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   FOUS11 KWBC 070816   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   316 AM EST Wed Jan 7 2026   
      
   Valid 12Z Wed Jan 07 2026 - 12Z Sat Jan 10 2026   
      
      
   ...Northeast...   
   Day 1...   
      
   Amplified but progressive shortwave will track over New England 12Z   
   Wed - 00Z Thu, exiting quickly to the southeast of Cape Cod   
   tonight. The two distinct vorticity lobes embedded within this   
   shortwave will drive the two surface lows (one moving across far   
   southern Canada while a secondary low develops off the New England   
   coast), with a wedge in-between the two features supporting cold   
   air and overrunning precipitation. The primary mechanism for   
   precipitation development will be the PVA/height falls and   
   increasing isentropic ascent, especially as the secondary low   
   development occurs, before the flow shifts to produce NW winds and   
   CAA. There is still some uncertainty into exactly how strong and at   
   what speed this transition to CAA will occur, but the models   
   continue to support slightly more cold air, keeping snow the   
   primary p-type for northern New York and northern New England, with   
   a mix including freezing rain as far south as CT/RI, but even here   
   precipitation may end as light snow as the column cools.   
      
   Overall ascent is transient and moisture is just slightly   
   elevated, so total accumulations are expected to be modest. This is   
   reflected by WPC probabilities for an additional 2+ inches after   
   12Z Wednesday that are 30-50% in the highest terrain of   
   northern/central ME, and reach above 50% for 0.01" of ice, focused   
   across the Worcester Hills and Monadnock Region of NH.   
      
      
   ...Pacific Northwest, Great Basin, Northern/Central Rockies...   
   Days 1-3...   
      
   Deep trough over the Pacific will shift its axis inland by Friday,   
   with longwave ridging expanding across the Pacific Coast on   
   Saturday. As this ridge blossoms, precipitation will finally wane   
   across the area, but until then an extended period of active   
   weather with significant snowfall will continue.   
      
   The strong jet currently in place will dig slowly southward in   
   tandem with an embedded mid-level impulse tracking towards the   
   Great Basin. This impulse will be generally within zonal flow,   
   although as the trough kinks to become more amplified, mid-level   
   divergence will also increase. Despite a weakening of the onshore   
   mid-level flow, the upper jet will maintain its intensity of around   
   100 kts, at least early D1, before weakening, providing sufficient   
   IVT to fuel heavy precipitation from the Pacific Northwest into the   
   northern Great Basin and northern Rockies. Ascent will be aided   
   both by upslope flow and a fast moving surface cold front. Although   
   forcing looks generally transient, WPC probabilities D1 into D2 are   
   high (>70%) for more than 4 inches of snow from the Cascades   
   through the Northern Rockies and into the Tetons, Wind Rivers, and   
   Uintas, as well as the Ruby Mountains of NV. During D2, forcing   
   along the front pushes into UT/AZ where additional snowfall   
   exceeding 4" is likely (>70%) in the Wasatch and Mogollon Rim.   
      
   Behind this cold front, snow levels will crash, potentially falling   
   to below 1000 ft across the Pacific Northwest, while a second surge   
   of moisture occurs. This second moisture plume is driven by the   
   upstream jet streak from the primary trough axis, with the   
   impressive LFQ diffluence overlapping a secondary vorticity lobe   
   for ascent. As this rides into the Cascades, additional forcing for   
   lift created by upslope flow will wring out moisture as additional   
   heavy snowfall in the Olympics and Cascades, with moisture spilling   
   into the Northern Rockies as well. Notably, with this second   
   impulse, lapse rates are exceptionally steep beneath the core of   
   the mid-level trough, so any heavier rates will likely pull snow   
   down to around 600ft (NBM 10th% snow level), suggesting even some   
   light accumulations are possible in the foothills surrounding   
   Seattle and Portland (although the true lowlands will likely miss   
   out on any snow this time around). Still, the greatest impacts will   
   again be across the Cascades and the Passes where WPC probabilities   
   D2 into D3 are high (>70%) for an additional 4+ inches, with 2-day   
   total snowfall of 2-3 feet likely in the higher terrain, with even   
   1-2 feet possible at the passes.   
      
      
   ...Four Corners into the High Plains...   
   Days 2-3...   
      
   Elongated vorticity lobe will spin southeast from the Pacific   
   Northwest and amplify into deep longwave trough as it approaches   
   the Four Corners Thursday night. The intensity of this trough   
   remains uncertain as it amplifies towards the region around 12Z   
   Friday, but in general the trends have been for a deeper amplitude   
   while remaining positively tilted. At the same time, the resultant   
   jet streak downstream of this trough axis will deepen and pivot   
   northeast, reaching as high as 140 kts across the Southern Plains,   
   leaving favorable LFQ diffluence into the High Plains and Four   
   Corners. Where this jet streak overlaps with the most robust height   
   falls, lee cyclogenesis will likely result, likely in the northeast   
   plains of New Mexico as reflected by most ensemble members.   
      
   This low will move slowly Friday morning before elongating into   
   faster flow to the northeast by the end of the forecast period.   
   Before this occurs, a cold front will dig southward through the   
   High Plains, with the resulting wind field becoming more E/NE and   
   upsloping into the terrain of CO and NM. This will provide   
   additional ascent to the region, which will already be influenced   
   by favorable synoptic lift, suggesting a period of moderate to   
   heavy precipitation Thursday night through Friday aftn. Most of   
   this will fall as snow, especially as the cold front sags south and   
   cools the column through favorable CAA. However, there is still   
   uncertainty into how much snow will fall, especially from the   
   Palmer Divide northward where a significant 850mb moisture gradient   
   is expected. The ECMWF and its ensemble members are much drier   
   farther north than the other camps, but locations across southern   
   CO and northern NM have higher confidence in impactful snowfall,   
   including along the I-25 urban corridor. While the Palmer Divide   
   will likely experience significant snowfall due to the higher   
   elevations and more pronounced upslope, areas north of there,   
   including Denver, may experience lesser snow. However, in an area   
   that has seen very little snow so far this winter, this could   
   still be an impactful event for much of the region, with heavy snow   
   pulling northeast into OK/KS by the end of the period beneath the   
   upper jet streak.   
      
   Although confidence is modest for this time range, current WPC   
   probabilities are high D2 for at least 6 inches of snow across the   
   CO Rockies and into the White Mountains of AZ. Late D2 into D3 as   
   the surface low consolidates and upslope flow improves, WPC   
   probabilities indicate a greater risk, (50-70% chance), for at   
   least 6 inches of snow in the Front Range, Palmer Divide, Raton   
   Mesa, Sangre de Cristos, and into the high plains of southeast CO.   
      
      
   Weiss   
      
      
   $$   
      
   = = =   
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   --- SBBSecho 3.28-Linux   
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