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   FOUS11 KWBC 300739   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   239 AM EST Sun Nov 30 2025   
      
   Valid 12Z Sun Nov 30 2025 - 12Z Wed Dec 03 2025   
      
      
   ...Great Lakes to the Interior Northeast...   
   Day 1...   
      
   The major winter storm plaguing the Upper Midwest will wane as the   
   parent shortwave de-amplifies into a positive tilt and becomes more   
   embedded in the westerlies across Canada. This will cause a more   
   rapid progression of the driving low pressure, leading to moderate   
   continued snow accumulations across the Great Lakes, and expanding   
   across the higher terrain of the interior Northeast.   
      
   The surface low is progged to track from the L.P. of Michigan this   
   morning into the Canadian Maritimes by Monday morning /end of D1/.   
   This will result in two areas of additional snowfall accumulation.=20   
      
   The first is expected across the Great Lakes as NW flow develops in   
   the wake of the surface low. While brief deformation snow is likely   
   as the low departs Michigan, most of the additional snowfall is   
   likely to be of the lake effect snow (LES) variety as CAA   
   strengthens across the lakes. 850mb temps falling to -10C or lower   
   will support steep lapse rates and inversion depths rising to   
   around 800mb to support at least a brief period of heavy LES with   
   rates exceeding 1"/hr (40-60% chance from the HREF). The most   
   likely belts of heavy LES D1 will be across the eastern U.P., the   
   northern L.P., SW MI, and then east of Lake Ontario. In these   
   areas, WPC probabilities are 50-70% for more than 4 inches, with=20   
   locally as much as 8 inches east of Lake Ontario.   
      
   The other area of heavy snow from this system will be across the   
   higher terrain of Upstate NY and Northern New England. WAA   
   expanding northward downstream of the surface low will spread into   
   Canada, drawing a narrow column of moisture from the Gulf to cause   
   expanding precipitation in New England. The WAA is impressive but   
   transient, and regional forecast soundings suggest persistent mid-   
   level dry air which will somewhat preclude precipitation rate   
   intensity. Additionally, the cold high pressure over the region   
   early will rapidly retreat east, leading to funneled southerly=20   
   flow with no cold air entrenchment. This suggests that the heaviest   
   snow will be confined only to the higher terrain of the=20   
   Adirondacks, Greens, and Whites, where WPC probabilities are=20   
   moderate (30-50%) for 4+ inches of snowfall.   
      
      
   ...Central Rockies/Four Corners...   
   Days 1-2...   
      
   A closed 500mb low dropping out of the Interior Northwest will move   
   progressively southeast today while opening into a positively   
   tilted trough. The base of this feature will reach the Four Corners   
   Monday morning before continuing to eject into the Central Plains,   
   and the overlap of height falls with an amplifying jet streak will   
   produce large scale ascent across the Central Rockies and Four   
   Corners today. Available moisture is somewhat scarce (PWs only   
   around 0.25" and around the 10th percentile according to NAEFS) but   
   the impressive, albeit transient, deep layer lift will overcome   
   that to produce a swath of heavy snowfall. Snow should begin across   
   the Great Basin this morning, but intensify in response to better   
   lift aided by upslope flow over the Wasatch/Uintas this afternoon,   
   and then the CO Rockies/San Juans/Sangre de Cristos this evening   
   and tonight. Snow should end early Monday morning. WPC=20   
   probabilities suggest a high risk (>80% chance) of more than 6=20   
   inches across most of these mountain ranges, with snow levels=20   
   generally 4000-5000 ft.   
      
      
      
   ...Central Plains & Ozarks through the Northeast...   
   Days 2-3...   
      
   ...Widespread impactful snow and ice likely, but uncertainty with   
   respect to timing and track remains...   
      
   A complex evolution of mid-level features will create the first   
   widespread winter precipitation event from the Ohio Valley into the   
   Appalachians, Mid-Atlantic, and Northeast. However, uncertainty   
   remains considerable at this time range and model guidance   
   continues to feature a variety of solutions which will affect the   
   accompanying impacts.   
      
   This system will develop initially in response to a shortwave   
   diving out of the Central Rockies Monday afternoon, and this   
   feature is expected to become neutrally tilted as vorticity lobes   
   swing through its base by the time it reaches the Ohio Valley   
   Tuesday morning. Thereafter, the shortwave tracks rapidly east   
   while maintaining its modest amplitude, exiting New England by 12Z   
   Wednesday. The global guidance has come into much better agreement   
   this morning with the progged evolution, maintaining a flatter and   
   faster wave, and in general, this upper pattern does not=20   
   conceptually support a widespread significant snowfall.=20   
      
   However, there are caveats that may make this a bit more=20   
   impressive than it would otherwise appear at 500mb. This is=20   
   primarily due to the amplification and phasing of upper jet=20   
   streaks: one amplifying downstream of the shortwave and a second=20   
   subtropical jet streak emerging from Mexico. The interaction of   
   this jet energy is progged to occur across the Mid-Mississippi   
   Valley Monday night, with the strengthening result then arcing   
   poleward through Tuesday to provide more impressive lift both   
   through the RRQ and LFQ. The strengthening jet streak will provide   
   sufficient upper ventilation, in conjunction with the mid-level   
   height falls, to produce surface cyclogenesis along the Gulf Coast   
   Tuesday morning, with this low then deepening as it tracks rapidly   
   northeast to off the coast of the Carolinas and then towards the   
   40/70N benchmark. The SLP trends of the various ensemble camps=20   
   have been for a faster solution with also some latitudinal gain,=20   
   suggesting this will not be a heavy snow event for the I-95=20   
   corridor, which is supported by an unfavorable surface high=20   
   position as well, but could still cause impactful wintry=20   
   precipitation across a large area.   
      
   As the synoptics intensify and the surface low moves east,   
   increasing WAA on a 30-50 kts 850mb LLJ will spread northward   
   leading to an expansion of precipitation. Where this interacts with   
   the strengthening jet streak, a stripe of heavy banded   
   precipitation is likely, which will fall as snow in many areas.=20   
      
   The first band is expected Monday into Monday night from the=20   
   Central Plains through the Ohio Valley where the ageostrophic=20   
   response to the LFQ of the upper jet will help intensify 700-600mb=20   
   fgen, crossing directly the deepening DGZ (SREF DGZ 50mb depth=20   
   probabilities > 50%). This should produce a stripe of heavy snow   
   rates from KS through the Ohio Valley, and although the band will   
   be progressive, WPC probabilities indicate a moderate risk (50-70%   
   chance) for at least 2 inches from central KS through eastern OH,   
   with locally higher amounts of 4+ inches possible (10-30%) aided by   
   fluffy SLRs. South of this band, increasing moist isentropic=20   
   ascent atop the retreating cold air will result in an axis of=20   
   freezing rain across the Ozarks where WPC probabilities are modest   
   (10-30%) for at least 0.1" of ice accretion, highest in central AR.   
      
   Then, during D3 /12Z Tue to 12Z Wed/ the moist isentropic ascent   
   maximizes leading to widespread heavy precipitation along and ahead   
   of the strengthening low pressure system. With the surface high   
   retreating and a lack of mid-level confluence to lock in cold air,   
   many areas of the Mid-Atlantic into the Northeast will begin as a   
   brief period of mixed precip, but should quickly change to rain   
   Tuesday morning, especially along and east of I-95. Inland,   
   however, the strong WAA, especially in the 850-700mb layer, will=20   
   result in front end heavy snow, most likely from Kentucky northeast   
   into interior New England. There is still uncertainty into how far   
   north the warm air will spread to cause changeover, but=20   
   significant snow accumulations are likely as reflected by WPC   
   probabilities that have increased, and now suggest a greater than   
   70% chance for 4+ inches of snow from the Poconos through Downeast   
   Maine. Locally more than 8 inches is possible, most likely in the   
   higher terrain of central New England. It is prudent to note that=20   
   while most of the guidance does not support an I-95 snow event, the   
   ECMWF AIFS ensemble, and even to some degree the EFI, suggest some   
   heavier snow farther south than most of the other camps, which=20   
   could bring more impactful weather to I-95 and is worth continuing=20   
   to monitor.   
      
   Finally, south of the heavy snow and across the terrain of the   
   Southern and Central Appalachians, a period of light to moderate   
   freezing rain is likely which could cause impactful ice   
   accretions, especially in the higher terrain. Current WPC   
   probabilities are as high as 30-50% for 0.1" of ice, with local   
   amounts approaching 0.25" possible in the vicinity of the Blue   
   Ridge of NC and VA.   
      
   Key Messages remain in effect for this system and are linked below   
   (Key Message #3)   
      
      
   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_3.png__;!!DZ3fjg!_c96Fsm1kuyRpsYRqexGYrwZpDlGi30W0sU9cwbGm13v6=   
   4qFD27sEGEjMb_FSIQmd90IuTeJ7bqYRbSGk1Vc1yHTKf4$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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