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   FOUS11 KWBC 162056   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   356 PM EST Tue Dec 16 2025   
      
   Valid 00Z Wed Dec 17 2025 - 00Z Sat Dec 20 2025   
      
      
   ...Cascades and Northern Rockies...=20   
   Days 1-3...   
      
   An active period of winter weather persists across the Pacific   
   Northwest and Northern Rockies as pinched onshore flow pushes   
   considerable moisture onshore as an atmospheric river (AR). There   
   are some notable timing differences amongst the various global   
   ensembles as to when the most impressive IVT will move onshore, but   
   in general a core of elevated IVT (>80% chance to exceed 750   
   kg/m/s) will lift into WA/OR D2 ahead of a secondary front.   
      
   Before this occurs, a cold front will be moving across the Pacific   
   Northwest within another axis of elevated IVT D1, spreading   
   moisture from the coastal regions, across the Cascades, and as far   
   east as the Northern Rockies before 00Z Thursday. Impressive ascent   
   within the strong Pacific jet will combine with intensifying 700mb   
   fgen and periods of upslope flow to produce heavy precipitation   
   across the terrain. Snow levels during this time will rise ahead of   
   the cold front from around 4000 ft to 6000 ft, but then drop   
   rapidly in its wake to as low as 2000 ft. The heaviest snow is   
   likely ahead of the cold front so the most impactful snow will be   
   above pass levels, but impactful snow is likely at the Cascade   
   passes behind the cold front. WPC probabilities D1 for 6+ inches   
   are expansive across the Olympics, Cascades, Northern Rockies from   
   Glacier NP through the Salmon River/Sawtooth range, and around   
   Yellowstone NP, reaching above 70% across most of these areas with   
   snow levels varying from above 5000 ft east, to just 2000-3000 ft   
   west.   
      
   Moisture briefly decreases early D1 behind the first wave, but then   
   increasing rapidly again as the second and more impressive IVT   
   surge lift E/NE into the area. This will again be accompanied by a   
   frontal wave so snow levels will rise rapidly, reaching 6000-8000   
   ft within the strongest WAA/AR across OR/southern ID/southern   
   MT/WY, while remaining low at 2000-4000 ft farther north. While   
   snow levels will be lower north, the heaviest precipitation is   
   expected south/along the approaching front, with a heavy and wet   
   snow impacting much of the area before snow levels rise. This=20   
   could be impactful despite the rising snow levels due to the snow=20   
   load potential of a sub-climo SLR snow event within the AR, so=20   
   although the heaviest snow accumulations are likely above 7000 ft,=20   
   notable impacts are likely well below that elevation. WPC=20   
   probabilities D2 are high (>70%) for 6+ inches of snow in the=20   
   Cascades, Olympics, and Blues, expanding into much of the NW=20   
   terrain from the Cascades through the area around Yellowstone NP in   
   NW Wyoming D3. 3-day total snowfall of 3-4 feet is likely in the=20   
   higher terrain of much of the NW, with locally more than 6 feet in=20   
   the highest Cascades likely.   
      
      
   ...Northern Plains, Upper Midwest, Great Lakes...=20   
   Days 2-3...   
      
   A strong low pressure moving along or near the Borderlands of   
   ND/MN/Canada will bring impactful winter weather (as well as a   
   variety of other hazards) to the region through late week.   
      
   A potent shortwave emerging from the Pacific Northwest/British   
   Columbia Wednesday afternoon will be the driver of this developing   
   system as it tracks rapidly eastward embedded within pinched zonal   
   flow. This shortwave will amplify as it dives into the Northern   
   High Plains, closing off to manifest as 700-500mb height anomalies   
   falling below the 1st percentile according to NAEFS. As this low   
   tracks east it will interact with a powerful 160kt Pacific jet   
   streak diving across the Northern Rockies to create an intense   
   surface low pressure. While there continues to be some intensity   
   and latitudinal spread among the ensemble systems of this surface   
   low (ECENS north and strong, GEFS/CMCE farther south and a bit   
   weaker) the general trend is for a strong low moving along the   
   Canadian border from Wednesday aftn through Friday morning.=20   
      
   Downstream of this surface low, impressive theta-e advection will   
   surge northward from the Gulf, with a modest TROWAL potentially   
   developing as moisture curls back to the NW of the system and lifts   
   isentropically. Beneath this modest TROWAL, which will be well   
   positioned into the LFQ of the strong jet streak, a band of heavy   
   snow is possible from north-central MT through far northern MN. The   
   progressive nature of this system will limit the potential for   
   heavy snow, but intense ascent into a deep DGZ will result in at   
   least a short period of heavy snow within a possible fgen-forced   
   band. This will result in an axis of at least 2" of snow, but the   
   greatest potential for 4+ inches will be from far northern ND into   
   the Arrowhead of MN D2, and then across the U.P. of MI D3 where WPC   
   probabilities exceeding 50%.   
      
   Although the total snowfall from this event will be modest.   
   Increasingly strong winds, which will likely eclipse 50 mph in   
   gusts, will create dangerous travel due to blowing-snow or even   
   near-blizzard impacts. Additionally, the NAM SnSq parameter is=20   
   elevated along a cold front crossing MT and the Dakotas, producing   
   additional dangerous travel where any snow squalls develop late   
   Wednesday and Thursday.   
      
      
   ...Great Lakes...   
   Day 3...   
      
   Secondary low pressure developing across Wisconsin will rapidly   
   strengthen and lift progressively northeast through MI and into   
   Canada on Friday. The intensity of this low will result in an   
   impressive surge of WAA and precipitation downstream, but this will   
   all fall as rain in the warming column. In the wake of this low,   
   however, strong CAA leading to rapid column cooling will move   
   across the Great Lakes to produce periods of lake effect snow (LES)   
   especially across the eastern U.P., northwest L.P., and east of   
   Lakes Erie and Ontario. Inversion depths are forecast to be   
   generally modest (850mb) and plentiful moisture will be somewhat   
   transient. Still, efficient forcing into the DGZ will likely   
   produce at least short periods of heavy LES, reflected by WPC   
   probabilities that reach 30-50% for 4+ inches east of Erie/Ontario,   
   and 10-30% in the other favored N/NW snow belts.   
      
      
   The probability of significant icing is less than 10 percent.=20   
      
   Weiss   
      
      
   ...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!5zfyW7z50atNGCSNMWCRW7K3CpVFbHrJXRdkMBRxn1_d7=   
   VeGKD6e6yxZQFOZHxKnsIWNKrxXT1T8nfNkMGRS_-PpD1w$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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