Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 235 PM EST Sat Mar 1 2025 Valid 00Z Sun Mar 02 2025 - 00Z Wed Mar 05 2025 ...Great Lakes & Northeast... Day 1... Strong surface low pressure system exiting northern New England this evening will continue to usher potent CAA behind an arctic cold front clearing the East Coast tonight. Outside of lingering wrap-around snow across northern Maine, heavy accumulating snowfall through Sunday will be confined to localized lake-effect bands. With 20-35 kt 850 mb winds out of the northwest, the best chances for additional moderate snowfall is across the eastern MI U.P. and areas southeast of Lake Ontario. Here, WPC probabilities are low (10-30%) for an additional 4"+ of snow. ...California & Great Basin... Days 1-2... A closed upper-low tracking into California on Sunday will direct IVT topping out around 250 kg/m/s at the Sierra Nevada and Southern California ranges. While SWrly low-level winds may provide some marginal upslope flow enhancement, this IVT is not particularly strong, thus limiting most heavy amounts to the highest/more remote elevations. Snow levels are expected to be as low as 4,000ft from the Sierra Nevada on east into the Great Basin, but the heavier amounts will generally be confined to elevations above 6,000ft. Snow will fall heaviest over the Sierra Nevada and as far north as the Shasta/Siskiyou Sunday afternoon and into Sunday night. As the upper low approaches Sunday night, Pacific moisture will spill over into the Great Basin with moderate-to- heavy snowfall along the 6,000-7,000ft ridge lines of central Nevada Sunday night and into Monday. Moisture and a potent upper low will then eventually reach the Central Rockies late Monday. WPC probabilities depict moderate- to-high chances (50-80%) for snowfall >8" at elevations above 7,000ft in the Sierra Nevada through Monday morning. High chances (70-90%) for >6" of snowfall exist along the ridges of central Nevada through Monday evening. ...Northern & Central Rockies to the Central Plains... Day 3... The upper low responsible for the mountain snow in the highly elevated ranges of California and the Great Basin will make its way into the Central Rockies on Monday and eventually the Central Plains on Tuesday. Falling heights and residual Pacific moisture will support moderate to locally heavy snowfall in the Wasatch, Uinta, southern Wyoming Rockies, and northern Colorado Rockies. WPC probabilities show moderate-to-high chances (50-80%) for snowfall >8" at elevations above 9,000ft. Farther north, temperatures will gradually cool as a cold front ushering in Canadian high pressure moves south. Lingering Pacific moisture, combined with weak easterly upslope flow will allow for light-to- moderate snowfall in the Northern Rockies between Monday afternoon and Tuesday morning. There remains a fair amount of spread regarding the snow potential in the Central and Northern Plains. The synoptic-scale jet stream setup is generally agreed upon; a strong jet streak over northern Mexico will place its divergent left-exit region over the central Plains Monday night. Along with strong PVA ahead of the 500mb low in the Rockies will result in rapid cyclogenesis in lee of the Colorado Rockies that will organize and strengthen a surface low by Tuesday. Where guidance differs are on the storm track, the rate at which the storm deepens, and the extent to which the colder air to the north is in place soon enough to result in a swath of heavy snow in the Northern Plains. Today's guidance is starting to come into better agreement with a dynamically colder band of precipitation breaking out across the central High Plains Tuesday morning, but with above average spread remaining on exact placement. The GEFS members remain generally little faster and therefor deeper/north with the storm by 00Z Wednesday compared to the ECENS which is slower and farther south. This leads to some differences regarding the location and magnitude of a potential swath of snow on D3 extending from eastern Colorado through the central Plains into eastern North Dakota. The one thing these ensembles have in common is focusing on the Palmer Divide and southeast WY with strong enough NErly upslope flow, along with stronger dynamic cooling aloft to support heavier snowfall. Interestingly, NAEFS mean IVT reaches >97.5th climatological percentile even though flow is out of the north. However, any faster storm motion could mean less snowfall, and the opposite for heavier snow should the storm slow down. Through 00Z Wednesday, WPC probabilities show moderate chances (40-60%) for snowfall >6" along the Palmer Divide and parts of southeast Wyoming, with lesser chances for >4" (10-30%) over parts of western Nebraska and south- central South Dakota. With a lack of sufficiently cold air east of the Rockies, snowfall potential will be more directly tied to the deformation axis placement and dynamic cooling aloft from the Denver metro on north and east. Given the potentially record-strong low pressure over the Midwest by Tuesday night, greater impacts could be related to strong winds. Residents in the Central and Northern Plains will want to monitor the forecast closely over the weekend as this is a complex storm system. Snell/Mullinax