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   FOUS11 KWBC 240846   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   346 AM EST Wed Dec 24 2025   
      
   Valid 12Z Wed Dec 24 2025 - 12Z Sat Dec 27 2025   
      
   *** Strong Atmospheric River followed by low pressure systems to   
    impact California and the Sierra Nevada through Friday ***   
      
      
   ...Maine...   
   Day 1... Maximum impact level from WSSI: Moderate   
      
   Heavy snow is expected to continue from overnight along a narrow=20   
   section of the Midcoast and interior sections of Maine for the=20   
   first part of Day 1 as a surface (Norlun) trough remains in place.=20   
   Over 6 inches of snow had already been reported within this band=20   
   over the coastal plains northeast of Portland as of 05Z. Heavy=20   
   snowfall with rates around 1-2"/hr are expected to continue through   
   at least 15Z as this mesoscale areas of low pressure form and=20   
   pivot within this surface trough. Once this area of low pressure=20   
   begins pivoting southward, snowfall should quickly ease in=20   
   intensity. WPC probabilities for an additional 4 inches of snow=20   
   after 12Z today are 20-40% from Portland east along the Midcoast,   
   but not after upwards of a foot of storm total snowfall occurs   
   across the region.   
      
      
   ...Great Lakes to Northeast...   
   Days 2-3... Maximum impact level from WSSI: Moderate   
      
   An impressive plume of Pacific and subtropical moisture ejecting=20   
   out of the robust longwave trough off the West Coast will travel=20   
   around the northern periphery of the ridge axis over the south-   
   central U.S. and head for the Great Lakes Thursday night before   
   reaching the northern Mid-Atlantic/Northeast on Friday-Friday   
   night. Guidance is in agreement on strong 850-700mb WAA over the=20   
   north- central U.S. Thursday afternoon with a band of snow=20   
   developing along the 700mb front. This is likely to result in a=20   
   combination of snow and some embedded sleet/freezing rain across=20   
   northern ND and northern MN Thursday night, then across northern WI   
   and the U.P. of MI early Friday morning. The strong but transient=20   
   850-700mb WAA will cause a stout warm-nose of >0C temperatures over   
   MI that cause a wintry mix to engulf much of Michigan's Mitten on=20   
   Friday, with the same icy potential just east of Lakes Erie and=20   
   Ontario Friday afternoon. The progressive nature of this=20   
   disturbance will help to cap snowfall totals to generally <4", with   
   the lone exceptions being the MN Arrowhead and northern half of=20   
   MI's U.P. where WPC probabilities show moderate chances (40-70%)=20   
   for snowfall totals >4". Otherwise, ice is of concern across=20   
   central and southern MI, where WPC probabilities show moderate-to-=20   
   high chances (50-80%) for ice accumulations of at least one-tenth=20   
   of an inch in these areas on Friday, including the greater Detroit=20   
   metro area. There are also up to 40% chances for a more impactful=20   
   event with over one- quarter of an inch.   
      
   In wake of a fast moving clipper system over northern New England   
   on Christmas Day, strong 850mb CAA ahead of a strengthening dome of   
   Canadian high pressure will result in a cold-air-damming signature   
   (CAD) to become positioned briefly over the Northeast by Friday   
   morning. To the west, a progressive 500mb shortwave trough over the   
   Great Lakes will approach and strong 850mb WAA will accompany   
   sufficient 290K isentropic ascent to produce precipitation over the   
   Northeast. Healthy mid-to-upper level ascent will also be supplied   
   in the form of healthy 700mb Q-vector convergence Friday afternoon   
   and evening. This classic "over-running" setup with a burgeoning   
   warm nose of >0C air at low-levels but sub-freezing wet-bulb   
   temperatures at the surface (due to a cold and very dry boundary   
   layer at the onset) will allow for an icy wintry mix of   
   sleet/freezing rain to fall from western PA and northern MD on east   
   across southern PA and towards the Delaware Valley. Given the warm   
   nose elevated in model cross sections to around 800-750mb, a deep   
   cold layer below it could mean precipitation falling mostly as   
   sleet across the northern mid-Atlantic (northern MD,   
   southern/central PA, and into southern/central NJ). This may cut   
   down on snowfall and freezing rain accumulations here, but still   
   could lead to slippery travel. Meanwhile, areas farther north of=20   
   the storm track and areas with more elevation; such as northern PA,   
   the Poconos, NY Finger lakes, Catskills, northern NJ, and=20   
   potentially the NYC metro on north and east through southern New=20   
   England, are showing better odds of remaining mostly snow for=20   
   Friday afternoon and into Friday night.   
      
   Through 12Z Saturday, WPC probabilities show moderate chances=20   
   (40-70%) for snowfall totals >4" in northeast PA, southern NY,=20   
   northern NJ, western MA, and western CT. Greatest chances for at   
   least 6" of snowfall (30-50%) are across the Poconos and Catskills   
   of northeast PA and NY. Meanwhile, WPC ice probabilities show=20   
   moderate chances (50-70%) for ice accumulations over a quarter of=20   
   an inch along the Laurel Highlands and central Appalachians. This=20   
   amount of ice would have the potential to produce scattered tree=20   
   damage and power outages. Light ice accumulations and some=20   
   accumulating sleet is expected as far east as northern MD, southern   
   PA, and the DC/Baltimore metro areas. The Philadelphia metro could   
   see more snow than sleet/freezing rain, but some wintry mix=20   
   combination can be expected with high uncertainty on amounts at the   
   moment. There remains ongoing fluctuations in the forecast track=20   
   which could result in additional changes in which areas see the=20   
   most snow and ice. Regardless, WPC's WSSI-P show >50% odds for=20   
   Minor Impacts across a large region from western PA to as far east=20   
   as the Tri- State area between 1AM Fri - 1AM Sat. Residents and=20   
   travelers following Christmas will want to keep a close eye on this   
   forecast over the next couple of days.   
      
      
   ...California/Sierra Nevada...=20   
   Days 1-3... Maximum impact level from WSSI: Extreme   
      
   The next strong and quick-hitting Atmospheric River (AR) is=20   
   entering CA early this morning, with heavy precipitation ongoing=20   
   across the central/southern Sierra Nevada by 12Z with 3-4"/hr rates   
   (per the 00Z HREF) above the snow levels of 7500ft/8500ft through=20   
   the heaviest precip midday today. Snow becomes more showery with=20   
   moderate rates and lower snow levels of 6500/7500ft Wednesday=20   
   evening/night in the continued southerly flow behind the main AR=20   
   plume. The next moisture plume arrives early Thursday morning with=20   
   snow levels around 6000ft and heavy snow rates of 2-3"/hr into the=20   
   afternoon before rates decreased a bit in showery conditions and=20   
   snow levels dipping below 5000ft through Friday morning.   
   Precipitation is expected to finally wane by Friday night into   
   early Saturday morning as upper-level ridging builds into the   
   eastern Pacific and the meandering upper low over CA weakens into a   
   positively-tilted trough extending across the Great Basin.   
      
   These rounds of heavy snow with lowering snow levels through Friday   
   will lead to increasing travel and infrastructure impacts. Storm=20   
   total snowfall in excess of 4ft can be expected above 6500ft along=20   
   the extent of the Sierra Nevada.   
      
   Far northern CA ranges (Shasta Siskiyou/Klamath/etc) can expect   
   heavy snow above 5000ft this afternoon through Thursday. Moderate=20   
   snow above 4000ft can be expected Thursday night through Friday as=20   
   low pressure lingers near Cape Mendocino. Some snow impacts to I-5=20   
   near Mt Shasta are possible.   
      
   Snow levels remain around 10,000ft in Southern CA through the main   
   AR plume Wednesday before dropping to around 7000ft Wednesday night   
   into Friday. Impactful snow looks to remain higher than Big Bear   
   Lake.=20   
      
      
   Key Messages for the CA atmospheric river and subsequent low that=20   
   maintains impacts through Friday are linked below.   
      
      
   ...Pacific Northwest, Great Basin and Rockies...=20   
   Days 1-3... Maximum impact level from WSSI: Moderate to Major   
      
   Area of low pressure riding up the Pacific Northwest coastline   
   today will bring a surge of moisture and increasing snow levels.=20   
   Snow levels rise on the WA Cascades to 5000ft by 12Z this morning   
   and quickly fall back to around 3500ft by tonight, but also as   
   moderate precipitation ends. Moderate snow rates around 0.5"/hr=20   
   can still be expected through this morning above those snow levels   
   on the WA Cascades.   
      
   The aforementioned strong AR surging through CA and will also=20   
   spill into the Great Basin today, reaching the western Rockies=20   
   tonight. Snow levels are high in this moisture shield, 8000-9000ft=20   
   over the Great Basin and the Rockies through central ID and WY with   
   much lower precip and snow levels of 6000-7000ft in northern ID=20   
   and MT. Day 1 snow probs for >6" are 40-80% in the highest central=20   
   NV ranges, the Sawtooths, Wind Rivers, and Uintas.=20   
      
   The following moisture surge through CA Thursday brings widespread   
   precip at lower rates than before across the Great Basin and=20   
   western Rockies with lower snow levels around 6000-7000ft. Day 2=20   
   snow probs for >6" are 30-70% again for the highest central NV=20   
   ranges, the Sawtooths, Wind Rivers, Uintas, as well as western CO=20   
   ranges and the Absarokas.   
      
   The upper trough comes ashore Friday in northern CA up through the   
   PacNW with moderate precip rates and Cascades snow levels around=20   
   2500ft in WA and 3500ft in OR. Day 3 snow probs for >6" are 40-70%   
   in the WA Cascades, including Snoqualmie Pass, and around 30% for   
   the OR Cascades including Santiam Pass. Snow levels also fall   
   across much of the Great Basin and Rockies, with levels below   
   3000-4000ft in the northern Rockies and 5000-7000ft in the central   
   Great Basin and remaining around 7000ft in the central Rockies.   
   This allows for more widespread moderate snowfall from the ranges   
   of northern ID and northwest MT to northern UT, where WPC   
   probabilities on day 3 for >6" are 50-80% above the aforementioned   
   snow levels.   
      
      
   ...North-Central Montana...   
   Days 1-1.5... Maximum impact level from WSSI: Minor   
      
   Freezing rain is likely and may exceed 0.1" across parts of north-   
   central MT the night of Christmas Eve into Christmas morning as the   
   strong moisture plume associated with today's CA AR attempts to   
   spill into the northern High Plains. IVT values across the interior   
   West above the 90th climatological percentile along with the   
   favorable left-exit region of a quickly traversing 130kt jet streak   
   will work to squeeze a few tenths of an inch of QPF (at most)   
   across a region with cold low-level air being reinforced by strong   
   high pressure to the northeast. WPC day 1.5 probabilities for at=20   
   least 0.1" of freezing rain are currently 20-50% across north-   
   central MT. This amount of freezing rain may not be enough for   
   impacts to infrastructure, but could lead to slippery holiday   
   travel.   
      
      
      
   Snell   
      
      
      
      
   ...Winter Storm/Atmospheric River Key Messages are in effect.=20   
   Please see current Key Messages below...   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_1.png__;!!DZ3fjg!-qtGErMs0aLaJ73UGP_o77Cl93lnVgZl3k0GZBYwfbYQ8=   
   qonqKfKWUckd8fr9smCsFtAVUCrx9eG-X6WZR3axB_5YwI$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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