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   FOUS11 KWBC 192022   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   322 PM EST Fri Dec 19 2025   
      
   Valid 00Z Sat Dec 20 2025 - 00Z Tue Dec 23 2025   
      
      
   ...Great Lakes & Northern New England...=20   
   Days 1-2...   
      
   Negatively-tilted upper trough and associated strong cold front=20   
   will exit New England this evening with a sharp drop in=20   
   temperatures behind it. 850 mb temperatures as low as -15C to -20C=20   
   are expected behind the strong cold front as lake temperatures=20   
   remain largely around 5C. This temperature delta and brief surge of   
   west- northwesterly flow will support moderate to heavy lake=20   
   effect snow in the typical snowbelts downwind of Lake Superior and=20   
   Michigan, with additional heavier snow possible downwind of Lake=20   
   Erie and especially Lake Ontario given the greater westerly=20   
   component to the low-level winds. The highest WPC probabilities for   
   at least 8" of snow through Saturday are across the Tug Hill=20   
   Plateau downwind of Lake Ontario (70-90%). Additionally, some light   
   to moderate upslope post-frontal snowfall is possible across=20   
   northern New England over elevated west-northwest-facing terrain.=20   
   Here, WPC probabilities for at least 4 inches of snowfall through=20   
   Saturday range between 40-70%.   
      
      
   *** Atmospheric River to continue in the West into and through next   
    week ***   
      
   ...Cascades, Northern Rockies, & Northern California...=20   
   Days 1-3...   
      
   An anchored -WPO/-PNA teleconnection pattern is the culprit in the   
   endless barrage of atmospheric rivers and moist Pacific flow into=20   
   the western U.S. This will feed copious amounts of snow throughout=20   
   the mountains of the Pacific Northwest and stream as far east as=20   
   the northern Rockies. The strongest IVT signal has shifted=20   
   southward today and will focus over northern California ENE across=20   
   the Great Basin to the WY ranges this weekend. It is within this=20   
   moisture plume where the highest snow levels above 8000-9000ft=20   
   reside, with much lower snow levels to the north in the colder air=20   
   mass across the Pacific Northwest (WA into OR) tonight around=20   
   1000-2000ft. These lower snow levels will continue to overspread=20   
   the northern and central Rockies tonight/Saturday after starting in   
   the 5000-7000ft range within the core of the ongoing Atmospheric=20   
   River. For the Tetons, Absaroka, and Wind River Ranges, this region   
   will reside closer to stronger 700mb jet and anomalous PWATs/IVT=20   
   (>98th climatological percentiles), giving these ranges both an=20   
   abundance of moisture and strong upslope enhancement to keep heavy=20   
   snow in the forecast through Saturday morning for elevation at and=20   
   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.   
      
   North of the AR, westerly upslope flow into the WA Cascades will=20   
   support much lower snow levels -- around 1,500ft on Saturday and=20   
   2,500 ft on Sunday. The bulk of the heaviest snowfall in the WA=20   
   Cascades and Olympics is expected above 2500ft, so this will=20   
   include nearly all the passes (e.g., Snoqualmie). WPC 72-hour=20   
   probabilities show high chances probabilities (>70%) for snowfall=20   
   totals >18" between 3000-4000ft, while similar high chance=20   
   probabilities exist above 4000ft for snowfall totals >30".   
      
   In the Rockies, snow will start to lessen tonight over the=20   
   Tetons/Wind River Range with additional light/modest accumulation=20   
   Day 1 of several inches, but perhaps more than a foot (>50%=20   
   probabilities) over the Uintas. For Days 2-3, additional snowfall=20   
   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=20   
   remote reaches of these ranges as another surge of moisture=20   
   associated with the long-lived AR moves through. Major Impacts=20   
   (significant travel disruptions) are expected. Some light snow is=20   
   also expected in the CO Rockies tonight into Saturday.   
      
   ...Sierra Nevada...   
   Days 1-3...   
      
   Precipitation along the southern periphery of the AR plume begins=20   
   to sink southward towards the central Sierra Nevada tonight, but=20   
   with snow levels near 9000ft. Multiple AR impulses through Monday=20   
   are expected with snow levels potentially dropping down to around=20   
   7500ft as these moisture plumes are aimed at the northern/central=20   
   Sierra Nevada. This may impact some of the passes across the crest=20   
   (e.g., Donner Pass at 7239ft) as snow levels dip/oscillate. The AR=20   
   event will continue beyond this forecast period into the medium=20   
   range. WPC probabilities for at least 12 inches of snowfall on days   
   1-3 (ending 00Z Tuesday) are between 50-70% and primarily above=20   
   9000ft elevation. Snow levels will trend a bit lower into Monday=20   
   and WPC probabilities for at least 4 inches of snow are >30% to=20   
   elevations around 7500ft. This is possibly the start of a very=20   
   active and impactful weather pattern across California, with=20   
   additional high elevation heavy snow likely through the middle to=20   
   end of next week. See our latest Key Messages that cover the event=20   
   through the end of next week, and CPC's Key Messages into week 2.   
      
      
   The probability of significant icing is less than 10 percent.=20   
      
      
   Fracasso/Snell   
      
      
   ...Atmospheric River Key Messages are in effect. Please see=20   
   the link below...   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_1.png__;!!DZ3fjg!8sYtn0Wp774cnXpWXeT7AtfkTrDIam3_gR8MIUE4ki1Bk=   
   zV6PsnZfiUuTi3j4YSONvYfbDAVkptvoOQj63yLjG5gmDs$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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