Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 312 AM EST Sun Dec 20 2020 Valid 12Z Sun Dec 20 2020 - 12Z Wed Dec 23 2020 Days 1-3... ...Pacific Northwest to the Northern and Central Rockies... An ongoing atmospheric river event is expected to continue on Sunday from the Pacific Northwest into the northern Rockies, with additional locally heavy precipitation likely along and ahead of a front pushing east through the northern Rockies while lingering near the Washington-Oregon border. This is likely to produce locally heavy snow accumulations across the higher peaks of the northern Cascades and the northern Rockies, from the northern Idaho and western Montana ranges to western Wyoming. This front is expected to linger across the region into early Monday then begin to lift north as a wave developing offshore along the front approaches the coast. This will bring additional additional heavy precipitation into the Olympic Peninsula and northern Cascades. This will include mountain snows, although rising snow levels early Monday ahead of the wave will keep heavy accumulations confined to the higher peaks. Meanwhile, developing southeasterly flow along the preceding front as it lifts across the northern Rockies and High Plains is expected to support locally heavy snow along the Lewis Range in northwestern Montana east of the Divide. Models continue to show a well-defined wave with a strong trailing cold front, driven by an amplifying upper trough, moving into the Northwest on Monday. This front is expected to plunge southeast across the Great Basin, and the northern and central Rockies on Tuesday -- bringing a round of precipitation and much colder temperatures across the region. While portions of the higher terrain could see more than a few inches of accumulating snow, the progressive nature of the system will help limit the potential for widespread heavy accumulations. For Days 1-3, the probability of significant icing (0.25-inch or more) is less than 10 percent. Pereira