Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 443 AM EDT Sun Oct 31 2021 Valid 12Z Sun Oct 31 2021 - 12Z Wed Nov 03 2021 ...Central Rockies to the central High Plains... Days 1/2... An upper trough over the northern Plains will be reinforced by a shortwave trough tonight into Monday. A surface cold front will continue to spread southeast across the Great Lakes and Great Plains while remaining hung up over the Front Range of the Rockies through Monday. Along this front and beneath the upper jet, local enhancements of ascent atop increased moisture from return flow out of the Gulf will lead to areas of moderate to heavy snow from near the WY/CO border east through the Neb Panhandle tonight into Monday. Here, an overlap will occur between the low-level baroclinic zone along the front and the favorable right entrance of an intensifying upper jet streak pivoting across the Northern Plains into the Upper Midwest. These features together will drive ascent for a plume of moderate to heavy precipitation. Further increase in snowfall potential is noted as guidance depicts an area of theta-e lapse rates less than zero within which -EPV exists in the lower portion of the saturated DGZ. This implies not only a potential east-west band of heavy snowfall, but potentially convective rates through CSI, as shown by approximately 50% probability for 1"/hr rates on the WPC prototype snowband tool and 20% probability of 1"/hr snow rates from the 00Z HREF. Day 1 snow probabilities for 4 or more inches are now around 30% over southeast WY into the southern Neb Panhandle with 2" probabilities spreading east to 100W longitude in central Neb. Also, on Day 1.5 there are 40% probs for 4 or more inches in the Park and Front Ranges of CO. On Tuesday, a shortwave trough from the CA/OR border undercuts a ridge over the western Canadian Prairies and shifts east across the central Rockies. The combination of modest height falls/PVA and right entrance diffluence will produce periods of moderate snow above 7000 ft across northern CO, where Day 3 WPC probabilities for 4 or more inches are as high as 40%. ...Great Lakes... Days 2-3... Lake effect precip begins behind the cold frontal passage off Lake Superior today with higher elevations in the UP having an accumulating risk starting tonight, lowering closer to lake level by late Monday. Water temperatures near +10C will drive steepening lapse rates and a deepening inversion to above 9000 ft. This should allow for lake enhanced precipitation to expand across the U.P. of Michigan, but marginal low-level thermals may lead to a rain/snow mix outside of the higher terrain such as the Huron and Porcupine Mountains. WPC probabilities for 4 inches peak on Day 1.5 in the Huron Mtns just south of the Keweenaw Peninsula around 30%. The cold northeasterly flow spreads across the rest of the Great Lakes Tuesday with LES for all typical Great Lake snow-belts by Tuesday night. Day 3 snow probabilities for 2 or more inches are limited to the UP and northern LP of MI though. For Days 1-3, the probability of significant icing is less than 10 percent. Jackson