Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 457 PM EDT Thu Oct 11 2018 Valid 00Z Fri Oct 12 2018 - 00Z Mon Oct 15 2018 Day 1... ...Central Rockies... Models show a strong upper ridge remaining in place over the eastern Pacific over the next three days -- allowing a series of shortwaves to dig south from western Canada into the western U.S. Ongoing snow showers associated with a positively tilted shortwave that extends from the northern High Plains back through the Great Basin may continue into the evening and overnight hours, resulting in some additional, mainly light accumulations across the western and central Colorado ranges. As the upper trough splits and the northern branch moves east out into the Plains, showers are expected to diminish across the region Friday morning. The probability of significant icing (0.25 inch or more) is less than 10 percent. Day 2... ...Northern Rockies... Models show the next in the series of shortwaves digging south through western Canada, with low to mid level frontogenesis and upslope flow increasing across the northern Rockies as the wave drops through British Columbia Friday night. This will support snow showers extending south along the Continental Divide, with WPC probabilities continuing to highlight the significant potential for accumulations of 4-inches or more across the higher elevations of the northwest Montana ranges, including areas within Glacier National Park. Snow showers will continue to extend further south along the mountains, reaching the southwest and northwest Wyoming ranges during the day on Saturday. While WPC probabilities indicate that amounts of 4-inches or more are likely for portions of the region, including the Absaroka and Big Horn mountains, the progressive nature of the system and marginal moisture values are expected to limit the potential for widespread heavier amounts. The probability of significant icing (0.25 inch or more) is less than 10 percent. Day 3... ...Central Rockies and High Plains... The previously noted trough will continue to dig south, with the models showing a sharp, positively-tilted upper trough centered over the Four Corners region by the period's end. Strong low level northeasterly flow along with low to mid level frontogenesis centered over the central Plains and Rockies will support snow showers extending south from the southeast Wyoming ranges along the Colorado Front Range and into the Sangre De Cristos Saturday night into Sunday. WPC probabilities are highlighting the potential for heavier amounts on the order of 8-inches or more along some of the eastern slopes. As the cold air deepens, rain changing to snow can be expected out across the High Plains, with significant accumulations extending as far east as western Kansas and the Oklahoma Panhandle -- where WPC probabilities indicate at least a Slight Risk for accumulations of 4-inches or more by 00 UTC Mon. The probability of significant icing (0.25 inch or more) is less than 10 percent. Pereira