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   FOUS11 KWBC 220827   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   327 AM EST Mon Dec 22 2025   
      
   Valid 12Z Mon Dec 22 2025 - 12Z Thu Dec 25 2025   
      
      
   ...Northeast...   
   Days 1-3... Maximum impact level from WSSI: Minor to Moderate   
      
   Upper level pattern over CONUS by the end of Day 1 includes an=20   
   anomalous ridge centered over the central U.S. and strong jet=20   
   soaring to the north along the U.S-Canadian border before aiming=20   
   southeastward across the Northeast, where slight troughing remains.   
   An embedded shortwave crossing north of the Great Lakes on Tuesday   
   within a divergent left-exit region of a strengthening 150kt jet=20   
   streak will aid in blossoming precipitation across the Northeast by   
   early 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   
   freezing rain on the southern edge of the precipitation shield=20   
   (central PA and perhaps into parts of WV). The highest QPF=20   
   associated with the strongest low-level WAA will lie over=20   
   Northern/Upstate/Central NYS into central New England as the=20   
   surface warm front lifts into the region. With the fast flow, the=20   
   parent low over Canada will get left behind as the front occludes=20   
   and stretches across New England, ultimately forming a new low in=20   
   the Gulf of Maine. This will likely lead to a separate and higher=20   
   maximum in QPF due to an inverted trough or quickly developing=20   
   TROWAL maximizing snow potential across southern/coastal Maine, but   
   some uncertainty remains with the details as this trough could=20   
   still shift somewhat east or southwest along the Gulf of Maine.=20   
   This setup has the potential to produce a narrow corridor of very=20   
   heavy snowfall given the strong low- level convergence potential=20   
   from roughly Portland to Bar Harbor, ME. In fact, the NBM 75th=20   
   percentile currently sits around 12 inches, highlighting the higher   
   end potential. The system will pull away from eastern New England=20   
   late Wednesday afternoon.   
      
   WPC probabilities for at least 4 inches of snow are >50% over=20   
   portions of the Finger Lakes region northward to the Tug Hill and=20   
   Adirondacks, Green Mountains, and into the White Mountains and=20   
   Maine Midcoast. The highest probabilities (40-70%) for at least 6=20   
   inches of snow extend between the southern and midcoasts of   
   Maine. The National Weather Service in Gray, ME has issued Winter=20   
   Storm Watches in this region for the potential for total snow=20   
   accumulations greater than 6 inches. Lighter, festive snowfall is=20   
   expected elsewhere in New England on Tuesday and even as far south=20   
   as northeast PA/northern NJ for the Tuesday morning commute.   
      
      
   ...Interior West & Rockies...   
   Days 1 & 3... Maximum impact level from WSSI: Moderate to Major   
      
   Heavy snowfall continues to impact the high terrain of the Rockies   
   and Intermountain West, but will see the overarching pattern begin   
   to shift away from the recent west-east oriented Atmospheric River   
   spillover towards more southerly flow and approaching height falls   
   by midweek. With that said, the Day 1 period starts with the   
   aforementioned AR and anomalous PWs extending from the northern   
   Sierra Nevada into the Northern Rockies, but this moisture stream   
   is expected to wane today as an associated shortwave ejects   
   eastward with ridging building in its place across the Rockies   
   through Tuesday. Snow levels will remain high within the axis of   
   greatest precipitation located across central ID into southwest MT   
   and western WY, with levels generally in the 7000-8000ft range. The   
   most impact snowfall with amounts measured in feet continue to be   
   located across the Wind River Range, Tetons, and Absarokas.   
      
   A brief lull in heavy snowfall is expected on Tuesday as IVT   
   weakens and reorients into a more north-south direction into the   
   northern Great Basin as height falls approach the West Coast.   
   However, by Day 3 a broad surge of tropical moisture lifts   
   throughout the interior West due to a near-record deep upper trough   
   off the West Coast. Given the moisture source it isn't too   
   surprising that snow levels across the Great Basin and Rockies will   
   be rather high, above 9000ft in Utah and above 10,000ft in western   
   Colorado, with slightly lower values around 7000-8000ft in the   
   Northern Rockies during this time period.   
      
   WPC 72-hr snowfall probabilities for at least 12 inches of snow are   
   highest across the Sawtooths of ID and Wind River/Absarokas of WY,   
   with 70-90% chances mainly above 8500ft.   
      
      
   *** Strong Atmospheric River to impact California and the Sierra=20   
    Nevada over the next several days ***   
      
   ...Sierra Nevada/Northern California/Cascades...   
   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). For Day 1 (ending 12Z Tuesday),   
   the ongoing AR will lift north into northern CA and weaken with   
   limited snowfall impacts across CA. Meanwhile, much lower snow   
   levels and a potent shortwave entering southeast British Columbia   
   will allow for heavy snow across the Olympics and Cascades today   
   into early Tuesday. Here, snow levels will vary between   
   1500-2000ft, which puts accumulating snow below the major WA   
   Cascade passes.   
      
   By late Day 2 into Day 3, the next surge of moisture impacts the   
   West Coast and continues beyond into late this week. Deepening   
   height falls off the West Coast and a potent shortwave lifting on   
   the eastern periphery of this deep upper trough rapidly surges   
   moisture inland Tuesday night. IVT directed into the Sierra will be   
   strong (>800 kg/m/s) Tuesday night, but relatively short-lived as   
   broad onshore flow and moderate precipitation continues on Day 3.   
   Snow levels during this strong AR are expected to generally remain   
   around 8000ft (closer to 8500ft in the core of the AR across the   
   central/southern Sierra Nevada), but quickly fall to between   
   5000-6500ft (lower north and higher south) across CA on Day 3.=20   
   This will allow for heavy snow to impact all major passes by=20   
   Wednesday.   
      
   For the most impactful day (Day 3) WPC probabilities for at least=20   
   12 inches of snowfall are 70-99% above around 7000ft and >70% for=20   
   at least 24 inches of snowfall above 8000ft in the Sierra Nevada.=20   
   Strong winds combined with very heavy-wet snowfall could make=20   
   travel difficult to impossible at times in the high terrain of the=20   
   Sierra Nevada, with damage to infrastructure also possible. This=20   
   very active and impactful weather pattern across California is=20   
   expected to continue through much of this week, with additional=20   
   high elevation heavy snow likely into the end of this week. See our   
   latest Key Messages that cover the event through the end of next=20   
   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!4Zad0Gdrr51OXkfTGAdILviKN5pcYLPC0kOOau9ldhSpK=   
   c8rBebO9pfTYNjHgGi_HaSZmFTnH2fhUaUDCBGGcnbudQc$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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