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   FOUS11 KWBC 290829   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   329 AM EST Sat Nov 29 2025   
      
   Valid 12Z Sat Nov 29 2025 - 12Z Tue Dec 02 2025   
      
      
   ...Northern High Plains through the Midwest into the Northeast...=20   
   Days 1-2...   
      
   ...Significant winter storm ramps up across the Midwest today...   
      
   A positively tilted shortwave trough diving out of Montana will   
   sharpen today across the Central Plains, and eventually take on a   
   negative tilt by this evening in response to secondary vorticity   
   energy rotating through its base. This evolution will result in   
   intensifying deep layer ascent in response to more significant   
   height falls overlapping mid-level divergence, and this will   
   additionally be aided by increasingly coupled upper jet streaks.   
   The surface low developing within this impressive ascent will then   
   deepen as it lifts northeast from Kansas /12Z Saturday/ to Michigan   
   /12Z Sunday/. This system will become increasingly progressive   
   through D2, reaching the Canadian Maritimes by 12Z Monday.   
      
   This evolution will yield an impressive swath of snow from the   
   Northern Plains through much of the Midwest and then into the Great   
   Lakes. As the low develops, increasing warm/moist advection will   
   surge PWs out of the Gulf, and PW anomalies above the 90th   
   percentile are progged by NAEFS to reach as far north as MO/IA.   
   This will occur during a period of intensifying 290K isentropic   
   ascent, which when occurring beneath the larger scale synoptic lift   
   will cause snowfall to expand quickly, and become heavy, especially   
   where 700-600mb fgen intensifies (both due to the WAA and the   
   ageostrophic response of the upper jet position). At the same time,   
   cross-sections continue to indicate a threat for CSI due to folding   
   theta-e surfaces, which additionally will support periods of heavy   
   snowfall rates as laterally translating bands shift northeast   
   through the region. Snowfall rates of 1-2"/hr are likely (>80%   
   chance according to the HREF), which, although won't be constant,   
   will help rapidly accumulate snowfall even with SLRs that will be   
   around to slightly below climo most of the event.   
      
   This intense snowfall will result in widespread significant   
   impacts, and the WSSI-P indicates a 60-80% chance of major impacts   
   for parts of IA, IL, and WI, including the Des Moines, Chicago, and   
   Milwaukee metro areas. This will snarl travel during the busy   
   post-Thanksgiving holiday weekend. Where snow is the most intense   
   and longest lasting, record or near-record November snowfall is   
   possible, especially across eastern IA, southern WI, and northern   
   IL. This is also where the ECMWF EFI is above 0.9 with an SoT of   
   1-5, further indicating the potentially record-breaking November   
   snowfall.   
      
   WPC probabilities D1 are high (>70%) for at least 6" of snow from=20   
   central IA though southern MI, with locally more than 12" possible   
   (10-30% chance) in eastern IA, southern WI, and northern IL.   
   Isolated higher totals are possible as well where any banding can   
   persist due to local convergent features such as indicated by many   
   of the high-res models near Lake Michigan.   
      
   The most significant snowfall should wane quickly from west to east   
   by 12Z Sunday, but post-system NW flow will create some modest lake   
   effect snow (LES) across the U.P. and L.P. of MI, as well as east   
   of Lakes Erie and Ontario Sunday night. Additionally, some modest   
   warm-advection snowfall is likely across the higher terrain of the   
   interior Northeast before snowfall comes to an end by 12Z Monday.   
   Additionally snowfall of more than 4 inches is likely (50-90%   
   chance) according to WPC probabilities from the Chautauqua Ridge   
   northeast through the higher terrain of the Adirondacks, and into   
   VT/NH/ME.   
      
   Key Messages remain in effect for this system (Key Message 2=20   
   below).   
      
      
   ...Central Rockies through the Ohio Valley...=20   
   Days 2-3...   
      
   The next system follows quickly behind the powerful storm across   
   the Midwest as progressive flow dominates the CONUS.   
      
   This system begins as a positively tilted shortwave emerging from   
   the Pacific Northwest and then digging rapidly into the Four   
   Corners states by 12Z Monday. As this impulse dives southeast, the   
   associated vorticity lobe will swing rapidly east, forcing the   
   evolution into a neutral tilt over the Central/Southern Plains D3,   
   and potentially becoming negatively tilted late in the forecast   
   period across the Mid-Mississippi Valley. During this evolution, a   
   downstream upper jet streak will amplify and pivot poleward over   
   the Central Plains to enhance deep layer lift. This should cause an   
   area of heavy snowfall D2 across the Central Rockies, including=20   
   the Wasatch, Uintas, and CO Rockies where snow levels of around   
   4000-5000 ft are expected. Although the system is progressive, PW   
   anomalies above the 90th percentile will support periods of heavy   
   snow, aided by upslope into the terrain, and WPC probabilities for   
   more than 4 inches of snow have increased to 70-90% across the Ruby   
   Mountains in NV, as well as the expanse of the Wasatch, Uintas, CO   
   Rockies, and San Juans. Locally as much as 12" of snow is possible   
   in the highest terrain of Colorado.   
      
   The forecast becomes more challenging into D3 as this system=20   
   pushes east. Although the surface wave is likely to wane across the   
   Southern/Central Plains, the downstream impressive jet streak and=20   
   height falls invof the upper trough will persist. Late Monday, some   
   of the guidance indicates that the downstream jet and a secondary=20   
   sub-tropical jet streak emerging across Mexico will phase, leading=20   
   to secondary low pressure development across the Mississippi=20   
   Valley. As this occurs, precipitation will expand from the Gulf=20   
   Coast through the Mid-Mississippi Valley in response to=20   
   intensifying moist isentropic ascent along the 290K-300K surfaces.=20   
   Mixing ratios within this plume are progged at 4-8 g/kg, around the   
   75th-90th percentile for the date, indicative of the impressive   
   moisture availability in this system.=20   
      
   While there are still important timing, location, and thermal=20   
   differences among the various global members and their associated=20   
   ensembles, the trends are for a wetter and colder system which=20   
   could produce significant snow and mixed precipitation before 12Z=20   
   Tuesday. The AIFS and ECMWF EFI are both well southeast of the=20   
   highest NBM probabilities during D3, but the D3/D4 clusters=20   
   indicate high variability due to timing differences of the trough.=20   
   Despite this uncertainty, current WPC probabilities for at least 2"   
   of snow are moderate (30-70% chance) from central Kansas through=20   
   the western Ohio River Valley, with the potent jet streak aloft=20   
   helping to produce potential banded snow in this region.=20   
      
   Additionally, with warm moist air flooding northward from the Gulf   
   and a retreating cold high pressure in place, a zone of mixed   
   precipitation, including freezing rain, is likely from the Ozarks   
   through the southern Appalachians. There continues to be much   
   uncertainty in the axis of heaviest ice accretions, but   
   probabilities for at least some modest ice have come up tonight,   
   leading to increasing confidence in areas of impactful freezing   
   rain. At this time, WPC probabilities for 0.1" on D3 are as high as   
   30% in the Ozarks, and 30-50% in the southern Appalachians.   
      
   This system will likely continue beyond this forecast period while   
   strengthening, and may create heavy snow across the interior Mid-   
   Atlantic and Northeast on Tuesday. For this reason, key messages   
   have been initiated (Key Message 3 below).   
      
      
   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_2.png__;!!DZ3fjg!9tHzHRjIkt5Wlu4_tiXy9Sv1ZC2dufc5i37Fx8TxUhNW7=   
   OiJVNq0TjnWroPuqfJkGLQTip36UtlIlRHH1vKCqFKDIUk$=20   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_3.png__;!!DZ3fjg!9tHzHRjIkt5Wlu4_tiXy9Sv1ZC2dufc5i37Fx8TxUhNW7=   
   OiJVNq0TjnWroPuqfJkGLQTip36UtlIlRHH1vKCQsKH6jU$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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