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   FOUS11 KWBC 050758   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   258 AM EST Mon Jan 5 2026   
      
   Valid 12Z Mon Jan 05 2026 - 12Z Thu Jan 08 2026   
      
      
   ...Upper Midwest/Great Lakes/Northeast...   
   Days 1-3...   
      
   Shortwave currently over Wisconsin will track rapidly eastward   
   while continuing to de-amplify this morning, likely moving off the   
   Maine coast by Tuesday morning. The weakening trend of this impulse   
   driven by the absorption into more pinched westerlies will   
   gradually weaken ascent such that ongoing heavy snow near the Great   
   Lakes will transition to primarily light snow from Upstate NY   
   through central/northern New England. Light accumulations of 1-3"   
   are expected in this region, with locally as much as 4" possible in   
   the Tug Hill Plateau due to lake enhancement and southern Greens   
   thanks to upslope flow. In general, however, the most substantial   
   impacts from this shortwave will end before the new D1 period   
   begins.   
      
   Almost as quickly as this first shortwave exits New England, a   
   second impulse following quickly in its wake will begin to   
   organize over the Upper Midwest Tuesday morning. This next impulse   
   stems from a weak vorticity lobe which ejected from the Pacific   
   trough late Sunday, crested the western ridge, and begins to deepen   
   as it moves towards the Great Lakes Tuesday. Unlike the lead   
   shortwave which weakens with time, this feature is expected to   
   gradually strengthen and gain amplitude as it moves eastward into   
   Wednesday. Additionally, this shortwave will be joined by an   
   impressive jet streak to its south (reaching 160 kts from the   
   Central Plains to the Ohio Valley) and accompanied by Pacific   
   moisture (PWs above the 90th climatological percentile according to   
   NAEFS). The resulting surface low which will develop will   
   additionally enhance ascent, and a swath of moderate to heavy   
   precipitation is likely from the western Great Lakes through New   
   England.   
      
   Thermally, the environment is marginal for wintry precipitation due   
   to antecedent ridging aloft and strengthening WAA, so p-type from   
   WI, across the Great Lakes, and into at least central New England   
   will likely be a mix of rain, snow, sleet, and freezing rain. The   
   mixed precip will result in some impacts, but WSSI-P indicates only   
   minor impacts expected due to the progressive nature and mixed   
   p-type in this region. Still, WPC probabilities for ice accreting   
   to at least 0.1" reach 30-50% from near the Twin Cities through   
   central WI and into lower MI. Locally 0.2" of ice is possible, but   
   snow amounts are expected to be very light as the guidance has   
   trended a bit warmer tonight.   
      
   During D3, secondary low pressure development in the Gulf of Maine   
   could make snowfall and impacts a bit more impressive over New   
   England. While there remains considerable uncertainty into the   
   timing of this secondary low development, a surge of cold air   
   wrapping behind (and a developing in-situ CAD) should keep p-type   
   primarily snow in northern New England and mixed   
   snow/sleet/freezing rain in the higher elevations of Upstate NY and   
   central/southern New England. This is reflected by an increase in   
   WPC snowfall probabilities, which, while still modest, are as high   
   as 10-30% D2 into D3 in the higher terrain of NH and ME. Icing to   
   the south of the snow may locally reach above 0.1" (30-50% chance   
   in the Adirondacks, Greens, and Monadnocks).   
      
      
   ...California...   
   Day 1...   
      
   Amplifying trough off the Pacific Coast will deepen into a closed   
   low today, with this closed center drifting southeast towards Baja   
   California by Wednesday morning. Downstream of this evolution,   
   continued SW mid-level flow and aligned Pacific jet energy will   
   pump moisture northeast, reflected by high probabilities (>80%   
   chance) for IVT exceeding 250 kg/m/s surging into CA. Although this   
   IVT will be pushed onshore via WAA, snow levels will generally   
   remain around 5000 ft as the WAA is offset by slowly sinking   
   heights downstream of the trough axis. At the surface, of wave of   
   low pressure will drop along the CA coast, and while there is   
   considerable spread in the longitudinal placement of this feature,   
   sufficient additional ascent into the moistening column will   
   result in periods of heavy snow above 5000 ft in the Sierra and   
   northern CA ranges D1, and WPC probabilities are high (>90%) for at   
   least 6 inches more of snow, with more than 12 inches possible in   
   the higher terrain, especially around Mt. Shasta.   
      
      
   ...Olympics, Cascades, and Northern/Central Rockies...   
   Days 2-3...   
      
   Mid-level flow becomes increasingly zonal across the Pacific,   
   directing energy and moisture into the Pacific Northwest beginning   
   on Tuesday. Aloft, a jet streak will intensify directly atop the   
   zonal mid-level flow, providing additional funneling of moisture   
   onshore before the jet starts to slowly sink southward on   
   Wednesday. This is reflected by a brief period of elevated IVT   
   (>90% chance exceeding 250 kg/m/s but less than 25% chance of 500   
   kg/m/s) Tuesday into OR/WA. As forcing for ascent increases D2 and   
   D3 through gradual height falls, left-exit diffluence, and   
   impressive upslope into the Olympics, Cascades, and Northern   
   Rockies, heavy snowfall will result in these areas, with snow   
   levels Tuesday 2000-3000 ft, falling to as low as 1000 ft late   
   Wednesday (higher but still just around 2500 ft in the Central   
   Rockies).   
      
   This cooling column, increasing ascent, and impressive moisture   
   indicates that heavy snowfall will become increasingly widespread   
   above these snow levels, especially in the upwind regions of the   
   Olympics, Cascades, and Northern Rockies. Forecast soundings   
   suggest steepening lapse rates up to as high as 750mb, which   
   indicates accumulating snow could occur even lower than these snow   
   levels, and the NBM 10th% fall to as low as 500 ft in the Pacific   
   Northwest, so lowland snow is possible late in the forecast period.   
   However, the most significant accumulations, which are likely to   
   be impressive, will be above 2000 ft, but will encompass most of   
   the Cascade and Northern Rockies Passes leading to challenging   
   travel later this week. WPC probabilities D2 for more than 6 inches   
   of snow are high (>70%) in the Olympics, Cascades of WA and OR,   
   the Northern Rockies, and as far south as the NW WY ranges (Tetons,   
   Wind Rivers), with 1-2 feet likely in the higher elevations of   
   these ranges. During D3, precip wanes a bit across the   
   Intermountain West, but persists in heavy fashion across the   
   Olympics and Cascades where WPC probabilities for 12+ inches remain   
   above 50%. As noted above, the falling snow levels will result in   
   dangerous travel across all of the important Cascades Passes this   
   week.   
      
      
      
   Weiss   
      
      
   $$   
      
   = = =   
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