Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 224 PM EST Thu Dec 29 2022 Valid 00Z Fri Dec 30 2022 - 00Z Mon Jan 02 2023 ...The West... Days 1-3... Confluent mid-level flowing driving an impressive atmospheric river (AR) with IVT exceeding 500 kg/m/s according to both the ECENS and GEFS ensembles from CW3E, onshore the Pacific coast starting tonight. This AR is progged to persist for several days ahead of an amplifying mid-level trough which will pivot onshore CA Saturday night and then deepen into a negatively tilted trough over the Great Basin by the end of the forecast period. Moisture within the AR will be additionally supported by persistent Pacific 150kt jet stream energy shifting eastward through Saturday, before the core jet streak finally moves onshore Sunday ahead of the trough axis, while an upstream jet streak re-energizes to the west. While the persistent downstream upper diffluence, mid-level divergence, and height falls will provide plentiful ascent within the enhanced moisture Friday and Saturday, it is likely the heaviest precipitation rates will occur Sunday as the trough finally moves onshore. Within the strong WAA associated with the leading edge of the AR, snow levels on D1 will climb steadily to as high as 9000 ft in CA, 600-8000 ft across the Great Basin and Four Corners, and generally above 4000 ft farther north. This should limit heavy snowfall to above most of the passes, but WPC probabilities for more than 6 inches are still above 50% in the central Sierra, WA and OR Cascades, and into parts of the Great Basin. Additionally, these snow levels are nearing December highs for CA and the Great Basin, suggesting a very heavy and wet snow with SLR likely near or below the 25th percentile according to the Baxter climatology. This indicates that any places that do receive heavy snow could experience significant impacts due to snow load, reflected by high probabilities in the PWSSI for extreme impacts in the Sierra. As the trough axis moves onshore D2, it will drive an associated occluded surface low and accompanying fronts into CA to help enhance ascent and spread more significant moisture eastward. This will be most impressive on D3 when there will also be a surge in instability across CA and the Great Basin to increase precipitation rates and spread heavy snow farther south and east as the overlap of moisture and ascent expand. Snow levels will again be quite high D2, but then begin to fall as the low and cold front surge eastward, remaining anomalously high for D3 only across the Four Corners, but 2000-4000 ft elsewhere. Multiple waves of precipitation will continue through the weekend, but the heaviest snow is likely to spread west to east from the Sierra through the CO Rockies, including the Wasatch, Uintas, Nevada Ranges, and into NW WY. WPC probabilities both D2 and D3 are high for more than 6 inches each day across these areas, with event total snowfall exceeding 3 feet likely in the Uintas, northern Wasatch, and CO Rockies, with more than 5 feet likely in the higher terrain of the Sierra. Additional heavy snow is likely, primarily on D2, across the WA Cascades where WPC probabilities for more than 6 inches are 30-40%. ...Central Plains & Upper Midwest... Day 1... Weakening low pressure ejecting from the Central Plains will race northeast within fast mid-level flow and beneath a filling shortwave and associated vorticity streamer. A pinching temperature gradient along and behind this low track will allow precip to change from rain to briefly heavy snow, driven by this increased fgen, modest deformation, and overlapped near-0C/km theta-e lapse rates. The speed of this system and slowly weakening forcing will likely limit any significant snowfall accumulations, but a translating band of heavy snowfall with rates that may reach 1"/hr at times according to the WPC snow band prototype tool will result in at least modest accumulations from across far eastern MN through the Arrowhead. However, WPC probabilities for snow exceeding 1" are just around 10%, with some light icing also possible in this area. Weiss