Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 439 PM EDT Mon Oct 07 2019 Valid 00Z Tue Oct 08 2019 - 00Z Fri Oct 11 2019 Days 1-3... ...Pacific Northwest/Northern and Central Rockies/Northern Plains... Models remain in good agreement - indicating that an amplifying upper trough and associated cold front will sweep south across the northwestern U.S. Tuesday and Wednesday - bringing sharply colder air and another strong, early-season winter storm to portions of the northern Rockies. As the upper digs into the Pacific Northwest on Tuesday, snow levels are forecast to drop to around 3000 ft, with some significant accumulations likely for areas above 8000 ft in the northern Cascades. Meanwhile, expect lighter accumulations farther west across the eastern Oregon and central Idaho ranges. By late Tuesday, snows will begin to develop and spread south along the northern Rockies. Strong low to mid-level frontogenesis/upslope flow will support heavy snows developing across the northwestern Montana ranges, with locally heavy accumulations expected within Glacier National Park by Tuesday evening. As the upper trough continues to amplify and its associated frontal band pushes farther southeast, snows will continue to develop and spread south and east across Montana through Tuesday night into Wednesday. With snow levels continuing to drop through the day, a prolonged period of strong low to mid-level frontogenesis and upslope flow late Tuesday into early Wednesday is expected to enhance the threat for heavy snows across the central into southwestern Montana and northwestern Wyoming ranges. WPC probabilities continue to show a High Risk for accumulations of 8-inches or more from the Big Snowy and Little Belt Mountains south into the Absaroka, Madison and Big Horn Ranges ranges, with accumulations well above a foot possible across portions of the higher, orographically favored terrain. Meanwhile, lighter amounts are expected to spread northeast along an axis of strong low to mid-level frontogenesis extending from central into northeastern Montana and northwestern North Dakota, where WPC Day 2 probabilities (ending 00Z) indicate at least a Slight Risk for accumulations of 4-inches or more. From late Wednesday into Thursday, with the upper trough continuing to amplify and shift east, model spread increases - lowering forecast confidence from the northern High Plains through the Dakotas. While confidence in the details is limited, it is expected that low to mid-level frontogenesis, coupled with favorable upper jet forcing, will support widespread snows from the central Rockies to the northern Plains, with locally heavy totals possible. WPC solution preferred the more westerly ECMWF, EC Ensemble Mean and NAM solutions - keeping the heavier totals farther northwest across the northern Plains into late Thursday. The probability of significant icing is less than 10 percent. Pereira