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   FOUS11 KWBC 242044   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   344 PM EST Wed Dec 24 2025   
      
   Valid 00Z Thu Dec 25 2025 - 00Z Sun Dec 28 2025   
      
   *** Continued Heavy Snow on the Sierra Nevada through Friday ***   
      
      
   ...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 traverse the   
   northern periphery of the ridge axis over the south-central U.S.=20   
   and head for the Great Lakes Thursday night before reaching the=20   
   northern Mid-Atlantic/Northeast Friday night. Guidance is in=20   
   agreement on strong 850-700mb WAA over the north-central U.S.=20   
   Thursday afternoon with a band of snow developing along the 700mb=20   
   front. This will result in a combination of snow and some embedded=20   
   sleet/freezing rain across northern ND and northern MN Thursday=20   
   night, then across northern WI and the U.P. of MI early Friday=20   
   morning. The strong but transient 850-700mb WAA will cause a stout=20   
   warm-nose of >0C temperatures over MI that cause a wintry mix to=20   
   engulf much of Michigan's Mitten on Friday, with the same icy=20   
   potential just east of Lakes Erie and Ontario Friday afternoon. The   
   progressive nature of this disturbance will help to cap snowfall=20   
   totals to generally <4", with the lone exceptions being the MN=20   
   Arrowhead and northern half of MI's U.P. where WPC probabilities=20   
   show moderate chances (40-70%) for snowfall totals >4". Otherwise,=20   
   ice is of concern across central and southern MI, where WPC=20   
   probabilities show moderate-to- high chances (50-80%) for ice=20   
   accumulations of at least one-tenth of an inch in these areas on=20   
   Friday, including the greater Detroit metro area. There are also up   
   to 40% chances for a more impactful 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) that becomes positioned briefly over the Northeast by Friday.   
   To the west, the same progressive 500mb shortwave trough over the=20   
   Great Lakes will approach and strong 850-700mb WAA will accompany=20   
   sufficient 290K isentropic ascent to produce precipitation over the   
   Northeast and Mid-Atlantic. Healthy mid-to-upper level ascent will   
   also be supplied in the form of healthy 700mb Q-vector convergence   
   Friday afternoon and evening. This strong mid-level convergence is   
   further bolstered by the region being located beneath the diffluent   
   left-exit region of a 110kt 300mb jet streak. Guidance is keying=20   
   in on a narrow area of strong 700mb FGEN and vertical velocities=20   
   directly beneath the 300mb jet streak's left-exit region over=20   
   northeast PA, the Southern Tier of NY, the Lower Hudson Valley,=20   
   Poconos and Catskills, and even as far south as the NYC metro area=20   
   Friday evening. It is here where snow is likely to remain the=20   
   primary precipitation type with potential for hourly snowfall rates   
   >1"/hr.   
      
   Farther south, this classic "over-running" setup with a burgeoning   
   warm nose of >0C air at low-levels but sub-freezing wet-bulb=20   
   temperatures at the surface (due to a cold and very dry boundary=20   
   layer at the onset) will allow for an icy wintry mix of=20   
   sleet/freezing rain to fall from western PA and northern MD on east   
   across southern PA and towards the Delaware Valley. Model cross   
   sections and soundings continue to show a warm nose elevated=20   
   around 800-750mb, meaning as precipitation falls into a deep cold=20   
   layer between the surface and 850mb would force sleet to fall across   
   the northern mid-Atlantic (northern MD, southern/central PA, and=20   
   into southern/central NJ). This would cut down on snowfall and=20   
   freezing rain accumulations here, but still could lead to slippery=20   
   travel conditions Friday night and Saturday morning given some   
   guidance members show the potential for as much as 2" of sleet in=20   
   some locations where prolonged periods of sleet occur.   
      
   Through 12Z Saturday, WPC and NBM probabilities show moderate=20   
   chances (50-70%) for snowfall totals >6" in northeast PA, southern=20   
   NY, northern NJ, western MA, and western CT, and the NYC metro=20   
   area. Greatest chances for >8" of snowfall (around 30%) are across   
   the Poconos and Catskills of northeast PA and NY down into northern   
   NJ. On the flip side, WPC ice probabilities show moderate chances=20   
   (50-70%) for ice accumulations over a quarter of an inch along the=20   
   Laurel Highlands and central Appalachians. This amount of ice would   
   have the potential to produce scattered tree damage and power=20   
   outages. Light ice accumulations and accumulating sleet is=20   
   expected through southeast PA, the Balt-Wash metro areas and into   
   NJ/DE. The Philadelphia metro could see more snow than=20   
   sleet/freezing rain, but some wintry mix combination can be=20   
   expected that results in slick travel conditions through Saturday=20   
   morning. 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 EST Fri - 7AM EST Sat. Residents=20   
   and travelers following Christmas will want to keep a close eye on=20   
   this forecast over the next couple of days.   
      
      
   ...California/Sierra Nevada...=20   
   Days 1-3... Maximum impact level from WSSI: Extreme   
      
   Extreme snow rates currently on the High Sierra will diminish=20   
   later this afternoon as the AR plume shifts to eastern Nevada.=20   
   Snow becomes more showery with more moderately heavy rates and=20   
   lower snow levels of 6000/7000ft by 00Z in the continued southerly   
   flow behind the main AR plume. The next moisture plume arrives=20   
   early Thursday morning with snow levels of 55000/6500ft and heavy=20   
   snow rates of 2-3"/hr through midday before rates again decrease in   
   showery conditions through the afternoon. The last larger punch of   
   moisture arrives Thursday evening with persistent snow levels of=20   
   5500/6500ft through the night before dropping to 5000ft for Friday   
   with continued moderately heavy rates (1-2"/hr). Precipitation is=20   
   expected to finally wane by Friday night as upper-level ridging=20   
   builds into the eastern Pacific and the meandering upper low over=20   
   far northern CA weakens into a positively-tilted trough that shifts   
   east across the Great Basin.   
      
   The lower snow levels and rounds of heavy snow through Friday will   
   lead to increasing travel and infrastructure impacts. Additional   
   snowfall in excess of 3ft can be expected above 6000ft along the=20   
   extent of the Sierra Nevada.   
      
   Far northern CA ranges (Shasta Siskiyou/Klamath/etc) can expect   
   moderately heavy snow above 5000ft through tonight with snow levels   
   dropping to 4000ft on Thursday with the higher rates continuing   
   through Thursday night before easing through Friday as low=20   
   pressure lingering near Cape Mendocino finally shifts east. Some=20   
   snow impacts to I-5 near Mt Shasta are possible mainly early Friday.   
      
   Snow levels drop to around 7000ft tonight where they linger through=20   
   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   
      
   The aforementioned strong AR plume shifting east from the Sierra   
   Nevada this afternoon will continue to shift over the Great Basin=20   
   and reach the western Rockies by this evening before the intensity   
   greatly diminishes overnight. Snow levels are high in this=20   
   moisture shield, 8000-9000ft over the Great Basin and the Rockies=20   
   through central ID and WY with much lower precip rates and snow=20   
   levels of 6000-7000ft in northern ID and MT through tonight. Day 1=20   
   snow probs for >6" are 40-80% in the higher Sawtooths, Wind=20   
   Rivers, and Uintas, with 30-60% in the Absarokas, western CO   
   ranges, southern UT ranges, and highest central NV ranges.   
      
   The next moisture surge through CA early Thursday brings=20   
   widespread precip at lower rates than before across the Great Basin   
   and western and now northern Rockies with lower snow levels around   
   6000-7000ft Thursday night through Friday. Day 2 snow probs for=20   
   >6" are 40-80% again for the Sawtooths, Wind Rivers, Tetons, and=20   
   Absarokas with 30-60% for the central NV ranges, Uintas, and   
   northern ID/western MT ranges.   
      
   The upper trough comes ashore over the Pac NW Friday with moderate   
   precip rates and Cascades snow levels around 2500ft in WA and=20   
   3500ft in OR. Day 2.5 snow probs for >6" are now 50-90% in the WA=20   
   Cascades, including Snoqualmie Pass, and 40-70% for the OR=20   
   Cascades including Santiam Pass. Snow levels also fall across much=20   
   of the Great Basin and Rockies on Friday, with levels below=20   
   3000-4000ft in the northern Rockies and 5000-7000ft in the central=20   
   Great Basin and remaining around 7000ft in the central Rockies.=20   
   This allows for more widespread moderate snowfall from the ranges=20   
   of northern/central ID and northwest MT to northern UT and western   
   WY, where WPC probabilities on day 2.5 for >6" are 50-80% above=20   
   the aforementioned snow levels.   
      
   The inland pushing trough axis will bring decent snows across the   
   northern and central Rockies Friday night/Saturday.   
      
      
   ...Northern Montana...=20   
   Days 1-2... Maximum impact level from WSSI: Minor   
      
   Strong 850-700mb WAA and anomalous Pacific moisture overrunning a=20   
   sub-freezing boundary layer will provide a setup for freezing rain   
   tonight and into Christmas morning over northern MT though QPF is a   
   question. Surface temperatures will remain well below freezing=20   
   through 12Z Thursday while a burgeoning warm nose of >0C air=20   
   resides within the 850-750mb layer per CAMs soundings. While some=20   
   sleet may mix in at times, freezing rain will be the primary=20   
   concern through Christmas morning. Based on some soundings Thursday   
   afternoon, however, there is the potential for lingering freezing=20   
   fog across northeast MT and northwest ND. In fact, some model=20   
   members suggest prolonged moisture advection and sub-freezing temps   
   near the MT/ND and Canada border may keep freezing drizzle in the=20   
   forecast Christmas night and into Friday morning. WPC probabilities   
   show low chances (<30%) for ice accumulations over one-tenth of an   
   inch in north- central MT, but most accumulations will be less=20   
   than one- tenth across the northern tier of "Big Sky Country".=20   
   Expect potentially hazardous travel conditions across much of=20   
   north- central and northeast MT tonight and through Christmas=20   
   morning, as depicted by the Minor Impacts on the WSSI.   
      
      
   Mullinax/Jackson   
      
      
      
      
   ...Winter Storm Key Messages are in effect. Please see current=20   
    Key Messages below...   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_1.png__;!!DZ3fjg!_TGyp3Ju54N-UcQ8HVhlEwuc26EkDO2dwLP3MMwRwDfNA=   
   Wm2qQYg4ms8WStXcEBZSQHu0Z6O9E0XwoiDPybqQ-vgKIQ$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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