Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 513 PM EST Sat Feb 18 2023 Valid 00Z Sun Feb 19 2023 - 00Z Wed Feb 22 2023 ...Long-duration, widespread, major winter storm expands from the Northwest Monday night to much of the West and northern tier states through the middle of next week... ...Pacific Northwest, Northern/Central Rockies, Northern High Plains... Days 1-3... A very active pattern is underway as directed by a sprawling vortex low centered north of Hudson Bay and a strong ridge over the northeastern Pacific. Shortwave energy continues to round the ridge and track from Alaska to the Northwest until a potent wave breaks the ridge, tracking over the Gulf of Alaska Sunday night and diving across the PacNW Monday night, becoming a full-latitude trough over the West Coast Tuesday night. Ahead of the pattern changing wave is a shortwave trough currently moving southeast over the interior PacNW and a shortwave trough currently over southwest AK that is discussed further in the section below. Both of these waves will provide ascent in an increasingly strong north-northwesterly jet that also brings Pacific moisture that is sourced from Hawaii (PW anomalies are progged to exceed +2 standard deviations above the climo mean with the wave from the PacNW to the Great Basin), setting up a long duration snow event when combined with the Mon/Tue wave that brings an Arctic plunge to the Plains and Western U.S. The particularly heavy snow event that spreads over much of the Western and Northern U.S. begins late Monday in the PacNW as the potent trough shifts south from BC. The anomalous PWs being acted upon by the deep synoptic lift through height falls, PVA, and left exit jet streak diffluence, with additional ascent developing along the cold front through strengthening low and mid-level frontogenesis, as well as increasing upslope flow for eastern slopes of the northern Rockies behind the front. Snow levels through the period will waver between 2000 and 3000 ft, and may briefly rise to 3500 ft with pre-frontal warm air advection, but will crash to the surface after the cold front as increasing omega spreads heavy snow across the terrain and lower elevations. WPC probabilities for more than 6 inches of snow on Day 1 are 50-70% over the higher peaks of the northern WA Cascades and down the Northern Rockies to the WY/CO border since this time is generally between the two preceding troughs. For Day 2, more expansive and heavier snow from the second wave spreads southeast over the WA Cascades through the Northern Rockies to the NW WY ranges where WPC probabilities for more than a foot are 50 to 80%. The for Day 3, with the big show starting, the probabilities for a foot or more expand down the WA/OR Cascades and the Northern Rockies. The Arctic cold front behind the clipper over the northern Plains shifts southwest over MT Monday with day with lift from the strong NWly jet allowing moderate Day 3 snow probabilities of six inches or more over much of the Plains from central MT west to the foothills. The long duration, widespread, major winter storm which spread across much of the Western U.S. and the north-central Plains to the central/northern Great Lakes is described in the key messages below. ...North Dakota to the Northern Great Lakes... Days 2/3... A shortwave trough currently over southwest AK rounds the strong ridge currently over the northeast Pacific, diving southeast around the sprawling low pressure vortex centered north of Hudson Bay and emerging from Alberta as a clipper Sunday night, crossing the northern Great Lakes Monday night before swinging up the St Lawrence Valley Tuesday. Lift in a deepening DGZ will be supportive of larger aggregates, which will help produce above climo SLR and moderate snow accumulations. Day 2 snow probabilities for 4 or more inches are low to moderate from northeastern ND across northern MN with Day 2.5 probs moderately high along the North Shore of Lake Superior where lake enhancement followed by wrap around snow should result in a max north from Duluth (but still decent confidence in less than 6" in 24hrs). Moderate Day 2.5 snow probabilities for 4 or more inches are also low to moderate over the western U.P. (shores from the Keweenaw and west) for that wrap around snow that lasts into Tuesday. Significant icing is not forecast across the CONUS Days 1-3. Jackson ...Key Messages for Major Coast-to-Coast Winter Storm... --A major winter storm will spread a large swath of heavy snow from the West Coast to the Upper Midwest Monday through Thursday of next week. --The combination of heavy snow and gusty winds will likely result in widespread hazardous travel and cause impacts to infrastructure. --While subtle shifts in storm track are possible, confidence is increasing that this winter storm will be extremely disruptive in affected areas. --Very cold temperatures are likely from the West Coast into the Northern Plains behind this system, with record lows possible. The potential also exists for a flash freeze for portions of the northern Rockies on Tuesday. --Additionally, a heavy wintry mix of snow, sleet, and freezing rain may impact the Great Lakes and Northeast later next week.