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   FOUS11 KWBC 120908   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   408 AM EST Fri Dec 12 2025   
      
   Valid 12Z Fri Dec 12 2025 - 12Z Mon Dec 15 2025   
      
      
   ...Montana...   
   Day 1...   
      
   NWly jet downstream of an amplifying ridge axis off the PacNW coast   
   continues to intensify through this morning with 250mb winds   
   increasing above 150kt over south-central Alberta. Pacific moisture   
   continues to shift inland south of this jet which meets   
   particularly cold air from the Canadian Prairies that is banking=20   
   up against the eastern slopes of the MT Rockies. The amplifying=20   
   ridge maintains the jet dynamics with banding over central MT now=20   
   expected to drift north to northern MT through this evening before=20   
   ejecting southeast over the north-central Plains late=20   
   tonight/Saturday morning.=20   
      
   Ongoing freezing rain along the southern periphery of the banding   
   in valleys in the eastern slopes will continue this morning with   
   only a narrow strip of wintry mix shifting north with the southern   
   extension rest of today. Otherwise this is fluffy with consistent   
   0.5"/hr rates across the MT Plains through this evening (per 00Z   
   HREF) with SLR in the 15 to 18:1 range. Day 1 PWPF for >6" snow is   
   over 40% across much of north-central to eastern MT, including=20   
   around Havre where probabilities are above 70%.=20   
      
      
   ...Northern Plains through the Midwest to the central=20   
   Appalachians... Days 1-2...   
      
   A reinforcing trough over the Gulf of Alaska early this morning   
   rides an increasing NWly jet today, reaching southern Saskatchewan   
   tonight. This trough is the catalyst for allowing ongoing snow   
   banding over MT to eject southeast across the Dakotas late tonight   
   and the Midwest Saturday. Strong jet dynamics and ample cold air   
   with Arctic origin allows for decent snow rates through this swath,   
   consistently over 0.5"/hr per the 00Z HREF. Day 1.5 PWPF for >4"   
   are lower over South Dakota where the bands are fairly progressive,   
   generally 30%, but higher over Iowa through north-central IL and   
   central IN where the band pivots east with values of 40-60%.   
   Probabilities continue to ramp up to the east over Ohio later   
   Saturday before orographic lift brings increasing probabilities=20   
   for >6" to northern WV, peaking for areas just west of the   
   Allegheny Front where Day 2 PWPF for >6" are 40-70%.=20   
      
   This swath is forecast to be a bit north of the current snow   
   tracking east from southern IN and eastern KY. These two   
   clippers will produce decent swaths of snow and combine to cover   
   large areas of the Midwest with snow through Saturday.=20   
      
      
   ...Mid-Atlantic through the coastal Northeast...=20   
   Days 2-3...   
      
   The Saturday shortwave trough/clipper amplifies an upper low over   
   Lake Superior Saturday allowing it to further develop and dig   
   south over the eastern Great Lakes Saturday night. The surface   
   component of this low develops as a trough off the Mid-Atlantic   
   coast to the Outer Banks by 12Z Sunday. Right entrance jet dynamics   
   and this developing surface trough allow moderate to locally heavy   
   banding to develop over the central/northern Mid-Atlantic through   
   southern New England Saturday night into Sunday. Placement of   
   banding with a key area of 0.25" QPF remains a bit uncertain at   
   this time, but areas from Baltimore through New Jersey and=20   
   possible Long Island are of most concern now. Day 2.5 PWPF for >4"=20   
   are 30-40% around Philly and around 20% over southern Connecticut=20   
   through southeast Mass. Confidence has increased on the snow=20   
   advancing and reaching the NYC metro late Saturday evening.=20   
      
   Rapid low pressure development is expected off the Northeast coast   
   Sunday which will maintain offshore flow and should allow ocean/bay   
   enhanced snow for the Cape and Islands through Sunday night. Day 3   
   PWPF for >4" are around 30% for Cape Cod.   
      
      
   ...Great Lakes...=20   
   Days 1-3...   
      
   Persistent troughing across the east will maintain cold conditions   
   as a storm brings another round of lake enhanced and effect snow=20   
   across the Great Lakes tonight through Sunday. Low pressure=20   
   currently over Manitoba dives southeast to Lake Superior tonight=20   
   before amplifying by an aforementioned shortwave trough. Westerly=20   
   flow ahead of this low brings synoptic snow with lake enhancements=20   
   for the U.P. and tip of the L.P. mitt where Day 1.5 PWPF for >6"=20   
   are 30-50%.=20   
      
   This westerly flow then crosses the eastern Great Lakes early   
   Saturday bringing renewed cold air advection and transient single-   
   banding to snow belts east of Cleveland through the Chautauqua   
   Ridge and across the Tug Hill where Day 2 PWPF for >6" are 40-70%.   
   LES then continues on NWly flow behind the low on Sunday with Day 3   
   PWPF for >4" 40-80% in northwest PA and around Syracuse.=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!47yZ9O3RTM9AE6Rtm8R9BQEbLvVg3byGcA07p9m73MzIA=   
   1eUO2G1MfH3v-DCGZA6hRJFaFZPvnR-6NLsfgEM7HTLZE8$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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