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   FOUS11 KWBC 161905   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   205 PM EST Sun Nov 16 2025   
      
   Valid 00Z Mon Nov 17 2025 - 00Z Thu Nov 20 2025   
      
   ...Northeast/Great Lakes...   
   Days 1-2...   
      
   Amplifying closed mid-level low will continue its slow trek into   
   the Canadian Maritimes tonight through Monday, but wintry   
   precipitation will continue in earnest across portions of Northern   
   New England and Upstate New York.   
      
   While the primary warm conveyor will pivot well east and away from   
   the United States, a modest TROWAL will persist into northern Maine   
   bringing some pivoting/comma head snowfall with rates of 0.5-1"/hr   
   at times. This combined with increasing northerly winds will bring   
   moderate impacts to the north and east portions of the state,   
   especially the first half of the period, before the DGZ dries out   
   and precip wanes. WPC probabilities for additional snowfall   
   exceeding 4" in northern/eastern Maine are generally 30-50%.   
      
   The more significant snowfall is expected farther west into the   
   terrain of central ME and across NH, VT, and Upstate NY. This will   
   be in response to prolonged and impressive upslope flow as NW winds   
   develop and strengthen behind the departing low pressure system.   
   The guidance is in good agreement that unidirectional NW flow will   
   persist D1 and into D2, bringing periods of heavy snow as ascent is   
   forced into the deepening DGZ and moistening the lower half of the   
   resultant column. This should provide an environment favorable for   
   heavy snow into Tuesday, and with Froude numbers indicating   
   critical flow, the focus should be along the crests of the higher   
   terrain and just upwind. With snowfall rates from the HREF progged   
   to exceed 1"/hr at times, this will produce heavy accumulations for   
   which WPC probabilities are high (>70%) for an additional 6+ inches   
   of snow in the northern Adirondacks, Greens, and portions of the   
   Whites of NH and ME, with locally up to 1 foot of snowfall possible   
   (10-30%). Precipitation will wane on Tuesday with snow likely   
   ending Tuesday aftn.   
      
   Finally, the pronounced CAA on NW flow behind this system will   
   cross the Great Lakes and produce some narrow bands of lake effect   
   snow (LES), especially downwind of Lake Superior and Lakes   
   Erie/Ontario. While this direction is not ideal for elongated fetch   
   across the lakes, the high-res guidance is in good agreement that   
   an upstream connection will occur from Superior, to Huron, to Erie,   
   with an additional upstream band from Georgian Bay to Lake   
   Ontario. This should produce narrow corridors of heavy snowfall   
   exceeding 1"/hr on D1, leading to accumulations for which WPC   
   probabilities indicate have a 50-90% chance of exceeding 4 inches   
   across the Chautauqua Ridge, the Tug Hill Plateau, and portions of   
   the eastern Finger Lakes region, with the latter continuing through   
   D2.   
      
      
   ...California to the Central and Southern Rockies...   
   Days 1-3...   
      
   A pair of upper lows embedded within persistent troughing across   
   the West will bring periods of high-elevation heavy snow to   
   portions of the region.   
      
   The first will be associated with a compact shortwave lifting   
   across UT/WY/CO that will deepen into a closed feature as it moves   
   into the Central High Plains Monday afternoon. The amplification of   
   this wave will result in increased deformation to its west,   
   aligning with modest upper diffluence as a weak jet streak develops   
   in the vicinity. This paired ascent occurring within a moist column   
   (PWs above the 99th percentile) will result in a transition to   
   heavy snow through dynamic cooling, especially in the higher   
   terrain of CO and WY above 7000 ft. WPC probabilities for more   
   than 6 inches of snow are high (>70%) from the Wind Rivers through   
   portions of the Uintas and into much of the CO Rockies, with   
   locally as much as 12 inches possible in the Wind Rivers and the   
   Park Range.   
      
   Upstream of this feature, a more pronounced upper low will dig   
   slowly along the CA coast, tracking into Baja Wednesday before   
   finally beginning to eject eastward late in the forecast period.   
   Downstream of this feature, another round of impressive IVT   
   exceeding the 90th percentile according to NAEFS ensemble tables   
   will surge eastward into CA and the Great Basin, reaching the Four   
   Corners states Wednesday. Beneath the upper low, snow levels will   
   fall to 5000-5500 ft, but within the plume of IVT and accompanying   
   WAA, which is also where the heaviest precipitation is expected,   
   snow levels will remain elevated at 7000-9000 ft. This will result   
   in heavy snowfall, initially in the Sierra, then spreading east   
   into the Great Basin and Four Corners terrain including the San   
   Juans, Wasatch, and portions of the Mogollon Rim (White Mountains   
   of AZ area). WPC probabilities D1-2 are high (>70%) for 6+ inches   
   in the Sierra and the higher terrain of the Great Basin and into   
   the Wasatch and San Juans, where locally 1-2 feet of snow is likely   
   by the middle of the week.   
      
      
   ...Central Plains/Upper Midwest...   
   Day 2...   
      
   Compact mid-level shortwave will close off as it crosses from   
   Nebraska to the Great Lakes Monday night and Tuesday. Significant   
   mid-level moisture advection ahead of this feature will spread   
   precipitation northwards into the Dakotas and then eastward through   
   the Upper Midwest. On the north side of this precipitation shield,   
   soundings suggest the column will be marginally thermally favorable   
   for snowfall. However, cross-sections indicate an increasing threat   
   for CSI where theta-es decreases with height, directly into the DGZ   
   to support intense snow growth. Additionally a deep isothermal   
   layer beneath the DGZ indicates a favorable setup for aggregate   
   maintenance and large snow flakes with efficient accumulation.   
      
   The guidance, especially the available high- res, has become more   
   aggressive with this development, and it is likely that this ascent   
   will cause rapid dynamic cooling of the column to support a narrow   
   band of heavy snowfall which will track slowly from near the   
   Coteau des Prairies through far southeast Wisconsin. While there is   
   still pronounced latitudinal spread and timing differences,   
   confidence is increasing that a band of heavy snow, with rates of   
   at least 1"/hr, will move across the area. This is supported by   
   increasing snowfall probabilities in both the NBM and the AIFS, and   
   the updated WPC probabilities for at least 1" of snowfall have   
   increased to above 70%. Locally much heavier snow is possible, and   
   future updates will need to be monitored for this potential.   
      
      
   Weiss   
      
      
   $$   
      
   = = =   
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