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   FOUS11 KWBC 092030   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   330 PM EST Tue Dec 9 2025   
      
   Valid 00Z Wed Dec 10 2025 - 00Z Sat Dec 13 2025   
      
      
   ...Pacific Northwest through the Northern High Plains...   
   Days 1-3...   
      
   Atmospheric River (AR) event to continue through the rest of the   
   week with heavy rain/snow for the Pacific Northwest into/through   
   the northern Rockies and western High Plains.   
      
   The strong AR will continue this evening/overnight over the   
   Cascades and push high snow levels into the region from the   
   southwest, initially from 4000-5000ft over northern WA but rising   
   to over 7000- 9000ft by early Wednesday as the jet lifts northward   
   across British Columbia. Any snow at the higher passes will change   
   to rain even into the higher terrain of NW MT tomorrow with the   
   strong IVT push (exceeding the 99.5th percentile) as snow levels   
   rise to 7000ft. Farther southeast, mid-level shortwave will help   
   yield snow to the higher terrain of WY as well through Wednesday.   
   For Day 1, WPC probabilities for at least 8 inches of snow are >50%   
   above 5000- 6000ft over northern areas and above 8000-9000ft over   
   WY into northern CO.   
      
   Into Day 2, upper jet will wobble a bit across southern BC but   
   keep the Northwest in mild conditions with snow levels 7000-9000ft   
   to start. Through the day Thursday, ridging over the Northeastern   
   Pacific may allow snow levels to slip a bit, especially by Thursday   
   evening as the intense moisture plume subsides a bit. WPC   
   probabilities for at least 8 inches of snow are >50% above about   
   6000ft. Much colder air will dive southward east of the Divide   
   across Montana setting up a significant snowfall by Friday.   
      
   By Day 3, ridging will establish itself on a NNW axis from NorCal   
   into the Northeast Pacific, allowing the moisture plume to lift   
   northward through BC and also dig southward along/east of the   
   Divide across much of Montana. At the same time, the strong cold   
   front will have brought in much colder air to most of the state   
   east of the Divide, coincident with cold Canadian high pressure   
   from the north. With a continued moisture surge (albeit less than   
   earlier in the period but still a quite respectable >90th   
   percentile PW and IVT), snow will expand and strengthen over   
   central/eastern MT as snow ratios rise as well. Strong low/mid-   
   level FGEN may support a heavier axis of snow from central to   
   southeastern MT but there remains disagreement on the placement as   
   the upper jet meanders over northeastern MT. For just Day 3 through   
   00Z Sat, WPC probabilities for at least 6 inches of snow are   
   highest in areas above 6000ft in the northern Rockies. Over central   
   to eastern/southeastern MT, WPC probabilities for at least 6   
   inches of snow are >50% from just east of Great Falls southeastward   
   through Billings to near the WY border. Snow will continue past   
   this period as well.   
      
      
   ...Upper Midwest through the Great Lakes and Northeast...   
   Days 1-3...   
      
   An amplified shortwave and accompanying vorticity lobe will   
   quickly dive from the Upper Midwest to across the southern Great   
   Lakes and into the Northeast through Thursday morning. The   
   impressive height falls ahead of this wave will overlap efficiently   
   with the LFQ of a stout Pacific jet streak, producing intense deep   
   layer lift which will help deepen the resulting surface low. This   
   surface low / Alberta Clipper will scoot across lower Michigan by   
   Wednesday morning and northern Maine/Quebec by Thursday morning.   
   Above normal moisture in conjunction with the impressive dynamics   
   will lead to a swath of moderate to heavy snowfall along the   
   surface low track. WPC probabilities for snowfall >4" remains   
   moderate to high (45-75%) across the heart of WI and central Lower   
   Michigan, with low probabilities (10-30%) of snowfall >6". South of   
   the axis of snowfall, a stripe of freezing rain is possible across   
   portions of southeast MN, northeast IA, southern WI, northern IL,   
   southern MI, northeast IN and northwest OH, though any ice   
   accretions will be less than a tenth of an inch.   
      
   As this clipper system moves from the Upper Midwest and Great   
   Lakes and into the Northeast, the downstream strong WAA will   
   overrun a very cold antecedent airmass. This will create a   
   broadening plume of wintry precipitation, falling primarily as snow   
   from Upstate NY through northern New England, with some mixing,   
   including periods of freezing rain, expected farther south into the   
   northern Mid- Atlantic and southern New England. Total   
   accumulations of wintry precipitation should be less than areas   
   upstream, but WPC probabilities Day 1 into Day 2 for 4+ inches of   
   snow reach 60-95% for portions of western PA and across the   
   southern Adirondacks, Greens, Whites, and terrain of northern ME.   
   Meanwhile, WPC probabilities for freezing rain >0.01" are low to   
   moderate (30-60%) for the higher terrain of northeast PA and   
   northwest NJ.   
      
   Finally, behind this system a brief period of favorable NW flow   
   will create lake effect snow (LES) and upslope snow across the   
   eastern Great Lakes through Day 3. While some of this snow will   
   fall on top of the synoptic snow noted above (and is included in   
   the snowfall probabilities), an additional 4+ inches of snow has   
   moderate to high probabilities (50-95%) downwind of Lake Erie and   
   especially Lake Ontario. The Tug Hill Plateau, in particular, has   
   rather high chances (>70%) for snowfall totals surpassing 8" with   
   even low-to-moderate chances (30-60%) for snowfall topping 12   
   inches.   
      
      
   ...Midwest and Lower Ohio Valley to the Central Appalachians...   
   Day 3...   
      
   A vigorous Clipper system (mid level vort lobe) across the Midwest   
   Friday morning will pivot through the Lower OH Valley and into the   
   WV Mountains Friday afternoon and early evening. Overall the model   
   QPFs (at least the blended means) have come up at 12Z, though   
   latitudinal differences remain (not surprising given a day 3   
   forecast). WPC deterministic QPF incorporated a PMM of the latest   
   GFS/ECMWF/NAM/RRFS, which yields a rather elongated footprint of   
   0.25-0.35" liquid QPF from north-central IL east-southeast through   
   central IN, southern OH, far northern KY, and essentially the   
   middle 2/3rds of WV.   
      
   Highest QPFs (0.50+ liquid) again were noted across the most   
   elevated windward slopes in WV, where the westerly upslope is most   
   favorable. This is also where the latest WPC probabilities of at   
   least 4" in 24hrs are highest -- currently between 50-70+ percent.   
   Elsewhere across north-central IL into central portions of IND,   
   spotty 4"/24hr probabilities between 30-60% were noted.   
      
      
   Fracasso/Miller/Hurley   
      
   $$   
      
   = = =   
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