Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 452 AM EDT Mon Oct 19 2020 Valid 12Z Mon Oct 19 2020 - 12Z Thu Oct 22 2020 ...Northern Rockies/High Plains... Day 1... Waves of energy embedded within progressive, northwesterly flow aloft will continue to support periods of unsettled wintry weather across much of MT through the next week. The next shortwave trough will push southeast across BC today. Southerly flow over cold air in place to the north of a stalled frontal boundary is forecast to support snow across the higher elevations of the northern Rockies into the High Plains with freezing rain common for valleys and much of central MT through this evening. Snow levels are then forecast to increase from southwest to northeast with the heaviest snow accumulations are expected to fall across the higher elevations of the western and central Montana to the northern Wyoming ranges -- where WPC PWPF continues to show moderate probabilities for local accumulations of 6-inches or more. Ice probabilities for a tenth inch or more are limited to around 20% in south-central MT, though the probability for one hundredth is low to moderate for much of MT east of the Rockies. ...Northern Plains and Upper Midwest... Days 1/2... Shortwave energy swinging east from the northern Rockies is expected to amplify over the northern Plains on Tuesday. A weak surface reflection along with favorable upper forcing will help support a wide swath of light to increasingly moderate precipitation as it shifts across the Dakotas late tonight and across MN/WI/the UP Tuesday. Enough cold air will be in place to support accumulating snow on a southerly flow ahead of the shortwave. Days 1.5/2 snow probabilities are moderate for four or more inches across central MN, the MSP metro into western WI and for the North Shore of Lake Superior (enhanced from the southerly flow over the lake). A low-to-mid level frontal band lifting across the region looks to support some heavier totals across central to northern Minnesota and northern Wisconsin. As of now the Day 2 snow probs are up to 10% for 6 or more inches in central MN. ...Northern Rockies to northern Great Plains... Days 2/3... The next wave rounding the deep low over Hudson Bay amplifies as it digs over the Pacific NW Wednesday, reaching the northern Great Basin Wednesday night. This promotes less side surface cyclogenesis over WY on Thursday which shifts south to CO Wednesday night. This provides yet another opportunity (and perhaps the strongest yet) for snow in the northern Rockies as Gulf of Mexico sourced moisture lifts up the Plains and wraps around the developing lee-side cyclone. Furthermore, a ridge from a 1040mb high shifts south into northwestern MT Wednesday night, further promoting low level convergence and upslope flow. Day 3 snow probabilities for 6 or more inches are moderate to high for much of central MT. Probabilities for a foot or more are moderate to high for the northern Absarokas and Crazy Mtns of southern MT. An inverted surface trough under a developing southwesterly jet spreads this focus east over the Dakotas Wednesday night where there are moderate Day 3 probabilities for 4 or more inches over southern ND. For Days 2/3, the probability of significant icing is less than 10 percent. Jackson