Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 349 PM EST Thu Feb 01 2024 Valid 00Z Fri Feb 02 2024 - 00Z Mon Feb 05 2024 ...The West... Days 1-3... A full-latitude trough extends from a closed low west of Vancouver Island down the length of the West Coast with a reinforcing shortwave trough pushing into southern CA on the head of a powerful 150+kt Wly Pacific jet. An atmospheric river (AR) ahead of this trough is currently surging through San Diego and Pacifc-sourced moisture will continue pushing into the Southwest ahead of the trough axis that crosses the Four Corners Friday. The southern end of the trough closes into a low over northeast NM Friday night which greatly slows its eastward progress and it merely crosses OK Saturday into Sunday. Meanwhile, the parent upper low center drifts south off the Pacific Northwest coast through Saturday. Precip ahead of the leading trough will have snow levels over the Four Corners states starting around 8000ft, then dropping to about 6000ft before precip tapers off. Day 1 PWPF is 40-70% for >8" over the Mogollon Rim and southern UT ranges with those values for 12" over the San Juan Mtns in southwest CO. Continued onshore flow ahead of the upper low gives the Sierra Nevada and CA Cascades heavy snow with 30-60% Day 1 probs for >8" snow. The arrival and stalling of the negatively-tilted trough/becoming a low over the southern Rockies late Friday allows a lee-side inverted trough to develop from the southern High Plains through the northern High Plains. Gulf-sourced moisture streams north east of this trough and up over the Rockies with two areas of focus for Day 2 heavy snow in 1) the CO Rockies into southern WY and 2) the northern Rockies from NW WY through ID/western MT. Day 2 snow probs are 40-80% for >6" across the CO Rockies and more like 20-40% over the northern Rockies. On Saturday, a TROWAL rounding the developing low over the southern High Plains looks to surge into the central High Plains. A focused snow band should form in this with uncertainty whether this will favor northern CO (and Denver) or southern WY. Dynamics in this banding should allow the marginal thermals to be overcome and produce some accumulating snow over the High Plains. As of now the Day 2.5 snow probs for >2" are focused on the higher terrain of the Raton Mesa/Palmer Divide/Cheyenne Ridge, though the dynamics may strong enough to not need higher terrain to cause accumulating snow. Surface high pressure centered over south-central Alberta Saturday night, along with right entrance lift from a Sly jet lifting from MT into the Canadian Rockies aides a renewed focus for snow over north-central MT. Day 3 PWPF for >6" is 20-60% along eastern slopes of the MT Rockies through north-central MT. ...California and Nevada... Day 3... The aforementioned upper low drifting south off the Pacific Northwest Coast helps orient the next AR directly into central/southern CA Sunday and Monday. There has been uncertainty with positioning with the ECMWF/GFS in a more southern camp with the precip axis through Sunday over south-central CA and the NAM/CMC/UKMET farther north over the central to north-central CA coast. Preference is currently with the ECM/GFS. This AR will have much-elevated moisture/IVT, but the troughing/height falls through its arrival look to keep snow levels suppressed in the 5000-6000ft range, lower than the AR that passed last night. Day 3 PWPF for >12" are 40-70% over the central and southern Sierra Nevada. Lift aided by the low to the north and jet to the south looks to allow precip to cross the Sierra and push into Nevada with Day 3 probs for >4" extending into the western margin of Nevada. ...Northern Maine... Day 1... The clipper low crosses northern Maine this evening with snow continuing tonight. Day 1 PWPF are 30-60% for an additional 2" over far northern/northeastern Maine. The probability of significant icing greater than 0.10" is less than 10% for days 1-3. Key messages for next Atmospheric River into California are below: Strong Atmospheric River Another Atmospheric River (AR) event is expected to impact California beginning Sunday and continue through Tuesday. This AR will be of longer duration and potentially stronger than the one that just moved through California. Heavy Sierra Nevada Snow A significant winter storm is anticipated for the Sierra Nevada, with several feet likely in higher elevations. Disruptions to daily life, including difficult to impossible travel conditions are expected. Risk of Excessive Rainfall and Flooding Heavy rainfall is expected to impact lower elevation and coastal areas, and some of the rainfall totals could be locally significant from central to southern California including the Transverse Ranges. Flash, urban, and river flooding, along with debris flows, and mudslides, with locally considerable impacts are likely. Strong Winds and High Surf Powerful, damaging onshore winds are expected over the central CA coast and Coastal Ranges, with high and potentially damaging surf. Jackson