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   FOUS11 KWBC 111951   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   251 PM EST Thu Dec 11 2025   
      
   Valid 00Z Fri Dec 12 2025 - 00Z Mon Dec 15 2025   
      
      
   ...Montana...   
   Days 1-2...   
      
   A second round of heavy snowfall (although with minimal break from   
   the first) will develop across MT, generally east of the   
   Continental Divide and expand into the High Plains. This will be   
   the result of overlapping ascent slowly shifting northeast with   
   time into a cold and anomalously moist environment.   
      
   The event begins to expand as a powerful (150+ kt) Pacific jet   
   streak begins to pivot northeast, while a strong cold front   
   continues to waver NW to SE and bank into the terrain. The overlap   
   of this jet and its accompanying IVT (still > 90% probability of   
   exceeding 250 kg/m/s spilling east of the terrain) will help expand   
   precipitation once again across the area. This will fall   
   exclusively as snow thanks to rapid column cooling behind the   
   front. The accompanying westerly flow aloft, while not ideal for   
   column moistening, will lead to at least modest isentropic ascent   
   atop the cold front, primarily along the 290K-295K surfaces,   
   helping to expand the snow swath. At the same time, persistent   
   700-600mb fgen will drift across the area, aided by the entrance   
   region to the upper jet, and driving ascent efficiently into the   
   deepening DGZ (30% probability from the SREF for DGZ depth   
   exceeding 100mb) suggest the potential for periods of heavy   
   snowfall rates exceeding 1"/hr (up to 70% chance Friday evening).   
      
   Temperatures will be quite cold, so SLRs are expected to be fluffy   
   and above-climo which should result in rapid accumulation of   
   snowfall. This will be most pronounced where any jet-enhanced   
   banding can occur, especially due to the prolonged and nearly-   
   stationary nature of the upper jet aloft. This is reflected by WPC   
   probabilities that support a moderate risk (50-90%) for at least 6   
   inches of snow across central MT just east of the terrain, with   
   locally as much as 12" possible (10-30%), and locally higher   
   amounts in the mountains.   
      
      
   ...Great Lakes into Northeast...   
   Days 1-3...   
      
   Persistent troughing across the east will keep cyclonic flow in   
   place across the Great Lakes and into the Northeast through the   
   weekend, while periodic shortwave impulses rotate southeast through   
   the flow. Each of these shortwaves will result in transient   
   enhanced ascent, with the most impressive height falls likely D3 as   
   the last in this series of shortwaves amplifies sharply late in the   
   forecast period.   
      
   The result of this evolution will be periods of light synoptic snow   
   associated with each shortwave, followed by more impressive lake   
   effect snow (LES) or lake-enhanced snow. While many areas will   
   likely experience at least light synoptic snow, any significant   
   accumulations are expected to be confined to lake effect snow areas   
   on the persistent W/NW flow, with enhancement occurring behind   
   each shortwave and the accompanying CAA. On D1, modest LES, which   
   has a WPC probability of 10-30% for exceeding 4" is confined to the   
   Keweenaw Peninsula of MI and just east of Lake Ontario.   
      
   However, during D2 and D3, LES becomes more widespread and   
   intense. With 850mb temps crashing to potentially less than -20C   
   over the lake temperatures that are still generally +2C to +8C will   
   create deep inversion depths and impressive lake-induced   
   instability to support heavy LES. Snowfall rates of at least 2"/hr   
   appear probable, especially across the U.P. and then east of Lakes   
   Erie/Ontario where upstream connection may additionally occur. WPC   
   probabilities D2 for 4+ inches of snow are above 50% across much of   
   the southern shore of Lake Superior in the U.P., as well as near   
   Buffalo, NY and into the Tug Hill Plateau. By D3, these   
   probabilities expand to include the NW and SW L.P. of MI as well as   
   the breadth of the Chautauqua Ridge. 2-day snowfall totals from the   
   LES (D2-D3) could 18" in some areas.   
      
   ...Northern Plains through the Ohio Valley...   
   Days 1-3...   
      
   Pinched cyclonic flow on the periphery of an amplifying trough over   
   southeast Canada will keep flow progressive from the Northern   
   Plains through the Ohio Valley, while dual potent shortwave troughs   
   rotate through the flow.   
      
   The first of these will be accompanied by a strung out lobe of   
   vorticity racing from South Dakota early D1 to the Mid-Atlantic   
   Coast early D2. The speed of this system will somewhat limit the   
   potential for heavy precipitation. However, forcing for ascent will   
   maximize on the periphery of a potent upper jet streak digging   
   southeast coincident with a narrow corridor of impressive 290K   
   isentropic ascent and 925-700mb fgen. The most intense ascent is   
   expected from Iowa into Kentucky where a band of heavy snowfall is   
   likely to translate southeast on the edge of the strongest WAA.   
   This will create a stripe of moderate snowfall with briefly heavy   
   snow rates exceeding 1"/hr possible (30% chance). WPC probabilities   
   for this lead shortwave reach 30-50% for 4+ inches, with locally   
   6+" possible (10-30%) in IL/IN.   
      
   Immediately following this first wave will be a second shortwave   
   with more impressive amplitude, leading to a second streak of   
   moderate to heavy snowfall crossing almost the identical path.   
   Although there is considerable latitudinal spread among the   
   guidance, it is probable that the lead shortwave will push the   
   baroclinic gradient south to support a slight southward push of   
   heavy snow with the next wave, at least until amplification of the   
   mid-level pattern occurs Saturday evening which could result in the   
   heaviest axis again moving across central IN and then into the   
   lower Ohio Valley. Another stripe of moderate to heavy snow is   
   likely despite the transient nature of the event as the DGZ remains   
   elevated and 700-600mb fgen increases which crosses directly into   
   the DGZ to produce ample snow growth potential. Regional soundings   
   across this area also indicate a near-isothermal layer beneath the   
   DGZ, and while temperatures are cold, should still support fluffy   
   SLR with efficient accumulations. At this time, WPC probabilities   
   on D2 indicate a 30-50% chance of an additional 4+ inches in   
   central IL, reaching 30-70% D3 from IN through southern OH.   
      
   Although there will be a break between these two events, some areas   
   could receive double-digit snowfall by the time both of these   
   events are over, and with cold temperatures in place, this will   
   likely create hazardous travel for the region.   
      
      
   ...Central Appalachians and Mid-Atlantic...   
   Days 1-3...   
      
   The same impulses/clippers bringing snow to the Plains/Midwest will   
   track E/SE into the Central Appalachians and Mid-Atlantic states.   
   The first of these will be exiting the coast Friday morning, and   
   while snow will generally be modest, the addition of upslope flow   
   on W/NW winds into the Central Appalachians will result in moderate   
   accumulations across the terrain of WV. Here, WPC probabilities D1   
   into D2 support a low risk (10-30+%) of at least 4 inches of   
   snowfall.   
      
   The more significant event is with the secondary impulse which will   
   be intensifying D3 as the mid-level pattern amplifies and swings   
   the primary trough axis into the Central Appalachians and Mid-   
   Atlantic Sunday morning. This will drive rapid height falls into   
   the area, while the sharpening trough axis will help arc the   
   downstream jet streak poleward to produce rapidly intensifying   
   upper diffluence within the RRQ. Westerly mid-level flow will cause   
   impressive upslope flow into the terrain of WV/MD/PA once again,   
   but in this cause the available moisture will be more significant,   
   which when added to the robust deep layer ascent results in WPC   
   probabilities that exceed 50% for 6+ inches from the Laurel   
   Highlands through central WV, with locally more than 10 inches   
   possible.   
      
   The more challenging aspect of this forecast is what happens across   
   the coastal Mid-Atlantic states, including the I-95 corridor. As   
   the upper jet intensifies and arcs, it will likely provide enough   
   ascent downstream of the sharpening trough to produce a secondary   
   axis of moderate to heavy snow in the vicinity of I-95. The speed   
   at which this trough deepens and the corresponding jet amplifies   
   will drive the position of the heavy snow axis, which remains quite   
   uncertain at this time. However, with the PV anomaly swinging east   
   and potentially pushing the 1.5 PVU surface down below 450mb, this   
   could be a briefly intense event downstream.   
      
   Evaluation of the recent cluster analysis indicates that the   
   primary EOF /uncertainty/ revolves around the speed of this trough   
   amplification, and there is quite a bit of spread among the various   
   clusters. In general, the EC members are bit slower and deeper,   
   while the GEFS members are a little faster and weaker. Somewhere in   
   the middle is probably reasonable at this time, which results in a   
   stripe of heavy snow that will develop within increasing 850-700mb   
   fgen somewhere near I-95 from Washington, D.C. through NYC and   
   into Cape Cod, MA. There is still a lot of spread, but where this   
   band develops, which is currently expected near I-95, WPC   
   probabilities are moderate (50-70%) for at least 2 inches of snow   
   between DC and NYC, with locally more than 4 inches possible.   
   Considerable adjustments to the location of this band are still   
   possible, so further refinements to the location and accompanying   
   probabilities can be expected in the next few forecast cycles.   
      
   Weiss   
      
      
   $$   
      
   = = =   
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