988 FOUS11 KWBC 020719 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 319 AM EDT Wed Apr 2 2025 Valid 12Z Wed Apr 02 2025 - 12Z Sat Apr 05 2025 ...Northern Plains & Upper Midwest... Day 1... ...Major winter storm to bring heavy snow and gusty winds to portions of the Dakotas and Minnesota today. Key Messages have been issued for this system and are linked below... The storm system responsible for today's heavy April snow is currently organizing across the central Plains early this morning with an initial impulse of WAA snow showers lifting northward across the Dakotas and Upper Midwest. At the start of the forecast period, broad and deep upper troughing will be situated over the Intermountain West as an ejecting shortwave lifts northward in conjunction with a ridge building over the East Coast. As this shortwave amplifies, it will close off over eastern SD this afternoon and then continue to deepen while sliding northeast across northern MN this evening. For context, the 850-700mb heights associated with this low pressure system are forecast to drop below the 0.5 percentile and even near the 18Z record minimum within the NAEFS CFSR climatology. This indicates a strong system, and as the strongest height falls and PVA from the mid-level evolution overlap with the increasingly intense diffluence within the LFQ of a pivoting jet streak, a strong and intensifying surface cyclone will track from eastern Nebraska through the Arrowhead of MN before exiting into Canada Thursday aftn. With this track, the heaviest snowfall will occur just to the north and northwest of the low center. Two main areas most likely to experience heavy snowfall rates >1"/hr and accumulating snow despite the early-April sun are along the MN Arrowhead and north shores of Lakes Superior, as well as a corridor from the SD/ND/MN border intersection through northern MN. The latter region will fall with a potent deformation axis overlapped with an increasingly impressive TROWAL, especially across from west-central through northern MN, causing heavy snow rates for which the WPC prototype snowband tool and the HREF probabilities suggest will reach 1-2"/hr. Even these kinds of rates could overwhelm the warmth due to April sun, leading to rapid snow accumulations. These impressive snowfall rates may also occur for several hours (roughly 14Z-21Z per the 00Z HREF) as the upper low crosses overhead and pivots the TROWAL over west-central MN and the eastern Dakotas, which led to an increase in snowfall here compared to the prior forecast. Here, WPC probabilities are moderate (50-70%) for 6+ inches of snow, with locally 8-10 inches possible. The Arrowhead of MN will also be a focus for locally heavier snowfall as reflected by a 70-90% chance for 8 inches here, but there remains some uncertainty into how effectively lake enhancement can occur. South of this axis of heavy snow, a swath of mixed precipitation, including freezing rain, is likely. The heaviest icing is expected across and parts of lower MI, where WPC probabilities for 0.1" of ice or more range from 10-30%. ...Northeast... Days 1-2... The same system that will bring the major winter storm to the Northern Plains/Upper Midwest will bring some wintry weather to the Northeast as well. As the primary surface low over MN occludes to the east into the Great Lakes, an attendant warm front will begin to lift northeast downstream of this secondary low pressure. As this warm front lifts into the Northeast, it will spread increasing moisture across Upstate NY and northern New England, especially after 00Z Thursday. The accompanying lift through WAA will ascend isentropically, especially along the 290K-295K surfaces, which will push PWs to above the 99th climatological percentile according to NAEFS, while at the same time driving a warm nose above 0C northward. The impressive ascent along this warm front will result in axis of heavy precipitation, but with p-type gradually changing from snow to a mix to rain, even into northern Maine. The fast progression of this event, combined with heavy precipitation rates, and the transitioning p-type will limit overall amounts, but impacts will still be notable due to snow, sleet, and freezing rain. WPC probabilities for more than 4 inches of snow are modest at just 30-50% in the higher terrain of NH and ME, but more than 0.5" of sleet is possible across a larger portion of those areas. Additionally, freezing rain has a 50-70% chance of accreting to more than 0.1" of ice across the Adirondacks, southern Greens, the Northeast Kingdom of VT, and northern NH into neighboring ME. ...Rockies into the CO Front Range... Days 1-3... Widespread light to moderate snow D1-D2 potentially becomes more significant across the Southern/Central Rockies late D3. For D1 and D2 /12Z Wednesday through 12Z Friday/, the entire region will be covered by large but diffuse troughing extending from the Pacific Coast through the High Plains. Beneath this large and slow moving trough, waves of vorticity will rotate through, leading to periods of enhanced ascent within steep low level lapse rates and aided by upslope flow. In general, this forcing will be modest across the region, but a local exception will exist across the Northern Rockies and maybe as far south as Yellowstone NP as a cold front digs out of Canada leading to some increased fgen and enhanced upslope flow in its wake by the end of D1 into D2. This will cause heavier snow rates and at lower snow levels (falling to 2000-3000 ft) leading to accumulations that have a high risk (>70% chance) of exceeding 6 inches around Glacier and northern Absarokas. Elsewhere on D1 and D2, moderate to locally heavy snowfall across much of the terrain from the Four Corners and UT ranges into the CO Rockies has 30-50% chances of exceeding 6 inches, primarily above 8000 ft. This is in response to a shortwave rounding the base of the trough in the Southwest pivoting over the Four Corners. Then during D3, more impressive and consolidated ascent begins to manifest across the central and southern Rockies, generally from WY through CO and into NM as the tail shortwave within this larger trough begins to amplify. There is good agreement that this shortwave will close off and dig south into the Deserts of AZ, leading to increasingly impressive mid-level divergence downstream. At the same time, a subtropical jet streak will intensify and rotate around the base of this amplifying closed low to place favorable LFQ ascent into the eastern Rockies, while additionally the cold front from D2 across the northern Rockies drops south across WY and CO. The overlap of this baroclinic gradient with the increasing synoptic ascent could result in an impressive area of expanding precipitation D3 and bleeding into D4, with snow levels starting around 6000 ft and dropping below 4000 ft and snow into the High Plains. There is still uncertainty into the exact evolution of this event and how far heavy snow extends into the CO Front Range. Current WPC probabilities on D3 are high (60-90%) for more than 8 inches of snow in the Sangre de Cristos and eastern San Juans, with slightly lower into the central and northern CO Mts. Snell/Weiss ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png $$