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   FOUS11 KWBC 280707   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   207 AM EST Fri Nov 28 2025   
      
   Valid 12Z Fri Nov 28 2025 - 12Z Mon Dec 01 2025   
      
   ...Great Lakes & Northeast...=20   
   Day 1...   
      
   Cyclonic flow aloft will persist a long duration lake effect snow   
   (LES) event into Saturday morning, with prolific snowfall totals   
   likely in a few areas.=20   
      
   The period begins with an elongated but closed mid-level low   
   centered over Quebec driving lowered heights into the Northeast.   
   Rounding the base of this trough, a secondary shortwave and   
   elongated vorticity lobe will swing eastward through the Great=20   
   Lakes and into the Northeast tonight, bringing a brief period of=20   
   shortwave ridging to the area tonight. This will be the evolution=20   
   to finally shut off the LES.   
      
   However, until then, two significantly impactful winter weather   
   areas are expected. The first is in the favored WNW snow belts off   
   the Great Lakes: from the eastern U.P. through the NW L.P. of MI,   
   and then downstream along the Chautauqua Ridge and the Tug Hill   
   Plateau. 850mb temps falling to as low as -15C (-10C far east   
   portions) will drive steepening lapse rates across lake   
   temperatures that are still generally +6C to +10C according to   
   GLERL. The cooling column will keep the DGZ relatively shallow, but   
   favorable ascent crossing the DGZ efficiently will produce bands of   
   very heavy snow for which the HREF indicates has a >60% chance of   
   exceeding 1"/hr, and will likely reach 2-3"/hr at times in response   
   to deep inversion depths as high as 700mb. Although winds will   
   fluctuate just enough to prevent stationary single bands (plus the   
   fetch direction is not along the long-breadth of the lakes in many   
   areas), narrow corridors of heavy snow are expected, especially   
   where any upstream connection can occur from Huron/Superior/Georgian   
   Bay to Lakes Erie and Ontario. The heaviest snow is likely D1 just   
   southeast of Lakes Erie and Ontario where WPC probabilities for 8+   
   inches reach 50-90%, and storm total snowfall from this event=20   
   could be over 2 feet in a few locations.   
      
   In addition to the LES, a strong cold front pressing east from the   
   Lakes early this morning will reach the Atlantic coast this=20   
   evening. The guidance has become increasingly excited in the   
   potential for convective snow showers or snow squalls along this   
   front, as reflected by a snow-squall parameter well above 1. The   
   environment appears favorable for snow squalls from western PA   
   through northern ME as the front passes east, moving across an   
   environment with SBCAPE as high as 250 J/kg coincident with modest   
   0-2km fgen and theta-e lapse rates falling below 0C/km. Moisture   
   may be the limiting factor away from the Great Lakes, so the best   
   chance for squalls appears to be well inland of these states, but   
   any place that does receive a squall this afternoon would likely   
   experience dangerous travel due to rapidly changing conditions with   
   briefly heavy snow and gusty winds.   
      
   Key Messages remain in effect for this system (link #1 below).   
      
      
   ...Northern High Plains through the Midwest into the Northeast...=20   
   Days 1-3...   
      
   ...Significant winter storm to impact large portions of the=20   
   Northern Plains, Midwest, and Great Lakes through this weekend...   
      
   The first widespread significant winter storm of the season begins   
   in earnest today as a positively tilted shortwave trough emerges   
   from the Pacific Northwest and tracks southeast into the   
   Northern/Central Plains. As a secondary vorticity impulses rotates   
   into the trough, this will force downstream amplification of the   
   mid-level pattern, resulting in a negatively tilted trough and   
   increasing ascent downstream into the Upper Midwest and Great   
   Lakes. At the same time, jet streaks both upstream and downstream   
   of this amplifying trough will begin to couple, producing even more   
   intense deep layer lift, and a surface low will result - first in   
   the lee of the Rockies and then tracking gradually northeast into   
   MI by Sunday morning. This low will then continue to deepen as it   
   shifts across southeast Canada and exits into the Canadian   
   Maritimes by the end of the forecast period /12Z Monday/.   
      
   The guidance has come into much better agreement with the track and   
   intensity of this system, and while small temporal and spatial   
   differences among the ensembles still exist, the spread is minimal   
   compared to previous model runs leading to higher confidence in the   
   evolution. As the low deepens and moves northeast, impressive WAA   
   will develop downstream of the developing cyclone, spreading PWs   
   which will exceed the 90th percentile northward into MO/IA. The   
   accompanying theta-e ridge will surge in tandem and isentropically   
   ascent the region, and while guidance is still not suggesting a   
   strong TROWAL, at least subtle warm air aloft will help develop   
   modest conditional instability. The coincident and impressive 290K   
   isentropic ascent will lead to expanding and intensifying   
   precipitation, with moderate snow likely across much of the   
   Northern Plains, Upper Midwest, and Great Lakes, especially Friday   
   night through Sunday morning.   
      
   On D1, the heaviest snowfall accumulations are likely in a narrow   
   band driven by the jet streak aloft and the accompanying   
   ageostrophic/fgen enhancement between 700-600mb. This will cause a   
   stripe of periodically heavy snow from southern ND through western   
   IA, and although the band should be narrow, snowfall rates of   
   0.5"-1"/hr are likely which could cause 4-6" of snow as reflected   
   by WPC probabilities for 4+ inches that reach above 70%.   
      
   However, the most significant part of this event begins late D1   
   across IA and then spreads east through D2 into MI. Here, the   
   impressive isentropic ascent will be quite moist as reflected by   
   mixing ratios of 4 g/kg on the 290K surface, leading to a rapidly   
   saturating column. While in general this upglide will result in   
   moderate snowfall rates, there appears to be an increasing risk for   
   heavier snowfall across portions of MO/IA/IL as the WAA forces   
   an increasing isothermal layer beneath the DGZ which also begins to   
   deepen. Cross-sections are more robust featuring a corridor of   
   folding theta-es surfaces within the elevated RH, suggesting a   
   greater potential for CSI and convective snowfall rates. There is   
   still some uncertainty into this since this signal is much more   
   robust tonight than previous model runs, but locally 1-2"/hr rates   
   appear possible which is supported by the WPC prototype snowband   
   tool despite a modest overall appearance of the evolution with   
   respect to conceptual models for heavy snow bands. Still, multiple   
   bands lifting northward for a long duration will result in   
   significant snowfall accumulations and major impacts for the post-   
   Thanksgiving holiday travel.=20   
      
   This will be a significant system with several waves of moderate to   
   heavy snowfall. On D2, the heaviest accumulations are expected from   
   far eastern NE through IA, northern IL, and southern WI. WPC   
   probabilities in this region are 70-90% for more than 6 inches of   
   snow, and locally as high as 50% for more than 12 inches, highest   
   near the Quad Cities of IA. Later D2 into D3 the heaviest snow   
   shifts eastward and eventually wanes in intensity, but additional   
   heavy snowfall exceeding 6" is possible, especially around Lake   
   Michigan and into the L.P. of MI. Event total snowfall may reach   
   15" in isolated locations Saturday.   
      
   Moderate snowfall is also expected across the interior northeast=20   
   where WPC probabilities D3 are as high as 50-70% for 4+ inches in=20   
   the higher terrain of VT, NH, and ME.   
      
   Key Messages remain in effect for this system (Link #2 below).   
      
      
   ...Central Rockies...   
   Day 3...   
      
   A potent shortwave will drop out of the Pacific Northwest Sunday   
   morning and then move progressively southeast, reaching the Four   
   Corners Sunday night while maintaining a positive tilt. Late D3   
   this feature will begin to sharpen, with the resulting downstream   
   jet streak intensifying into the Central Plains. The overlap of the   
   RRQ of this jet streak with the greatest height falls ahead of the   
   trough axis will help spawn low pressure in the Great Basin, and   
   this low will help to increase ascent across the Central Rockies   
   Sunday evening. As the overall ascent increases, precipitation will   
   expand, and periods of moderate to heavy snow are likely, generally   
   above 5000 ft. This system is expected to remain progressive so   
   prolonged snowfall is not expected, but WPC probabilities indicate   
   a moderate risk (50-70% chance) of at least 6 inches of snowfall   
   across the CO Rockies and northern San Juans, with lighter   
   accumulations expected as far west as the Wasatch of Utah.   
      
      
   Weiss   
      
      
      
      
   ...Winter Storm Key Messages are in effect. Please see current=20   
    Key Messages below...   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_1.png__;!!DZ3fjg!6Sc2F0cnium5fdExFU2gmXHPKlYB1f8bP0Ql_VmqDRwDR=   
   tO2_L40P2nqWm2GPccQ_EI7PVD0CcUgKaNs5dlLL32TFvQ$=20   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_2.png__;!!DZ3fjg!6Sc2F0cnium5fdExFU2gmXHPKlYB1f8bP0Ql_VmqDRwDR=   
   tO2_L40P2nqWm2GPccQ_EI7PVD0CcUgKaNs5dlLAcOrDGE$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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