Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 353 AM EST Thu Nov 18 2021 Valid 12Z Thu Nov 18 2021 - 12Z Sun Nov 21 2021 ...Pacific Northwest to the Northern Rockies... Days 1-2... Shortwave ridging centered over the Northwest early today is expected to give way to a shortwave trough and its associated surface wave moving onshore tonight. This will bring widespread precipitation into the Northwest later today, extending into the northern Rockies tonight. The system is expected to steadily weaken as it moves onshore, with precipitation shifting east through the Rockies Friday into early Saturday. Dry conditions will spread from west to east across the Northwest into the Rockies through the remainder of the weekend as a strong ridge shifts east from the eastern Pacific. Snow levels are forecast to rise during the day today but remain at or below 4000 ft across much of northern Washington, the Idaho Panhandle, and northwestern Montana where much of the heavier precipitation is expected to occur through Friday. WPC 48 hour probabilities -- ending 12Z Saturday -- show the greatest potential for snow accumulations of 6 inches or more is along the Washington Cascades, and the northern Tetons and southern Yellowstone region in northwestern Wyoming. WPC probabilities also suggest the potential for locally heavy accumulations in the Wallowa Mountains in northeastern Oregon, the central Idaho ranges, and the Idaho Panhandle into the northwestern Montana ranges west of the Continental Divide. ...Great Lakes... Days 1-2... An amplified upper trough and strong surface cold front will move east from the Great Lakes and Ohio Valley today, reaching the Eastern Seaboard tonight. Winds becoming northwesterly across Lake Superior will support redeveloping lake effect snow showers with some potential for locally heavy amounts today and tonight across portions of the U.P. and northern Lower Michigan. By this evening, westerly winds across the eastern lakes will support lake effect snow showers from northeastern Ohio to western New York, as well as the Tug Hill region, with locally heavy amounts possible. Expect showers to diminish by late Friday as high pressure moves into the region from the Midwest. For Days 1-3, the probability of significant icing is less than 10 percent. Pereira