Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 431 PM EDT Tue Oct 08 2019 Valid 00Z Wed Oct 09 2019 - 00Z Sat Oct 12 2019 Days 1-3... ...Northern and Central Rockies to the Northern Plains... A powerful, early-season winter storm will continue to develop, bringing heavy snows to the northern Rockies through Tuesday night into Wednesday. Snows will then spread into the northern High Plains on Wednesday before continuing farther east across the Dakotas while moving south into the central Rockies on Thursday. Snows will continue across the Dakotas through Friday, where heavy accumulations are becoming more likely. Models remain in good agreement, showing an upper level trough continuing to amplify over the northwestern U.S. Tuesday night into early Wednesday. This will bring sharply colder air south out of Canada into the northern Rockies and High Plains. Heavy snows remain likely for portions of the northern Rockies, especially across the central to southwestern Montana ranges into far northern Wyoming, where strong dynamic forcing combined with upslope flow is expected to enhance snowfall rates. WPC probabilities for the 24-hr period ending 00Z Thursday (Day 1) continue to show a greater than 70 percent chance for accumulations of 12-inches or more across some of the higher, orographically favored terrain. Meanwhile, strong low to mid-level frontogenesis will support a band of light to moderate snows dropping southeast across eastern Montana and western North Dakota. WPC Day 1 probabilities continue to show a 30 percent chance or greater for snow accumulations of 4-inches or more extending across northeastern Montana into northwestern North Dakota. By late Wednesday, models show the upper trough beginning to move east of the northern Rockies, with a closed center developing over the High Plains along the eastern Montana-Wyoming border. Increasing divergence along the right-entrance region of a strengthening upper jet is expected to support increasing snowfall rates along what will remain a well-defined low to mid level front moving east across the Dakotas on Wednesday. With the models showing better agreement, probabilities for heavy snow accumulations have increased over the past 24-hrs - with WPC probabilities for the Day 2 period (ending 00Z Friday) showing a 50 percent chance or greater for amounts of inches or more from the Black Hills northeastward into central North Dakota. Meanwhile, locally heavy amounts are forecast to move south from the northern into the central Rockies, including the northern Colorado ranges. Snows will continue across the central into the eastern Dakotas through Thursday night into early Friday, with generally light accumulations expected. By Friday afternoon snows, are expected to increase as the upper low begins to deepen and assume a negative-tilt over the northern Plains. Strong upper forcing will bring a period of heavy snows that are forecast to develop and linger over eastern North Dakota through late Saturday. WPC Day 3 probabilities (ending 00Z Saturday) show a 50 percent chance or greater for accumulations of 8-inches or more across a good portion of central into eastern North Dakota. The probability of significant icing is less than 10 percent. Pereira