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   FOUS11 KWBC 130906   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   406 AM EST Sat Dec 13 2025   
      
   Valid 12Z Sat Dec 13 2025 - 12Z Tue Dec 16 2025   
      
      
   ...North-Central Plains, Midwest to the Central Appalachians...=20   
   Day 1...   
      
   Low to mid-level low is over north shore of Lake Superior with a   
   reinforcing shortwave trough over far northeast MT. This wave is   
   riding a powerful 140kt jet that extends across the Midwest and   
   through the Mid- Atlantic. Intensifying fgen (both through WAA and   
   the result of the upper jet streak position) will be sufficient to   
   drive omega through the snow growth zone through this evening.=20   
   With strong lift occurring into a DGZ that is deepening in the cold   
   airmass, periods of heavy snow are expected, especially within a=20   
   narrow translating band from Iowa through Ohio and northern WV   
   today. Forcing is strong enough to warrant a risk for thundersnow=20   
   particularly midday/early afternoon over central IL/IN where 1"/hr   
   rates are possible per the 00Z HREF.=20   
   The bands rise over the Allegheny Plateau this evening where   
   orography once again brings a risk for 1"/hr rates. Day 1 PWPF for   
   >6" are around 10% in central IL/IN and quickly increase from 40%   
   to 70% across far southeast OH through north-central WV. Snow rates   
   drop off Sunday morning over the central Appalachians, but the   
   upper trough does cross midday, so continued upslope snow showers   
   can be expected at least through the day light hours.   
      
      
   ...Mid-Atlantic through Southern New England...=20   
   Days 1/2...   
      
   Upper low stalls over northern Lake Superior today before ejecting   
   southeast tonight with the aid of a reinforcing shortwave. The   
   resultant upper trough axis reaches the Northeastern Seaboard   
   Sunday evening with downstream surface trough then low development   
   off the Mid-Atlantic coast Sunday morning. Right entrance dynamics   
   aid fgen banding to develop in the lee of the central Apps later   
   this evening, most likely over north-central MD east through NJ   
   before the surface trough/low offshore provides additional forcing   
   for more intense banding overnight to shift south and east. This=20   
   system is fairly progressive, but the frontal forcing in   
   increasingly cold air allows 0.75"/hr rates in the Mid-Atlantic,   
   extending to southern New England early Sunday. Marginal thermals=20   
   at the southern extend of the precip swath to begin with quickly=20   
   shift to all snow - this is aided the nighttime occurrence.=20   
      
   This event should last no more than 12 hours in any given location,   
   and the exact placement, timing, and intensity of the jet streak   
   and offshore trough will determine where the heavier bands setup   
   and pivot. Day 1.5 PWPF for >6" are over 10% from just north of DC   
   through Cape Co with a peak of around 30% for greater Philly.=20   
      
      
   ...Great Lakes...=20   
   Days 1-3...   
      
   Aforementioned low pressure system stalling over Lake Superior   
   today ejects southeast tonight with a reinforcing shortwave trough   
   passage on Monday. Subterranean DGZ under the low today makes for   
   coarse/low SLR snow, but some warming this evening brings back the   
   dendrites and 20:1 SLRs to the U.P.   
      
   Day 1 PWPF for >6" are 40-80% over NWly flow snow belts in the=20   
   eastern U.P. and east of Grand Traverse Bay. Wly flow snow belts   
   east of Lakes Erie/Ontario result in Day 1 PWPF >6" of 30-50% east   
   of Cleveland to the Chautauqua Ridge and over 80% for the Tug Hill   
   where single banding LES sets up this evening. Lull expected   
   for Sunday night as low level ridging shifts east of the Lakes. But   
   then shortwave crosses Monday with a decent, but transient swath of   
   lake enhanced snow. Light to locally moderate synoptically driven   
   snow across the Great Lakes region Monday with more single banding   
   in westerly flow over Lake Ontario into the Tug Hill where Day 3   
   PWPF for >6" is over 80%.=20   
      
      
   ...Pacific Northwest...   
   Days 2/3...   
      
   Potent atmospheric river arrives Monday morning to western WA/OR   
   with snow levels skyrocketing up to 9000ft over the Cascades. A   
   shortwave trough passage over WA late Monday brings snow levels on   
   the North Cascades down to 5000ft, reaching 4000ft on Tuesday.   
   Lower precip rates are expected behind the trough passage, but   
   moderate snow brings Day 3 PWPF for >6" in the 40-60% range in the   
   high northern WA Cascades.=20   
      
      
   The probability of significant icing is less than 10 percent.=20   
      
   Jackson   
      
      
      
   ...Extreme Cold 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!_jPxnzcgK9JB0jlxCEYkNgPmBqDB9AV_xArz815tBmqPn=   
   UJySnOPdk6IfueYnBtMZQh3ZGrHh9pNlUcp8ywHHzAOYSE$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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