Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 343 PM EDT Fri Oct 08 2021 Valid 00Z Sat Oct 09 2021 - 00Z Tue Oct 12 2021 Day 1... ...Great Basin to North-Central Rockies... A mid-level shortwave will continue to lift northeast along the leading edge of a broader-scale trough centered over the western U.S. This wave is expected to move from Nevada and Utah across the north-central Rockies overnight. Broad-scale ascent produced in part by left-exit region upper jet forcing will support organized precipitation, with high-elevation snow likely across portions of the north-central Rockies late today into Saturday. Accumulating snows are expected from the northeastern Nevada mountains to southwestern Montana, western Wyoming and western Colorado. The heaviest amounts of this event are most likely to include the Wind River Mountains in western Wyoming. WPC Day probabilities continue to indicate that accumulations of 4-inches or more are likely above 9000 ft, with heavier accumulations possible across the higher peaks. Day 2... ...Northern Cascades... A northern stream trough emanating over the Gulf of Alaska is forecast to dig south, driving a strong cold front across the Pacific Northwest on Sunday. With the frontal passage, snow levels are expected to lower on Sunday, bringing the potential for accumulating snows down to around 3000-4000 ft. Day 3... ...Northern Rockies... The previously noted trough in the Northwest on Day 2 will continue to amplify through Day 3, with energy digging south into the Great Basin, with a closed low beginning to form late in the period over Nevada. By late Monday, the GFS shows 500 mb heights that are 2.5 standard deviations below normal across much of the Great Basin. Favorable upper jet forcing, along with strong low-to-mid level frontogenesis will support locally heavy snows developing across parts of the northern Rockies on Monday -- particularly across southwestern Montana into northern Yellowstone N.P. WPC probabilities are greater than 70 percent for accumulations of 4-inches or more from parts of the Madison to the Beartooth Mountains in southwestern Montana and far northwestern Wyoming. Probabilities for accumulations of 8-inches or more are generally 10-30 percent across this same area. For Days 1-3, the probabilities for significant icing (0.25 inch or more) are less than 10 percent. Pereira