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   FOUS11 KWBC 212011   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   311 PM EST Sun Dec 21 2025   
      
   Valid 00Z Mon Dec 22 2025 - 00Z Thu Dec 25 2025   
      
      
   ...Upstate/Central New York...   
   Day 1... Maximum impact level from WSSI: Minor   
      
   Exiting trough will lead to an end to any lake effect snow over NY   
   east of ROC and near/south of Oswego to SYR. Additional snowfall=20   
   after 00Z tonight will be light, though some areas between Sodus=20   
   and Oswego could see another 4" of snow where WPC probabilities are   
   >50%.=20   
      
      
   ...Northeast...   
   Days 2-3... Probability of at least Minor Impacts: >70%   
      
   Upper level pattern over CONUS by Day 2 includes an anomalous=20   
   ridge centered over the central U.S. and strong jet soaring to the=20   
   north along the U.S-Canadian border before aiming southeastward=20   
   across the Northeast, where slight troughing remains. An embedded=20   
   shortwave crossing north of the Great Lakes on Tuesday within a=20   
   divergent left-exit region of a strengthening 150kt jet streak will   
   aid in blossoming precipitation across the Northeast by early=20   
   Tuesday. Broad WAA ahead of the system over the still cold=20   
   Northeast (especially just inland) will bring generally light snow=20   
   to much of the Northeast and northern Mid-Atlantic but some light=20   
   freezing rain on the southern edge of the precipitation shield=20   
   (central PA and perhaps into parts of WV). The highest QPF will lie   
   over Northern/Upstate/Central NYS into central New England within=20   
   the zone of strongest lower-level WAA as the surface warm front=20   
   lifts into the region. With the fast flow, the parent low over=20   
   Canada will get left behind as the front occludes and stretches=20   
   across New England, ultimately forming a new low in the Gulf of=20   
   Maine. This may lead to an inverted trough or quickly developing=20   
   TROWAL maximizing QPF/snow potential across southern Maine, but=20   
   uncertainty remains with the details. The system will pull away=20   
   from eastern New England late Wednesday afternoon and snow will=20   
   come to an end by the end of this forecast period. WPC=20   
   probabilities for at least 4 inches of snow are >50% over portions=20   
   of the Finger Lakes region northward to the Tug Hill and=20   
   Adirondacks, Green Mountains, and into the Maine Midcoast. The=20   
   highest probabilities for at least 6 inches of snow are over the=20   
   Tug Hill/Adirondacks.=20   
      
      
      
   ...Cascades, Northern Rockies, & Northern California...=20   
   Days 1-3... Maximum impact level from WSSI: Major to Extreme   
      
   Digging heights near 135W in the eastern Pacific will help lift a=20   
   shortwave over the Northwest tonight as well as direct a resurgent=20   
   Atmospheric River into northern California with spillover extending   
   into the Northern Rockies. Snow levels will remain on the lower=20   
   side north of the moisture plume, but continued upslope into the=20   
   Cascades and northern Rockies will still yield modest snow totals=20   
   for these regions, especially days 1-2. The incoming shortwave=20   
   tonight will briefly raise snow levels in the WA Cascades from=20   
   around 2000 to 3000ft, but then fall again with some CAA behind the   
   system. This will continue to impact the passes in the region=20   
   (e.g., Snoqualmie and Stevens) where significant snow is likely.=20   
   Heavy snow in these regions of the Northwest and Northern Rockies=20   
   may be focused around two waves -- the initial one tonight with=20   
   another Monday night into early Tuesday. WPC probabilities for at=20   
   least 12 inches of snow for the three-day period are >50% above=20   
   3000ft or so.   
      
   Farther east, snow levels around 4000-8000ft over the Northern=20   
   Rockies (north to south) will rise a bit more tonight in advance of   
   more snow. SW flow and sufficient moisture will support lighter=20   
   but still appreciable accumulations compared to the Cascades over=20   
   northern Idaho into northwestern Montana, generally above 5000ft.=20   
   The strongest moisture flux will be in the core of the AR where PW=20   
   and IVT anomalies lie near/above the 99th percentile, generally in=20   
   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 central Idaho and especially western Wyoming. High mountain=20   
   peaks/ridges above 9000-10,000ft may see several feet of snow=20   
   before precipitation begins to wane on Tuesday in response to=20   
   building heights along/east of the Divide.   
      
      
   *** Strong Atmospheric River to impact California and the Sierra=20   
    Nevada over the next several days ***   
      
   ...Sierra Nevada...   
   Days 1-3... Maximum impact level from WSSI: Major to Extreme   
      
   A long-lived Atmospheric River event will contain periods of=20   
   precipitation as embedded systems move inland from the Pacific over   
   the next several days (and beyond). A few areas of surface low=20   
   pressure have and will move inland across NorCal today/tonight and=20   
   will continue to bring heavier snow to the northern/central Sierra=20   
   into early Monday with snow levels around 8000-10,000ft from north=20   
   to south. Snow may impact some of the passes across the Sierra=20   
   Crest though the heaviest snow rates may be above 9000ft (2-3"/hr).   
   Monday night/early Tuesday, the moisture plume will lift northward   
   with less accumulation overall (except for the highest peaks in=20   
   the Shasta/Siskiyous and farthest north Sierra) as another system=20   
   in the Pacific deepens. This will allow the initial plume to weaken   
   a bit over NorCal into southern Oregon while the next surge of=20   
   moisture pushes into central CA in response to height falls just=20   
   off the West Coast. This will target most of the Sierra by early=20   
   Wednesday with increasingly heavier snow and snow rates (2-4"/hr=20   
   near the Crest) above 8000ft that will fall as the AR progresses=20   
   slowly southeastward.=20   
      
   For days 1-2, WPC probabilities for at least 12 inches of snowfall   
   are >50% above around 8000ft. For day 3 alone, with the next=20   
   strong moisture surge, WPC probabilities for at least 12 inches of=20   
   snow are >50% above 8000ft with >70% probabilities for at least 18=20   
   inches of snow above 9000ft. This very active and impactful weather   
   pattern across California is expected to continue through much of=20   
   this week, with additional high elevation heavy snow likely into=20   
   the end of this week. See our latest Key Messages that cover the=20   
   event through the end of next week, and CPC's Key Messages into=20   
   week 2.   
      
      
   The probability of significant icing is less than 10 percent.=20   
      
   Fracasso/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!8YxyrDcVV2VjIHL52sTTCq9ZhLfybUScoUs9uTHV_nFoe=   
   rz6D4vv4aCmjAfBzmSyI8UdxqgSYNSrs-zGtj35CF7gC7Q$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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