Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 122 AM EST Thu Feb 20 2025 Valid 12Z Thu Feb 20 2025 - 12Z Sun Feb 23 2025 ...Central Appalachians to New England... Day 1... A major winter storm which brought significant snow to the Tidewater area of Virginia and surrounding portions of VA/NC will continue to eject to the E/NE today while deepening. Snow will end quickly this morning over VA, but as the low deepens and tracks well southeast of the 40N/70W benchmark, snow will overspread portions of coastal southern New England, especially Cape Cod and the Islands. The system is progressive, but moisture spreading north ahead of it will maintain periods of snow in far southeastern MA, with some additional ocean enhanced snowfall likely across the Cape Thursday night. Total additional snowfall is expected to be modest, but may exceed 4" especially across the Outer Cape and Nantucket as reflected by WPC probabilities of 30-50%. Behind this potent surface low, an anomalous upper low will race from the Ohio Valley to off the Mid-Atlantic coast by this evening, bringing an arc of 500mb heights below the 2.5th climatological percentile according to NAEFS. Steep lapse rates beneath this upper low combined with moistening of the low-to-mid levels of the column will result in a pivoting axis of moderate snowfall, which may accumulate significantly in the higher terrain of WV where WPC probabilities for 4+ inches are locally 50-70%. Farther east, moderate snow rates may bring some light accumulations from near Greensboro, NC northeast through Hartford, CT. Although accumulations will be less than 1 inch in most places, this snow falling atop very cold ground could cause hazardous travel. ...Great Lakes... Day 1... Lake effect snow (LES) will develop in the wake of the potent upper low moving through the Ohio Valley and into the Mid-Atlantic today. NW flow and accompanying CAA will produce a favorable environment across the lakes, most notably SE of Lake Michigan early D1, and then shifting to downstream (SE) of Lakes Erie and Ontario tonight into Friday morning. The flow is not ideal along the long fetch of the lakes except Lake Michigan, and duration is likely to be relatively short, but WPC probabilities for 4+ inches of snow reach as high as 30-50% across far SW MI near the lakeshore of Lake Michigan, as well as along the Chautauqua Ridge east of Lake Erie. ...Central Rockies... Day 1... A weakening shortwave moving out of the Great Basin will dig E/SE into the Central Rockies Thursday, combining with a weak jet streak aloft to produce widespread deep layer ascent to support wintry precipitation from Idaho through Colorado. A surface low beneath the shortwave will traverse across Utah and Colorado, providing additional ascent, and although moisture is near normal according to NAEFS PW anomalies, this will result in a swath of moderate to heavy snow, especially in the higher terrain of the Wasatch, Uintas, and CO Rockies, primarily above 4000 ft. The waning amplitude of this upper trough will allow precip to wane towards the end of D1, but WPC probabilities for 6+ inches of snow are high (>70%) along the Wasatch, higher terrain of the Uintas, and into portions of the CO Rockies. ...Pacific Northwest... Days 2-3... A deepening trough well off the Pacific Coast will begin to advect moisture onshore the Pacific Northwest beginning Friday night. This moisture plume will be characterized by IVT within an atmospheric river (AR) which has a high probabilities (>70%) of exceeding 500 kg/m/s according to both the GEFS and ECENS. Additionally, this AR will have significant duration as probabilities for 500 IVT exceeding 50% persist for around 48 hours from Friday night through just beyond the end of this forecast period. The overlap of this robust IVT and accompanying WAA with intensifying jet level diffluence and mid-level divergence will result in expanding precipitation, likely occurring in two distinct waves: one Friday night into Saturday morning, and another Saturday night through Sunday. Within the first wave, snow levels will rise gradually to 4000-6000 ft along and west of the Cascades, but cold air remaining entrenched to the east will keep snow levels below 3000 ft across portions of interior OR and WA. With the second surge, however, stronger WAA will lift snow levels to 6000-8000 ft, generally keeping significant snow accumulations above pass levels. On D2, WPC probabilities for 4+ inches that are above 50% are confined to the highest terrain of the WA Cascades, although Washington Pass may receive a few inches of snowfall D2. During D3, WPC probabilities for 4+ inches increase in the WA Cascades, but again at only the higher elevations, while additionally spreading east into the Northern Rockies. Weiss ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png