Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 304 PM EST Sun Nov 07 2021 Valid 00Z Mon Nov 08 2021 - 00Z Thu Nov 11 2021 Pacific Northwest and Northern California to the Northern Rockies... Days 1-3... A series of low amplitude shortwaves troughs will continue to move across the Pacific Northwest through the overnight into Monday -- supporting showers, including mountain snows for the Olympics and northern Cascades. A building shortwave ridge is expected to support relatively dry weather across much of the Northwest during the day on Monday. For the Day 1 period -- ending 00Z Tuesday -- WPC probabilities greater than 50 percent for snow accumulations of 6 inches or more are largely confined to the Olympics and far northern Washington Cascades above 3000 ft. Monday night into Tuesday, a well-defined shortwave is expected to move inland ahead of a deep upper low settling southeast across the Gulf of Alaska. Strong upper forcing and onshore flow will support the development of widespread moderate to orographically focused heavy precipitation across southern Oregon and northern California. However, snow levels will be increasing on Tuesday. For the Day 2 period -- ending 00Z Wednesday -- probabilities greater than 50 percent for snow accumulations of 6 inches or more are largely confined to elevations above 5000 ft for the Klamath Mountains into the northern Sierra Nevada. For the central Sierra Nevada, these higher probabilities are largely for elevations above 9000 ft. As the upper level shortwave translates inland, locally heavy snows are forecast to develop over the northeastern Oregon and central Idaho ranges. WPC probabilities suggest widespread accumulations of 4 inches or more are likely for the Sawtooth Range and the Wallowa Mountains. Meanwhile, the potential for heavy snow is expected to increase across the Olympics and northern Cascades, with WPC probabilities showing high probabilities for accumulations of 6 inches or more for elevations above 4000 ft. Late Tuesday, this leading shortwave will shift into the Plains. In its wake, progressive northwest flow with embedded mid level energy will spread through the Northwest and Great Basin into the Rockies. This will support showers, including mountain snows from the Olympics and Cascades to the northern and central Rockies. In addition to additional heavy amounts across portions of the Olympics and northern Cascades, WPC probabilities should some potential for locally heavy accumulations for portions of the northern and central Rockies, including the northern Idaho, northwestern Wyoming and the northwestern Colorado ranges. For Days 1-3, the probability of significant icing is less than 10 percent. Pereira