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   FOUS11 KWBC 270840   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   340 AM EST Thu Nov 27 2025   
      
   Valid 12Z Thu Nov 27 2025 - 12Z Sun Nov 30 2025   
      
   ...Great Lakes...=20   
   Days 1-2...   
      
   Long duration and impressive lake effect snow (LES) event ramps up   
   today and continues through Friday with prolific snowfall totals   
   likely in the favored snow belts SE of the Lakes.   
      
   This event is driven by a large closed 500mb low which will be over   
   southeast Ontario to begin the period. This low will spin nearly in   
   place through this evening before secondary vorticity energy   
   rotating into its base helps pivot it off to the east on Friday. A   
   final piece of energy, a strung-out vorticity lobe, will then move   
   across the Great Lakes Friday aftn, pulling the entire system to   
   the east and resulting in warm-to-neutral temperature advection by   
   Friday night, bringing an end to this event.   
      
   With strong cyclonic flow covering the region, NW winds will bring   
   strong cold advection (CAA) across the lakes to produce this   
   impressive LES. 850mb temps are progged to fall to -10C to -15C,=20   
   which when moving across lake temperatures that are still generally   
   +6C to +10C according to GLERL, will produce increasingly steep=20   
   lapse rates, SBCAPE as high as 800-1000 J/kg, and inversion depths   
   potentially reaching 700mb from the surface. This all will support   
   intense snowfall rates for which both the HREF and WPC snowband   
   tool suggest will reach 1-2"/hr within narrow bands, and locally   
   heavier snowfall rates cannot be ruled out.   
      
   While the bands may twist and pivot at times due to wind   
   fluctuations, in general they should be pretty persistent and   
   focused across the northern U.P and far NW L.P. of MI through   
   Friday morning, with more focused and longer-duration bands   
   impacting the Chautauqua Ridge and Tug Hill Plateau into Friday   
   night. This will result in storm total snowfall that has a high   
   probability (>70%) of exceeding 12 inches near Traverse City, MI,   
   as well as east of Lakes Ontario and Erie. Locally, more than than   
   2 feet of snow is possible (10-30% chance according to WPC=20   
   probabilities) across the Chautauqua Ridge and Tug Hill Plateau.   
      
   Key Messages remain in effect for this system, and are linked below   
   (Key Message #1).   
      
      
   ...Northern High Plains through the Great Lakes...   
   Days 1-3...   
      
   ...Significant winter storm likely to impact portions of the   
   Northern Plains, Upper Midwest, and Great Lakes through this   
   weekend...   
      
   A closed low off the WA coast will dive rapidly southeast while   
   opening, reaching the Northern Rockies by Friday morning. This wave   
   interacting with a stationary front draped across the region and a   
   strengthening upstream jet streak will help spread precipitation   
   into the High Plains of MT very late D1 into D2 (around 12Z   
   Friday). Temperatures will be plenty cold for all snow as   
   precipitation continues to expand into the Dakotas, and the   
   evolution of this shortwave will drive the stationary front   
   southward as a cold front, leading to even colder temperatures and   
   dry fluffy snow into D2. Ascent during this period will be   
   generally modest, but as snowfall expands, WPC probabilities   
   suggest a moderate risk (50-70% chance) for at least 4 inches of   
   snow from northeast Montana through eastern South Dakota. This   
   jet-streak forced band has intensified in recent model progs, so   
   locally heavier snow is possible, especially across South Dakota   
   where a stripe of 700-600mb fgen gets intensified through the jet   
   structure.   
      
   Thereafter, the system amplifies to become much more impressive=20   
   with expanding heavy precipitation into the Upper Midwest Friday=20   
   night through Saturday. The mid-level trough diving into the=20   
   Central Plains will sharpen, and as the vort swings into the base=20   
   of the longer wave trough it will force a negative tilt coincident=20   
   with the development of coupled jet streaks (one strengthening over   
   the Central Plains and another exiting the Great Lakes) to produce   
   robust ascent. This overlapping synoptic lift will help strengthen   
   a surface low developing in the lee of the CO Rockies and then=20   
   tracking east, and eventually northeast, from KS through IL and=20   
   into MI by 12Z Sunday.   
      
   Downstream of this surface low, warm and moist advection will   
   rapidly intensify as 850mb winds surging to above 50kts draw   
   elevated PWs northward from the Gulf, reflected by PW anomalies   
   surging above the 90th percentile according to NAEFS into KS/MO/IA.   
   This will be accompanied by a theta-e ridge lifting northward as   
   well, and although a true TROWAL is not currently progged, this   
   theta-e ridge will still promote additional moisture and some   
   elevated warm air to promote modest potential instability. This   
   evolution suggests intense WAA, which is reflected as well by model   
   output, which will help expand precipitation from the Dakotas and   
   Iowa Friday night, to encompass most of the Upper Midwest Saturday,   
   and then reach the Great Lakes Saturday night.=20   
      
   As far as snowfall amounts and impacts, there is still considerable   
   uncertainty due to model placement and timing of this system. In   
   general, the models are in good agreement with the large-scale   
   evolution, but with such strong WAA in place, even minor timing or   
   latitudinal differences can result in significant changes to   
   impacts. Evaluation of D3 and D4 clusters suggests the GEFS is   
   potentially a bit under-dispersed with its faster trough   
   progression, while the ECENS/CMCE feature more spread to support a   
   variety of solutions suggesting a continued ensemble approach   
   especially by D3. However, some condensing of the spread D2 boosts   
   the confidence that light to moderate snow will expand into the   
   Corn Belt before 12Z Saturday, before pushing east thereafter.   
      
   In general, this event appears to be of longer-duration with=20   
   moderate snowfall rates driven by 280-290K isentropic ascent   
   combined with modest fgen beneath the weak theta-e ridge aloft.=20   
   However, as is the case with most strong WAA events, a narrow=20   
   corridor of heavier snowfall with rates greater than 1"/hr appears=20   
   likely, especially near MO/IA where conditional instability is in=20   
   place and strong fgen drives lift into the DGZ. The DGZ appears to=20   
   be elevated and shallow in most places (although some SREF=20   
   probabilities for 100mb of depth do eclipse 30%), but locally some=20   
   banding could produce more intense rates leading to more=20   
   substantial amounts and impacts.   
      
   Currently, WPC probabilities are extremely high (>90%) for more   
   than 4 inches in a large swath from eastern SD through most of IA,   
   northern IL, southern WI, and into parts of lower Michigan.   
   However, the axis of heaviest snowfall is likely from southern IA   
   through western Michigan, where WPC probabilities are high (>70%)   
   for more than 8 inches of accumulation, and locally 12+ inches of   
   snow is possible, and there are several members within the WSE   
   plumes which indicate more than 15 inches in some areas. The exact   
   placement of the heaviest snow is still in question due to a=20   
   continued southern trend in guidance, but the long duration of this   
   event, combined with locally heavier snow rates, leads to high=20   
   confidence in an impactful event. This is additionally reflected by   
   WSSI-P probabilities which exceed 90% (50%) for moderate (major)=20   
   impacts for portions of IA, IL, and WI. Post-Thanksgiving travel is   
   likely to be extremely disrupted during this event.   
      
   Key Messages are in effect for this system and are linked below   
   (Key Message #2).   
      
      
   ...Cascades and Northern Rockies...=20   
   Days 1-2...   
      
   Closed mid-level low west of WA will push onshore Thursday evening   
   while opening into a shortwave as it lifts into the Northern   
   Rockies. Downstream of this impulse, modest warm and moist   
   advection will surge IVT to above 250 kg/m/s into the Cascades,   
   leading to increasing coverage and intensity of precipitation. Snow   
   levels will climb to around 6000 ft within this IVT plume, but then   
   crash quickly as the cold front accompanying this system pushes   
   east as well. At the same time, a secondary front will dig   
   southward from Alberta, Canada into the Northern Rockies, and   
   westerly flow aloft pushing higher moisture into this region will   
   interact with the terrain and this front to produce ascent and   
   expanding precipitation. At the same time, the overlap of the   
   strongest height falls downstream of the filling shortwave with an   
   amplifying jet streak will likely cause surface low development   
   Friday morning, helping to additionally enhance ascent before   
   everything kicks off to the east by Friday night.   
      
   This evolution will likely cause moderate to at times heavy snow   
   (heaviest in the Northern Rockies due to fgen and upslope   
   enhancement), with snow levels fluctuating around 3000-4000 ft, but   
   remaining near the surface east of the Continental Divide and into   
   the High Plains. WPC probabilities are moderate (>50%) for 4+   
   inches D1 across the Cascades, but high (>70%) for 6+ inches near   
   Glacier NP and the surrounding Northern Rockies. During D2 snowfall   
   shuts off across the Cascades, but continues in the Northern   
   Rockies with an additional moderate chance (50-70%) for 6+ inches   
   of snow again. Travel across most of the Northern Rockies passes   
   will likely be impacted during this snow.   
      
      
   The probability of significant ice across the CONUS is less than=20   
   10 percent.   
      
   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!71PkPC9AEcSguo65jYfOcG0iB12Z-tOnE0FjdAaVTtrFj=   
   3ARvInbHUaxEL7Y-FFu1kMuU5m89_2mgxWYr5rimWePIrw$=20   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_2.png__;!!DZ3fjg!71PkPC9AEcSguo65jYfOcG0iB12Z-tOnE0FjdAaVTtrFj=   
   3ARvInbHUaxEL7Y-FFu1kMuU5m89_2mgxWYr5riXp5Fw8Y$=20   
      
      
   $$   
      
   =3D =3D =3D   
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