307 FOUS11 KWBC 230718 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 218 AM EST Thu Jan 23 2025 Valid 12Z Thu Jan 23 2025 - 12Z Sun Jan 26 2025 ...Great Lakes... Days 1-3... Cyclonic flow across the east will be amplified by a shortwave moving through the flow which will push a cold front eastward across the Great Lakes late Thursday into Friday, followed by renewed CAA. This CAA will be somewhat short lived as a brief period of shortwave ridging follows in its wake, primarily resulting in subtle WAA D2, before a second, but weaker and displaced farther north, shortwave digs across the region driving another cold front eastward. This will result in two rounds of CAA across the now cold lakes (GLERL total ice coverage up to 24%), so despite steepening lapse rates the duration and intensity of any subsequent lake effect snow (LES) will be modest. This results in the heaviest snow likely occurring D3 as reflected by WPC probabilities for more than 6 inches reaching 30-50% east of Lake Ontario and across the Keweenaw Peninsula, but D1 probabilities for 4+ inches are also high (70%) in the eastern U.P., along the eastern shore of Lake Michigan, and east of Lake Ontario. 3-day total snowfall may eclipse 12 inches in the most prolonged snow bands. ...The West... Days 1-3... An upper ridge dominating the flow across much of the West will quickly be replaced by an amplifying trough beginning the latter half of D1. This will occur in response to a shortwave trough digging across British Columbia/Alberta and connecting with secondary energy from the Pacific moving into the Pacific Northwest. Together, these will force a longwave trough to deepen, with height falls rapidly beginning Friday across the Pacific and Interior Northwest as secondary energy digs southward through the trough. With time, this feature is progged to become even more impressive, taking on a negative tilt near CA Saturday and then potentially closing off into an amplified low as reflected by both ECMWF and GFS deterministic 500mb fields, and supported by NAEFS 700-500mb height anomalies falling to below the 10th percentile over CA and portions of the Great Basin. This synoptic evolution will help push a cold front southward through the Central Rockies and Great Basin, while the placement of the upper low results in downstream divergence and pronounced SW flow atop the sinking front. The result of this will be increasing isentropic ascent and expanding precipitation, generally in the form of snow as the swath of precip pivots south from the Northern Rockies through the Great Basin, accompanied by snow levels falling from 1500-3000 ft ahead of the front to less than 500 ft below it. Most of the precipitation should be light to moderate as PW anomalies are generally normal to below normal, but some heavier snowfall is possible, especially D3 as a stripe of fgen develops in the LFQ of a strengthening jet streak collocated with the WAA/isentropic ascent from the Great Basin east to the Front Range of CO. Some enhanced ascent will also occur in this area due to increasing upslope flow on the NE flow around a high pressure to the north. This evolution will spread a swath of snowfall southward each day. On D1, WPC probabilities for 4+ inches are moderate (50-70%) across some of the higher terrain of central Montana. By D2 the coverage of moderate probabilities increases and spread across the Absarokas, NW WY ranges, and into the CO Rockies including the Park Range. By D3, WPC probabilities for 4 inches or more are highest across the Front Range and Park Range of CO, with some lower probabilities as far east as the Sierra. Days 2-3 snowfall could exceed 1 foot in parts of the Colorado Rockies. For the days 1-3 period, the probability of significant icing greater than 0.10" is less than 10 percent. 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 $$