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   FOUS11 KWBC 190739   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   239 AM EST Fri Dec 19 2025   
      
   Valid 12Z Fri Dec 19 2025 - 12Z Mon Dec 22 2025   
      
      
   ...Great Lakes & Northern New England...   
   Days 1-2...   
      
   Negatively tilted upper trough and associated strong cold front is   
   expected to swing across the Great Lakes and East Coast today   
   producing a rapid drop in temperatures and gusty winds. This setup   
   will also provide the opportunity for heavy lake effect and upslope   
   snow. 850 mb temperatures as low as -15C to -20C are expected   
   behind the strong cold front as lake temperatures remain largely   
   around 5C. This temperature delta and brief surge of west-   
   northwesterly flow will allow for heavy lake effect snow in the   
   typical snowbelts downwind of Lake Superior and Michigan, with   
   additional heavier snow possible downwind of Lake Erie and Lake   
   Ontario given the greater westerly component to the low-level   
   winds. The highest WPC probabilities for at least 6" of snow   
   through Saturday morning is across the Tug Hill Plateau downwind of   
   Lake Ontario, where chances sit at 70-90%. Additionally, some   
   light to moderate upslope post-frontal snowfall is possible along   
   the central Appalachians and northern New England elevated west-   
   northwest facing terrain. Here, WPC probabilities for at least 4   
   inches of snowfall through 18z Saturday range between 30-60%.   
      
      
   ...Cascades, Northern Rockies, & Northern California...   
   Days 1-3...   
      
   An anchored -WPO/-PNA teleconnection pattern is the culprit in the   
   endless barrage of atmospheric rivers and moist Pacific flow into   
   the western U.S. This will feed copious amounts of snow throughout   
   the mountains of the Pacific Northwest and stream as far east as the   
   Northern Rockies. The strongest IVT signal shifts southward over   
   the next few days and focuses over southern Oregon and especially   
   northern California this weekend. It is within this moisture plume   
   where the highest snow levels above 8000-9000ft reside, with much   
   lower snow levels to the north across the Pacific Northwest on Day   
   1 around 1000-2000ft. These lower snow levels also overspread the   
   northern Rockies by the end of Day 1 after starting in the   
   5000-6000ft range within the core of the ongoing Atmospheric River.   
   For the Tetons, Absaroka, and Wind River Ranges, this region will   
   reside closer to stronger 700mb jet and anomalous PWATs (>99th   
   ECMWF climatological percentiles through Friday afternoon), giving   
   these ranges both an abundance of moisture and strong upslope   
   enhancement to keep heavy snow in the forecast through Saturday   
   morning for elevation at and above 7000ft. For northern CA, snow   
   levels dip below 5,000ft in the Siskiyou and Shasta, but as   
   precipitation also wanes. These levels rise once again above   
   6000-7000ft with the next AR on Sunday.   
      
   The lone exception to the higher snow levels will be ongoing   
   westerly upslope flow into the WA Cascades as low pressure lingers   
   off the coast of British Columbia. Snow levels are also expected to   
   remain quite low and around 1,500ft on Saturday and 2,500 ft on   
   Sunday. The bulk of the heaviest snowfall in the WA Cascades and   
   Olympics is expected above 2500ft. WPC 72-hour probabilities show   
   high chances probabilities (>70%) for snowfall totals >18" between   
   3,000- 4000ft, while similar high chance probabilities exist above   
   4,000ft for snowfall totals >30". In the Rockies, high chance   
   probabilities (>70%) for >12" of snow on Day 1 are present above   
   5,000ft in the Blue Mountains, Bitterroots, and the Lewis Range,   
   while those same probabilities for >18" are present above 6,000ft   
   in the Sawtooth. Snowfall in the Tetons and Wind River Ranges are   
   likely to range between 1-3 feet with some localized amounts   
   approaching 4 feet in the more remote reaches of these ranges   
   including the next round on Sunday. The WSSI is keying in on   
   Extreme Impact potential (extremely dangerous travel, damages to   
   infrastructure in the Wind River, Teton, and Absaroka ranges   
   through Friday night, largely in the higher reaches of these   
   ranges. Still, Major Impacts (significant travel disruptions) at   
   intermediate elevations of these ranges are likely with heavy snow   
   and whiteout conditions contributing to the impacts in these ranges   
   to close out the week.   
      
      
   ...Sierra Nevada...   
   Days 2-3...   
      
   Precipitation along the southern periphery of the AR plume begins   
   to sink southward towards the central Sierra Nevada by the end of   
   Day 1, but with snow levels near 9000ft. Multiple AR impulses   
   through early Monday are expected with snow levels potentially   
   dropping down to around 8000ft as these moisture plumes are aimed   
   at the northern/central Sierra Nevada. WPC probabilities for at   
   least 12 inches of snowfall on days 2-3 (ending 12z Monday) are   
   between 50-70% and primarily above 9000ft elevation. This is   
   possibly the start of a very active and impactful weather pattern   
   across California, with additional high elevation heavy snow likely   
   through the middle to end of next week.   
      
      
   The probability of significant icing is less than 10 percent.   
      
      
   Snell   
      
      
   $$   
      
   = = =   
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