Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 300 PM EST Thu Nov 18 2021 Valid 00Z Fri Nov 19 2021 - 00Z Mon Nov 22 2021 ...Pacific Northwest to the Northern Rockies... Days 1-2... A fast moving shortwave digging southeast from the British Columbia coast will move onshore WA state Friday morning and then race eastward into the Northern and Central Rockies by Saturday evening while de-amplifying. This shortwave should drive a weak wave of low pressure and associated cold front onshore the WA/OR coast beneath it, which will then weaken with eastward advection into Saturday. Height falls, modest PVA, and at least weak upper diffluence in the LFQ of a Pacific jet streak will drive widespread ascent into a column characterized by increasing moisture as WAA drives PWs to 1-1.5 standard deviations above the climo mean. This will produce precipitation spreading from WA/OR/ID on Friday, into CO/WY/MT on Saturday. Snow levels within this moisture plume are progged to climb to above 6000 ft Friday during periods of most intense WAA ahead of the cold front except downstream of the WA Cascades where cold air will become wedged into the Columbia basin maintaining snow levels of 2000-3000 ft. On Saturday snow levels should climb above 5000 ft into the Northern Rockies. This will confine the heaviest snow above these levels, and WPC probabilities for at least 6 inches of snowfall are high in the WA Cascades D1 and moderate as far east as the Tetons of WY. By D2, the heaviest snow shifts east into the Northern Rockies and ranges of NW WY where pockets of high WPC probabilities for more than 6 inches of snow exist. Storm total snowfall will likely maximize in the WA Cascades and WY Tetons where event total may approach 18 inches in places. Snowfall at both Snoqualmie Pass and Stevens Pass will likely exceed 6 inches. Additionally, as precip overspread WA D1 on WAA, low-level cold air becomes trapped as weak isallobaric drainage occurs from high pressure to the northeast. This could lead to some light freezing rain as wet bulb temperatures remain below freezing in some of the sheltered valleys. WPC probabilities for 0.01" are above 50% in parts of the western Columbia Basin including the Kittitas Valley Friday morning. ...Great Lakes... Day 1... A potent upper trough swings eastward across the Great Lakes tonight, driving a cold front into New England and leaving robust CAA in its wake. This will produce a favorable environment for LES downstream of the Great Lakes which still feature warm surface temperatures. 850mb temps are forecast to fall to just -6 to -8C, not extremely cold, but with lake temperatures still above +10C in most locations, steep lapse rates will develop through an inversion approaching 8,000 ft. Westerly flow initially will produce the most impressive LES east of Lake Ontario and just south of Buffalo, before winds begin to back pushing the bands southward with less intensity by Friday aftn. This should produce accumulations in excess of 4 inches in far NW PA and SW NY, with snow exceeding 6 inches likely in parts of the Tug Hill Plateau. The coverage and intensity of LES should diminish by late Friday as high pressure moves into the region from the Midwest. Weiss