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   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   328 AM EST Thu Dec 18 2025   
      
   Valid 12Z Thu Dec 18 2025 - 12Z Sun Dec 21 2025   
      
      
   ...Blizzard conditions continue in the northern Plains today. See=20   
   Key Messages linked at the bottom of the discussion for more=20   
   information...   
      
   ...Northern Plains, Upper Midwest, Great Lakes...=20   
   Days 1-2...   
      
   By the start of the forecast period (12z Thurs.) a very strong and   
   mature mid-latitude cyclone is expected to be located across   
   northern Minnesota, with an estimated central pressure in the low   
   980s mb. This would place it within record low territory for the   
   region in mid-late December. The depth of this low pressure system   
   alone will continue to exhibit a tight pressure gradient on the   
   northern and western flanks, creating very strong winds (gusts up=20   
   to 70 mph) where snowfall is also the primary precipitation type   
   and some lingering snowpack still exists.   
      
   Strong theta-e advection from the Gulf combined with moist eastern   
   Pacific inflow will result in precipitable water anomalies above=20   
   the 97.5th percentile upstream of a developing TROWAL on the=20   
   northwest flank of the low center. With the right entrance region=20   
   of a strengthening jet streak situated over the Canadian prairie, a   
   heavy snow band is expected along the MN-Canadian border today. Some   
   isolated snow squalls are also possible underneath the mid-level   
   circulation as it dives southeast from the Dakotas into southern   
   MN tonight. The latest WPC probabilities for >6" on Day 1 highlight=20   
   20-50% chances across northern MN. Although the total snowfall from   
   this event may look modest for this region, strong wind gusts=20   
   upwards of 70 mph will lead to dangerous travel due to blowing snow   
   and potential blizzard impacts, even outside of the area receiving   
   the most snowfall.   
      
   As this system continues eastward tonight into Friday, gusty winds   
   and developing lake effect snow are the most likely winter-related=20   
   hazards. 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 heavy snow possible downwind of Lake Erie and Lake   
   Ontario. 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 40-70%.   
      
      
   ...Cascades and Northern Rockies...=20   
   Days 1-3...   
      
   An anchored -WPO/-PNA teleconnection pattern is the driver in the   
   seemingly endless barrage of atmospheric rivers that is delivering   
   copious amounts of snow to the mountains of the Pacific Northwest   
   and the Northern Rockies. The next surge of moisture is expected to   
   impact the Northwest during the day 1 period with a brief lull and   
   lowering snow levels on day 2 before the next round enters further   
   south across northern California on day 3. Meanwhile, moisture   
   continues to stream inland over the northern Rockies in order to   
   provide additional heavy mountain snowfall. Snow levels in the=20   
   core of this AR will rise to over 9000ft in Oregon this and 6000=20   
   to 7000ft in WA by this evening with a powerful cold front=20   
   shifting south over WA Thursday night crashing snow levels back   
   below 2000ft. Farther east, snow levels will rise to 6000t in the=20   
   Bitterroots by tonight and approach 7000ft in the Sawtooth. Before=20   
   the snow levels rise, however, strong 700mb FGEN will support=20   
   several hours of heavy snow at elevations as low as 6000ft in=20   
   central ID and 5000ft in the Blue Mountains through this evening.=20   
   Farther south, the Tetons, Absaroka, and Wind River Ranges will=20   
   reside closer to stronger 700mb jet and anomalous PWATs, giving=20   
   these ranges both an abundance of moisture and strong upslope=20   
   enhancement to keep heavy snow in the forecast through Saturday=20   
   morning for elevation at and above 7000ft.=20   
      
   Prolonged Pacific moisture flux continues to be directed at OR and   
   into the Northern Rockies on Friday with higher snow levels that   
   support heavy snow in the more remote reaches of the Cascades and   
   Northern Rockies. The lone exception will be ongoing westerly upslope   
   flow into the WA Cascades where snow levels as low as around 1500ft   
   are expected. The bulk of the heaviest snowfall in the WA Cascades   
   and Olympics is expected above 2500ft. WPC 72-hour probabilities=20   
   show high chances probabilities (>70%) for snowfall totals >24" at=20   
   elevations between 3000-4000ft, while similar high chance=20   
   probabilities exist above 4,000ft for snowfall totals >30". In the=20   
   Rockies, high chance probabilities (>70%) for >12" of snow on days   
   1 to 2 are present above 5,000ft in the Blue Mountains,=20   
   Bitterroots, and the Lewis Range, while those same probabilities=20   
   for >24" are present above 6,000ft in the Sawtooth. Snowfall in the   
   Tetons and Wind River Ranges are likely to range between 1-3 feet=20   
   with some localized amounts approaching 4 feet in the more remote=20   
   reaches of these ranges through Saturday. Strong winds are also   
   likely to be a concern across the Wyoming ranges, leading to   
   extreme impacts in the WSSI due to blowing snow.   
      
      
   The probability of significant icing is less than 10 percent.=20   
      
      
   Snell   
      
      
      
   ...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!-K8TNenTJXwRe3bckbpOSQRL-8xEEvGJg2vu5zjmjQ0HC=   
   tcZRTvD9tBTol3sa3LMKyTE9MwiP-TKn9hM0S6v-TfWe20$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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