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   FOUS11 KWBC 132003   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   303 PM EST Sat Dec 13 2025   
      
   Valid 00Z Sun Dec 14 2025 - 00Z Wed Dec 17 2025   
      
      
   ...Upper Ohio Valley to Central Appalachians...   
   Days 1-2...   
      
   Moderate to heavy snow will be ongoing at the start of the forecast   
   period (00z Sun) across the Ohio Valley into the central   
   Appalachians with upslope snow continuing along the higher terrain   
   through Monday morning. A mid- level low associated with this=20   
   snowfall is over northeast shore of Lake Superior with a=20   
   reinforcing shortwave trough over the Upper Midwest. This wave is=20   
   riding a powerful 140kt jet that extends across the Midwest and=20   
   through the Mid- Atlantic. Intensifying 700 mb fgen (both through=20   
   WAA and the result of the upper jet streak position) will be=20   
   sufficient to drive omega through a wide (greater than 100 mb) snow   
   growth zone through this evening. With strong lift occurring into=20   
   a DGZ that is deepening in the cold airmass, periods of heavy snow=20   
   and high snow ratios (around 20:1) are expected, especially within=20   
   a narrow translating band from Ohio into northern WV today,=20   
   southwest PA and far western MD tonight.   
   =20   
   The bands rise over the Allegheny Plateau this evening where   
   orography brings a risk for 1"/hr rates. Following this system a=20   
   strong arctic cold front pushes southward and promotes lake=20   
   enhanced upslope flow into the high elevations of southwest PA,=20   
   western MD, and WV. Day 1 PWPF for >6" are around 40% to 70% across   
   north- central WV into western MD and southwest PA.   
      
      
   ...Mid-Atlantic through Southern New England...=20   
   Day 1...   
      
   The same system impacting the Ohio Valley shifts towards the Mid-   
   Atlantic and Northeast tonight as the upper low stalls over=20   
   northern Lake Superior before ejecting southeast with the aid of a   
   reinforcing shortwave. The resultant upper trough axis reaches the   
   Northeastern Seaboard Sunday evening with downstream surface=20   
   trough then low development off the Mid- Atlantic coast Sunday=20   
   morning. Strong right entrance dynamics aid fgen banding to=20   
   develop in the lee of the central Apps later this evening, most=20   
   likely over north-central MD east through NJ before the surface=20   
   trough/low offshore provides additional forcing for more intense=20   
   banding overnight and shifts the band south and east. This system=20   
   is fairly progressive, but the frontal forcing in increasingly cold   
   air allows 0.5-1.0"/hr rates in the Mid- Atlantic, extending to far=20   
   southern New England early Sunday. Marginal thermals at the=20   
   southern extend of the precip swath to begin with quickly shift to=20   
   all snow - this is aided by the nighttime occurrence. Snow ratios=20   
   could tip the scales to higher end amounts across parts of eastern=20   
   PA through Long Island and southern New England as omega intersects   
   with a wide DGZ as the precipitation shield pulls eastward,=20   
   supporting ratios closer to 15-20:1 and much higher than=20   
   climatology.   
      
   This event should last no more than 6-12 hours in any given=20   
   location, and the exact placement, timing, and intensity of the jet   
   streak and offshore trough is still somewhat uncertain. Today's=20   
   guidance trended a tick north and wetter due to a more amplified=20   
   upper trough and related right entrance region of a 150kt upper=20   
   jet. Day 1 PWPF for >6" are 30-50% from north-central MD through=20   
   southeast PA and central NJ. Probabilities for >6" are 20-40%=20   
   across Long Island and Cape Cod, including the islands off southern   
   New England. Given most of this snow is expected to occur=20   
   overnight and during a weekend, impacts may be limited, but strong=20   
   cold air advection following the event and very cold temperatures=20   
   could pose a danger for individuals caught unprepared outdoors. Be=20   
   sure to check conditions before traveling in this region tonight or   
   tomorrow morning.   
      
      
   ...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=20   
   coarse/low SLR snow, but some warming this evening brings back the=20   
   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 2.5   
   PWPF for >8" 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 Days 2-3 PWPF for >8" in the 60-90% range in=20   
   the high northern WA Cascades. Additional snow is expected for=20   
   this region after the short range forecast period ends by 00z=20   
   Wednesday.   
      
      
   The probability of significant icing is less than 10 percent.=20   
      
   Snell/Jackson   
      
      
      
      
   ...Extreme Cold Key Messages are in effect. Please see current Key   
   Messages below...   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_1.png__;!!DZ3fjg!-y-EFOck4oiKPgryVMGcuJlO-9X4V4gIhgBlgpoo6TrGq=   
   8zrlWJ3ICxvhtyTseLzEz-LhdzxdCQUXLelwpLyVjJSkVU$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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