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   FOUS11 KWBC 041921   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   221 PM EST Sun Jan 4 2026   
      
   Valid 00Z Mon Jan 05 2026 - 00Z Thu Jan 08 2026   
      
      
   ...Upper Midwest/Great Lakes/Interior Northeast...   
   Days 1-3...   
      
   Over the next few days, two back-to-back progressive shortwave   
   troughs will bring a combination of snow and ice to the Upper   
   Midwest, Great Lakes, and interior Northeast.   
      
   The first disturbance is making its way over the Upper Midwest and   
   this afternoon and will be positioned over the Great Lakes tonight.   
   500mb height falls, sufficient PVA, and the left-exit region of a   
   strengthening jet streak will produce deep layer ascent. At the   
   same time, increasing and impressive 925-700mb WAA will spread   
   northeast ahead of the shortwave trough, creating a narrow FGEN   
   band over northern WI and the MI U.P. this evening. Moisture   
   associated with this feature comes from the highly active Pacific   
   jet stream pattern, reflected by NAEFS PWs that are above the 97th   
   percentile. With these synoptic and mesoscale ingredients at play,   
   a narrow band of heavy snow will develop which, according to latest   
   12Z HREF guidance, show high chances (>70%) of producing at least   
   1"/hr snowfall rates from far northern WI to the southern periphery   
   of MI's U.P. and the Tip of MI's Mitten this evening. Duration   
   will be limited given the upper-level disturbance's progressive   
   nature, but WPC probabilities for 4+ inches are above 50% from the   
   central MI U.P. to the tip of MI's Mitten, with locally 6-8"   
   possible (30-50% chance) in portions of the eastern U.P..   
      
   Farther south, an icy wintry mix is likely as WAA produces a warm   
   nose to around +3C at 850-750mb. The refreezing layer is quite deep   
   below 850mb, so some sleet may help to cut into the freezing rain   
   accumulations. Nevertheless, hazardous travel is expected this   
   evening from northern MN through central WI and central MI where   
   WPC probabilities for 0.01" of ice are 50-70%.   
      
   This first shortwave trough will race into the Northeast before   
   exiting to the Atlantic Monday night, but snowfall is expected to   
   be light. Snowfall totals over northern NY and as far east as the   
   Green Mountains are likely to receive 1-3" of snowfall, although   
   locally higher in the Tug Hill Plateau due to lake enhancement is   
   possible.   
      
   As snow concludes across the Northeast Monday afternoon, the next   
   Pacific shortwave reaches the Corn Belt Monday night. This   
   disturbance will have sufficient synoptic-scale forcing thanks to   
   a strengthening subtropical jet streak that strengthens to the tune   
   of 140 kts. Left-exit region dynamics and height falls downstream   
   of the shortwave will spawn low pressure over eastern NE Monday   
   evening, with the low then tracking east across the Midwest   
   Tuesday morning and over southern Ontario by Tuesday evening. There   
   remains some uncertainty on the placement of the heaviest QPF, but   
   increasing ascent will foster another round of wintry   
   precipitation from Minnesota through the Great Lakes Monday night   
   into Tuesday. As the low reaches southern Ontario Tuesday evening,   
   additional mixed precipitation spreads across New England Tuesday   
   night and into Wednesday. Northern New England has the better odds   
   for light-to- moderate snowfall on Wednesday as a coastal low may   
   support a deformation zone of snowfall that produces 1-4" of   
   snowfall.   
      
   While the first system had a fair amount of snowfall over the Upper   
   Great Lakes, the second system from the Midwest and Great Lakes to   
   the interior Northeast will largely be an ice producer. WPC   
   probabilities indicate a high risk (>70% chance) of at least 0.01"   
   of ice from central MN near the Twin Cities through the heart of   
   the Great Lakes and into the higher terrain of the interior   
   Northeast. The greatest risk for at least 0.1" of ice is likely in   
   parts of central MN, northern WI, and the higher elevations of the   
   Catskills, Adirondacks, and southern Greens.   
      
      
   ...Western U.S. Mountain Ranges...   
   Days 1-2...   
      
   The active west continues into the first half of the week as an   
   anomalous trough persists just off the Pacific coast through   
   Tuesday morning before amplifying into a closed low that heads   
   towards Baja by Wednesday. This should conclude the snow in the   
   Sierra Nevada by Tuesday, but lingering snow in the Northern   
   Rockies will continue into Wednesday.   
      
   Downstream of this trough, nearly continuous S/SW flow will shed   
   spokes of vorticity onshore, with the accompanying height falls/PVA   
   leading to rounds of precipitation across much of the western   
   CONUS. At the same time, a subtropical jet will amplify and waver   
   from CA through the Central Plains, providing additional   
   synoptically-induced ascent. The combination of persistent SW mid-   
   level flow will foster elevated IVT today and into Monday,   
   resulting in PW plumes that occasionally exceed the 90th   
   climatological percentile. While periods of enhanced ascent   
   will differ across different regions, the next 36-48 hours appear   
   to be synoptically forced with broad large-scale ascent and   
   through upslope enhancement. The strongest upslope flow continues   
   to be appear focused over the Sierra Nevada through tonight where   
   snowfall will be prolific at times, but heavy snow is likely to   
   wind down late Monday as the upper low pinches off from the polar   
   jet and dives south of CA Monday night.   
      
   With broad SW flow engulfing the region, ongoing WAA will surge   
   snow levels to as high as 6000-8000 ft in the Great Basin and   
   Rockies D1 (but generally 3000-5000 ft elsewhere), before steadily   
   falling through D2 as a cold front pushes eastward and the trough   
   axis finally swings farther east to the coast. By 12Z Tuesday, snow   
   levels are expected to be generally 1500-2500 ft in the north, to   
   5000-6000 ft in the south. 2-day WPC probabilities (00Z Monday   
   through 00Z Wednesday) feature a high risk (>70% chance) for 12+   
   inches in the Sierra, northern CA ranges, Uintas, Tetons, Wind   
   Rivers, and portions of the Salmon River/Sawtooth/Bitterroots, with   
   lesser snowfall expected at the other regional terrain. In the   
   Sierra, an additional 1-2 feet is expected in the higher   
   elevations.   
      
      
   ...Cascades & Olympics...   
   Days 2-3...   
      
   A powerful storm system traversing southern AK and an unusually   
   strong subtropical ridge north of Hawaii will create a strong   
   thermal gradient that that directs a strong >140kt 250mb jet   
   streak at the Pacific NW. The Olympics and Cascades will reside   
   beneath the left-exit region of this jet streak at the same time as   
   500mb height falls unfold on Tuesday. Between falling heights and   
   upslope enhancement driven by a westerly IVT topping 300 kg/m/s,   
   snow levels will fall to as low as 2,000ft Tuesday night in the WA   
   Cascades and Olympics and 3,000ft in the OR Cascades. By Wednesday,   
   a secondary disturbance ejecting out of the Gulf of AK trough will   
   bring another round of mountain snow to the Cascades on Wednesday.   
   WPC probabilities show high chances (>70%) for snowfall >12" above   
   3,000ft in the Cascades and Olympics with 2-4 feet of snow likely   
   in the northern WA Cascades above 4,000ft. Expect impacts at pass-   
   level in WA as the WSSI currently shows Moderate Impacts (hazardous   
   driving conditions and potential closures) for Snoqualmie and   
   Stevens Passes where as much as 1-2 feet of snowfall is forecast.   
   Note that the WSSI-P shows low-to-moderate chances (20-40%) for   
   locally Major Impacts, which imply increased odds for closures and   
   delays at these passes.   
      
      
   Weiss/Mullinax   
      
      
      
      
      
   $$   
      
   = = =   
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   --- SBBSecho 3.28-Linux   
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