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   FOUS11 KWBC 230833   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   333 AM EST Tue Dec 23 2025   
      
   Valid 12Z Tue Dec 23 2025 - 12Z Fri Dec 26 2025   
      
   *** Strong Atmospheric River followed by low pressure systems to   
    impact California and the Sierra Nevada through Friday ***   
      
      
   ...Northeast...   
   Days 1-2... Maximum impact level from WSSI: Moderate   
      
   A shortwave trough crosses northern Ontario today and New England=20   
   early Wednesday on a potent 150+kt NWly jet below a deep and=20   
   sprawling vortex centered north of Hudson Bay. Precip will be   
   ongoing this morning in warm air advection over the upper Ohio=20   
   Valley and the Northeast. Surface temperatures become marginal   
   quickly after sunrise and limits the potential for heavy   
   accumulations from PA to southern New England. Light freezing rain   
   (a few hundredths accretion) is also possible this morning on the=20   
   southern edge of the precipitation shield (from central PA to=20   
   central WV).   
      
   The surface low crosses New England this evening with continued=20   
   snow into Wednesday morning for the terrain including the Whites,=20   
   Greens and Adirondacks as the WAA pivots to deformation banding=20   
   with topographic enhancement (which should boost snow rates into=20   
   the 1"/hr range) on the back side of the low. Day 1 snow probs for   
   >6" are 40-80% over this terrain with 50% probs for over 8" in the   
   highest terrain.=20   
      
   Heaviest snowfall associated with this system is expected across   
   the coastal plain and areas just inland across Maine as low   
   pressure redevelops along a surface trough over the Gulf of Maine.   
   This surface trough, or also referred to as a "Norlun Trough", will   
   lead to very strong and narrow low-level convergence capable of   
   producing snowfall rates of 1-2"/hr. However, given the narrow axis   
   these setups can be extremely tricky to forecast. Current   
   expectations are for heavy snow to occur between Portland and Bar   
   Harbor, along with areas just inland. Surface temperatures could   
   also be a concern for regions right along the coast as southerly=20   
   flow draws both moisture and a slightly warmer marine airmass. Days   
   1-2 probs for >8" are 50-80% for much of the central Maine coast=20   
   west well into the remote interior portions of the state. Maximum   
   amounts within the heaviest band are likely to exceed a foot, which   
   is depicted in the NBM 75th percentile for areas just inland of the   
   coastal plain.   
      
      
   ...Great Lakes...   
   Day 3... Chances of at least minor impacts from WSSI-P: 40-50%   
      
   By late in the forecast period (Thursday night), the next shortwave   
   to round the top of the anomalous central U.S. upper ridge begins   
   to cross over the Northern Plains and Upper Midwest. Even though   
   the system is fairly unorganized through early Friday morning, the   
   origins of the shortwave and associated mid-to-upper level moisture   
   trace back to the tropical Pacific and Atmospheric River set to   
   impact California. In fact, NAEFS ESAT depict IVT above the 99.5th   
   climatological percentile Thursday evening over the western Great   
   Lakes. Still, the greatest forcing at this time appears along the   
   MN-Canadian border and just north into Canada, limiting snowfall   
   impacts. However, a potent warm nose around 800mb per model cross   
   sections will allow for precipitation throughout parts of the=20   
   Upper Midwest and Great Lakes to fall as sleet or freezing rain. As   
   850mb FGEN rapidly strengthens across southern MI Friday morning=20   
   in response to a developing low pressure center over the Midwest,   
   precipitation is expected to blossom and fall mostly as sleet or   
   freezing rain. WPC probabilities for at least 0.1" of ice accretion   
   through 12Z Friday (more possible afterwards as well) are currently   
   low (10-20%) across the central L.P. of MI. Expect higher chances   
   once all of Friday is in the forecast period.   
      
      
   ...California/Sierra Nevada...=20   
   Days 1-3... Maximum impact level from WSSI: Extreme   
      
   Strong Atmospheric River is set to impact CA during the Day 1   
   period with heavy snowfall across the Sierra Nevada. Additional   
   systems containing heavy high elevation snow and falling snow   
   levels are expected through the end of the week. In total, multiple   
   feet of snowfall are expected along the entire Sierra Nevada   
   (heaviest in central and southern sections), with snowfall also   
   impacting most major passes into northern CA.   
      
   The meteorological setup for this significant event includes a   
   currently north-south oriented AR combining with a potent shortwave   
   lifting along the WEst Coast today while on the eastern periphery   
   of an anomalous eastern Pacific upper trough. This shortwave and   
   associated potent surface low pressure will draw moisture northward   
   out of the tropics and produce IVT above the climatological record   
   for late December according to the NAEFS ESAT by early Wednesday.=20   
   Additionally, snow levels start out Tuesday night between   
   7500-8500ft across CA (highest in the southern Sierra) and fall to   
   below 6000ft in northern CA by Wednesday night as levels dip to   
   around 7500ft in the southern Sierra. Where snow does occur across   
   the Sierra Nevada it will come down extremely heavy as 00z HREF   
   depicts rates greater than 2-3"/hr after 09Z Wednesday and lasting   
   into Wednesday evening. Snow levels continue to drop under height=20   
   falls from the approaching trough through Wednesday night into=20   
   Thursday though as precip rates decrease on the Sierra Nevada as=20   
   the AR focuses over southern CA with 9000ft snow levels for the=20   
   SoCal ranges. This will lead to a dry slow until the next surge   
   ahead of a separate Pacific low approaches on Thursday.   
      
   By Thursday rates increase again ahead of the upper trough axis.=20   
   This time snow levels are around 6000ft on the Sierra Nevada under=20   
   the lower heights of an upper low centered off northern CA. Snow=20   
   rates of 2"/hr or more can be expected for much of Thursday.=20   
   Broader coverage of the >18" snow probs are depicted for Day 3 with   
   50-90% values along the full length of the Sierra Nevada as well=20   
   as the CA Cascades including Donner Pass on I-80. Snow levels also   
   drop to around 4000ft by the end of Day 3 across northern CA and   
   allow for accumulating snow potential for the I-5 pass near Mount   
   Shasta. 72-hr snow probabilities for >30" snow are between 50-90%=20   
   for much of the Sierra Nevada. Totals are likely to exceed 4-8 feet   
   above 7000-8000ft.   
      
      
   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 2-3... Maximum impact level from WSSI: Moderate=20   
      
   Outside of light snow across the northern Great Basin and Cascades   
   on Day 1, the next surge of moisture is forecast to enter the   
   interior West and Northwest ranges on days 2-3. The strong AR=20   
   extending the length of CA shifts east over the Great Basin and the   
   Rockies for Wednesday night/Thursday along with its anomalous   
   moisture. Snow levels are generally high, 8000-9000ft with this=20   
   moisture shield shifting inland outside of the Cascades where=20   
   levels increase to 5000ft Wednesday and drop back to around 3000ft=20   
   Thursday. Days 2-3 snow probs for >6" are limited to the highest=20   
   OR/WA Cascades, Great Basin ranges, the Sawtooths, Uinta, Wind=20   
   River, and San Juans where values are 40-70% (80% or greater for=20   
   the higher Wind Rivers and Sawtooths).=20   
      
      
   Snell/Jackson   
      
      
      
      
   ...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!-I0_CPZ0UePHURDQNcl63Rb55rx8NtluQA6D4C_6vmtRL=   
   qm2ELNT0ODX-yKMq99rSkPeDFD40nbB7L0gE6JKjNo8YlM$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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