Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 237 PM EST Sun Nov 28 2021 Valid 00Z Mon Nov 29 2021 - 00Z Thu Dec 02 2021 ...Great Lakes through the Northeast... Days 1-3... Broad cyclonic flow will persist across the east through the middle of the week, with embedded shortwaves producing periods of precipitation within the region. The first of these shortwaves will move across the Great Lakes tonight and then sharpen into a negatively tilted trough over New England Monday morning before pivoting eastward into Canada on Tuesday. This feature will be accompanied by an intensifying downstream jet streak which will pivot northward and place the favorable LFQ diffluent region atop the greatest height falls/PVA. At the surface, this will manifest as pressure falls and a wave of low pressure is likely to develop off Massachusetts and then lift northeast while deepening into the Gulf of Maine, and then towards Newfoundland, Canada. Depending on the exact track of this system, heavy snow is possible from eastern MA, southeast NH, and along the coast of ME, but there remains considerable spread in the track of this low. The models today have trended a bit west, and some of the guidance depicts significant snowfall near the coast where the best ascent through fgen couples with the ageostrophic response circulation of the upper jet streak. This has led to an increase in the WPC probabilities for 4+", which are now as high as 40% in northern Maine D1-2, with 10% probabilities for 2+" extending into southern NH. To the west, as the shortwave digs southward and leads to robust CAA in its wake. A stripe of moderate snowfall accumulation is likely from the Arrowhead of MN southeast towards the northern L.P. of MI, and WPC probabilities for more than 2 inches on both D1 and D2 are 50-70%, with event totals locally eclipsing 4". The greatest snowfall, however, is likely in the favored NW LES snowbelts, especially downwind of Lake Erie where deepening EL's combine with a lowering DGZ and intense ascent to produce convective snowfall rates of 1+"/hr. Here, WPC probabilities are high for more than 4 inches, with local amounts to 8" possible along the Chautauqua Ridge. Another shortwave trough racing across the Great Lakes late D2 into D3 will bring a renewed surge of CAA, with moderate LES once again developing. For D3 /Tuesday night into Wednesday/ WPC probabilities for more than 4 inches of snow is 20-30% in the Tug Hill Plateau. ...Pacific Northwest... Days 1-2... Modest shortwaves embedded within confluent mid-level flow will lift onshore WA state Monday morning, and then British Columbia Tuesday morning. The first will be accompanied by a brief surge in mid-level RH racing eastward to the Northern Rockies, while the BC trough advects a more robust AR into Canada and the Pacific Northwest. This airmass will generally be of tropical Pacific origin, and snow levels are progged to be 5000-7000 ft D1, and rising as high as 9000 ft on D2. The best moisture and most robust precipitation will remain north of 45N latitude, and snowfall on D1 has only a moderate WPC probability of exceeding 4" in the Northern Rockies. On D2, the more impressive moisture plume shifts into the Cascades, but with rising snow levels, WPC probabilities for more than 4" of snow are less than 30% and confined to the Northern WA Cascades. For Days 1-3, the probability of significant icing is less than 10 percent. Weiss