128 FOUS11 KWBC 061859 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 159 PM EST Thu Mar 6 2025 Valid 00Z Fri Mar 07 2025 - 00Z Mon Mar 10 2025 ...Great Lakes and Maine... Day 1... Negatively tilted longwave trough extending between two pronounced shortwaves will pivot northeast across New England and the Mid-Atlantic tonight. This trough is the remnant of the closed low which produced a powerful blizzard over the Upper Midwest, and the southern impulse will likely cause secondary cyclogenesis with rapid strengthening off of New England and into the Gulf of Maine tonight. As this low lifts into the Canadian Maritimes, it will spread heavy snow across Maine, with upstream resultant NW flow from the Great Lakes into New England driving a combination of lake effect snow (LES) and upslope snow into the Adirondacks and Greens/Whites. The heaviest snow is likely across portions of northeast Maine where WPC probabilities reach 50-70% for 4+ inches and 30% for 6+ inches. Moderate to heavy snow is also likely across the eastern U.P., southeast of lake Ontario, and into the Greens/Whites where WPC probabilities for 4+ inches are as high as 50%. ...Central Rockies through the Upper Midwest... Days 1-2... Lee cyclogenesis east of the Rockies will develop this evening downstream of a positively tilted but amplified upper level trough swinging through the Great Basin and towards the Central Rockies. An intensifying and meridionally shifting jet streak downstream of the primary trough axis will place favorable LFQ diffluence across the High Plains, additionally supporting the rapid development of this cyclone. As the low strengthens, it will move almost due east in response to confluent and increasingly zonal flow across the central/eastern parts of the CONUS. As this low pivots east from Colorado to Illinois by Friday night, it will drive a swath of heavy precipitation to its north, caused by an overlap of synoptic ascent through the left-exit of the jet streak aloft, and expanding 850-600mb frontogenesis. At the same time, moist isentropic ascent, especially in the 290-295K layer will surge northward creating PWs that will rise to above the 90th percentile within the CFSR climatology, upon which the ample ascent will drive a stripe of heavy snow. The warm/moist advection lifting northward and being acted upon by intense fgen will result in translating bands of heavy snow. Neutral static-stability reflected by theta-e lapse rates nearing 0C/km within the axis of strongest ascent could cause snowfall rates to reach 1+"/hr at times, which is additionally supported by the WPC prototype snowband tool, especially from eastern WY through NE. Where these bands linger, heavy snowfall exceeding 6 inches is likely (>70%) according to WPC probabilities across eastern WY and NE, with moderate probabilities (30%) for 4+ inches extending as far east as central IA. ...Four Corners States... Days 1-2... The forecast period begins with an amplified longwave trough positioned across the Great Basin and extending into the Northern Rockies. This trough will gradually pivot eastward through D1 and into D2 as two distinct shortwaves drop from the interior Northwest through the trough axis and then eject eastward into the Southern Rockies, eventually merging into a closed low over New Mexico on Saturday. Impressive height anomalies falling to below the 10th percentile according to NAEFS ensemble tables over the Southwest will combine with an increasingly meridionally shaped downstream jet streak to provide ample ascent across the Four Corners into Saturday. At the same time, a surface low will move along the CA coast beneath this trough evolution and then shift onshore into AZ and then NM, producing additional ascent to support widespread precipitation. Snow levels throughout the region will generally hover in the 4000-5000 ft range, but downstream of the low and within the most robust warm advection, they may at times rise to as high as 7000 ft, especially in the Sangre de Cristos D1 into D2 before crashing later Saturday. WPC probabilities D1 for more than 6 inches of snow are high (>70%) in the Ruby Mountains of NV, along the Wasatch, into the Uintas, CO Rockies, San Juans, and along the Mogollon Rim. During D2, the highest probabilities become confined to the Sangre de Cristos and along the Raton Mesa where locally as much as 12 inches of snow is possible. Weiss $$