   pressure will move inland across NorCal tonight/early Sunday and=20   
   bring heavier snow to the northern/central Sierra into early Monday   
   with snow levels around 8000-10,000ft from north to south. Snow=20   
   may impact some of the passes across the Sierra Crest though the=20   
   heaviest snow rates may be above 9000ft (2-3"/hr). Thereafter, the=20   
   moisture plume will lift northward and snow levels may lower a bit=20   
   into Tuesday with less accumulation overall except for the highest=20   
   peaks in the Shasta/Siskiyous and farthest north Sierra. This is=20   
   possibly the start of a very active and impactful weather pattern=20   
   across California, with additional high elevation heavy snow likely   
   through the middle to end of next week. See our latest Key=20   
   Messages that cover the event through the end of next week, and=20   
   CPC's Key Messages into week 2.   
      
      
   The probability of significant icing is less than 10 percent.=20   
      
   Fracasso   
      
      
   ...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!-8zQPUyCgrJcQnhUCXMvQ3b_nE5Ze81Ok5BTIcxjXD8Py=   
   r7EUaeUp7XWbO2laLsh-nfWXFH7q8lMAJhKbriU0lGuQyc$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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