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   FOUS11 KWBC 222058   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   358 PM EST Mon Dec 22 2025   
      
   Valid 00Z Tue Dec 23 2025 - 00Z Fri Dec 26 2025   
      
   *** Strong Atmospheric River followed by low pressure systems to=20   
    impact California and the Sierra Nevada through Friday ***   
      
      
   ...Northeast...   
   Days 1-2... Maximum impact level from WSSI: Moderate   
      
   A shortwave trough crosses northern Ontario Tuesday and New England   
   Tuesday night on a potent 150+kt NWly jet below a deep and   
   sprawling vortex centered north of Hudson Bay. Precip blossoms=20   
   late tonight in warm air advection over the upper Ohio Valley and   
   the Northeast Tuesday. Day 1 snow probs for >4" are 30-60% for the   
   Catskills west toward the Finger Lakes and the southern slopes of   
   the Adirondacks. Light freezing rain (a few hundredths accretion)=20   
   on the southern edge of the precipitation shield (from central PA=20   
   to central WV) is possible late tonight into Tuesday morning.=20   
      
   The surface low crosses New England Tuesday evening with continued   
   snow for Tuesday night in terrain including the Whites, Greens and   
   Adirondacks as the WAA pivots to deformation banding with=20   
   topographic enhancement (which should boost snow rates into the=20   
   1"/hr range) on the back side of the low. Day 1.5 snow probs for=20   
   >6" are 30-80% over this terrain with 50% probs for over 8" in the=20   
   highest terrain.=20   
      
   Most of Maine is north of the surface low track which will have=20   
   ocean enhanced snow Tuesday night before the back side banding=20   
   shifts down the coast to produce ocean enhanced snow for eastern=20   
   Mass Wednesday morning. Day 2 probs for >6" are 40-70% for much of   
   the central Maine coast west well into the remote interior   
   portions of the state.   
      
      
   ...California/Sierra Nevada...=20   
   Days 2-3... Maximum impact level from WSSI: Extreme   
      
   Atmospheric River (AR) over northern CA this afternoon pivots north   
   to OR and the northern Rockies tonight. High snow levels over   
   8000ft lift north with the core of highest precip from the   
   northern Sierra Nevada through the CA Cascades/Trinity Alps keeping   
   Day 1 quiet in CA from a snow perspective.=20   
      
   However, the now north-south oriented AR pushes across the CA coast   
   late Tuesday, reaching the full extent of the Sierra Nevada with=20   
   heavy precip and snow levels around 7000ft Tuesday evening with   
   particularly heavy snow starting roughly after midnight (with rates   
   exceeding 3"/hr per the 12Z HREF) above the snow level. Snow=20   
   levels drop under height falls from the approach trough through the   
   day Wednesday, though precip rates decrease on the Sierra Nevada=20   
   as the AR focuses over southern CA with 9000ft snow levels for the=20   
   SoCal ranges. Day 2 snow probs for >18" are 40% for the higher   
   central Sierra and 70-100% for the southern Sierra and the highest   
   White Mtns along the border with NV.   
      
   Expect a lull in Sierra Nevada snow Wednesday night as the dry=20   
   slot works its way across. However, by Thursday rates increase   
   again ahead of the upper trough axis. This time snow levels are   
   around 6000ft on the Sierra Nevada under the lower heights of an   
   upper low centered off northern CA. Snow rates of 2"/hr or more can   
   be expected for much of Thursday. Broader coverage of the >18"=20   
   snow probs are depicted for Day 3 with 50-90% values along the full   
   length of the Sierra Nevada as well as the CA Cascades including=20   
   Donner Pass on I-80.   
      
      
   Key Messages for the CA atmospheric river and subsequent low that=20   
   maintains impacts through Friday are linked below. CPC has further=20   
   KMs for next week.   
      
      
   ...Pacific Northwest, Great Basin and Rockies...=20   
   Days 1-3... Maximum impact level from WSSI: Major   
      
   Upper low that is directing AR into CA is centered over the Queen   
   Charlotte Sound (BC) and a reinforcing shortwave trough crosses   
   Vancouver Island by this evening bringing a round of moderate   
   precip to the WA Cascades this evening. Snow levels already down   
   around 2500ft drop to around 1800ft before rates decrease overnight   
   providing impactful snow well below pass level. Day 1 snow probs   
   for >6" are 40-60% roughly at pass level in western WA, but it's   
   really all for this evening.=20   
      
   AR across northern CA to western WY this afternoon pivots=20   
   north to southern OR to the Bitterroots through Tuesday. Snow   
   levels rise in this moisture axis to around 7000ft keeping Day 1   
   snow probs for >6" limited to the highest Sawtooths, Tetons, and=20   
   Absarokas. The AR continues to pivot north-south along the West=20   
   Coast through Tuesday night with snow levels rising above 4000ft on   
   the WA Cascades by Wednesday. Day 2 snow probs for >4" are 40-60%=20   
   in the WA Cascades.   
      
   The focus for precip shifts interior on Wednesday as the AR   
   extending the length of CA shifts east over the Great Basin and the   
   Rockies for Wednesday night/Thursday. Snow levels are generally   
   high, 8000-9000ft with this moisture shield shifting inland. Day 3   
   snow probs for >6" are limited to the highest Great Basin ranges,   
   the Sawtooths, Uinta, Wind River, and San Juans where values are   
   40-70% (80% or greater for the higher Wind Rivers).=20   
      
      
   Jackson   
      
      
      
   ...Atmospheric River/Winter Storm Key Messages are in effect for   
   California as linked below...   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_1.png__;!!DZ3fjg!-VZ02Vbi2p7uqvUSbXIOQH-rdR7tvmWmepI6Nz0W1prv3=   
   DGPmRKbpBHln8TEYs5vFPhnIwL8EQ9CGVhJtRgt-JyXbTM$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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