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   FOUS11 KWBC 170923   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   423 AM EST Wed Dec 17 2025   
      
   Valid 12Z Wed Dec 17 2025 - 12Z Sat Dec 20 2025   
      
      
   ...Cascades and Northern Rockies...=20   
   Days 1-3...   
      
   Active weather pattern for the Pacific Northwest and Northern=20   
   Rockies continues into next week. As of 09Z, a mid level low is   
   apparent in satellite imagery over the northern BC coast with a   
   trough axis extending south to the OR Coast. Pacific moisture had   
   surged inland ahead of this trough and is reaching the northern   
   Rockies now per regional NEXRAD returns. Snow levels on the   
   Cascades are quickly crashing to around 2000ft in WA and 3000ft in   
   OR with moderate precip rates persisting behind this cold front   
   today bringing impactful snow below pass level. The lower snow=20   
   levels spread across the northern Rockies through midday. Precip   
   rates briefly drop to light tonight. Day 1 WPC snow probs for >8"=20   
   are 60-90% for the WA Cascades as well as the higher western WY=20   
   ranges, and more like 40-80% for the northern OR Cascades, much of   
   western MT ranges through central ID and the Uinta in UT and=20   
   northern CO ranges.=20   
      
   A focused and powerful atmospheric river (AR) surges into the=20   
   northern OR coast Thursday before shifting south down the coast=20   
   through far northern CA Thursday night and Friday. Snow levels in=20   
   the core of this AR will rise to over 9000ft in OR and 5000 to   
   6000ft in WA through Thursday with a powerful cold front shifting   
   south over WA Thursday night. Moisture and higher snow levels push   
   over the northern Rockies ahead of this cold front Thursday night=20   
   with levels generally 5000 to 7000ft over MD/ID/WY with the cold   
   front pushing south over this area Friday/Friday night. Day 2 WPC=20   
   snow probs for >8" are 60-90% over the WA Cascades again and 50-80%   
   over the western MT/central ID ranges, and 40-80% over the western   
   WY ranges. Day 3 snow probs depict the southern shift with 40-80%   
   chances for >8" on the WA and OR Cascades along with the   
   Bitterroots with 60-90% for the Sawtooth and western WY   
   ranges. For three day totals: A few feet are likely over much of=20   
   the northern Rockies and several feet over the higher Cascades.   
      
      
   ...Northern Plains, Upper Midwest, Great Lakes...=20   
   Days 1-3...   
      
   Powerful and deep low pressure develops in the lee of the southern   
   Canadian Rockies today before tracking over northern North Dakota   
   tonight and northern Minnesota Thursday morning then across Lake   
   Superior Thursday afternoon and Ontario/Quebec Thursday   
   night/Friday.=20   
      
   The potent shortwave emerging from the Pacific Northwest/British   
   Columbia this afternoon will be the driver of this developing=20   
   system as it tracks rapidly eastward embedded within pinched zonal=20   
   flow. This shortwave will amplify as it dives over the Northern=20   
   High Plains, closing off to manifest as 700-500mb height anomalies=20   
   falling below the 1st percentile according to NAEFS. As this low=20   
   tracks east it will interact with a powerful 160kt Pacific jet=20   
   streak diving across the Northern Rockies to create the intense=20   
   surface low pressure.=20   
      
   Downstream of this surface low, impressive theta-e advection from=20   
   the Gulf, with a modest TROWAL developing as moisture curls back=20   
   to the NW of the system and lifts isentropically. Beneath this=20   
   modest TROWAL, which will be well positioned into the left exit of   
   the strong jet streak, a band of heavy snow is possible as far west   
   as north-central MT, but likely for northeast ND through far=20   
   northern MN. The progressive nature of this system will limit the=20   
   potential for heavy snow, but intense ascent into a deep DGZ will=20   
   result in at least a short period of heavy snow within a possible=20   
   fgen-forced band. The greatest risk for >6" snow is in the Day 1.5   
   range along the northern MN border where probs are around 60% with   
   greater than 30% probs for >2" over central ND through north-=20   
   central MN and the western U.P. Although the total snowfall from=20   
   this event will be modest, increasingly strong winds, which will=20   
   likely eclipse 60 mph in gusts, will create dangerous travel due=20   
   to blowing snow and potential blizzard impacts.=20   
      
   Additionally, the leading edge of comma head could trigger snow   
   squalls from eastern MT through the Dakotas into MN tonight through   
   Thursday.   
      
      
   ...Great Lakes and Interior Northeast...   
   Days 2/3...   
      
   The mid level low tracks across Michigan Thursday night with   
   reinforcing troughs swinging the trough axis around it to a   
   negative tilt as it lifts over the Northeast Friday. In the wake=20   
   of this low, strong CAA crosses the Great Lakes to produce periods   
   of lake effect snow (LES) especially across the eastern U.P.,=20   
   northwest L.P., and east of Lakes Erie and Ontario. Inversion=20   
   depths are forecast to be generally modest (850mb) and plentiful=20   
   moisture will be somewhat transient. Still, efficient forcing into=20   
   the DGZ will likely produce at least short periods of heavy LES,=20   
   reflected by Day 2.5 WPC probabilities for >4" that are 30-50%=20   
   east of Erie/Ontario. The wrap around flow and CAA cross the   
   Interior Northeast late Friday with Friday night snows bringing   
   30-60% Day 3 probs for >4" to the Adirondacks, Greens and Whites.=20   
      
      
   The probability of significant icing is less than 10 percent.=20   
      
      
   Jackson   
      
      
      
   ...Winter Storm Key Messages are in effect and linked below...   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_1.png__;!!DZ3fjg!-wkywK3UnTwBsBGOP5kzkOpPMSOCFlGrg9M87ZLgN5Vlt=   
   6rWNw8ghAX9HduWamnpI4EpMeeT4XumTYQ_H2Ci64kK_q0$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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