Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 358 AM EDT Mon Oct 29 2018 Valid 12Z Mon Oct 29 2018 - 12Z Thu Nov 1 2018 Cascades to the northern Rockies -------------------------------- A colder weather pattern has returned to the Pacific Northwest and the northern Rockies as a large scale upper trough has moved in from the Gulf of Alaska, in the wake of a strong cold front. Snow levels have dropped into the 4000-5500 foot range for the Cascades, and about 5500 to 7000 feet for the northern Rockies Sunday morning. Moist onshore flow orthogonal to the Cascade range will support a few inches of snow for the Washington Cascades during the day 1 period on Monday. Greater mid-upper level forcing over the northern Rockies, combined with orographic forcing, will likely lead to some 4+ inch amounts on day 1 across parts of the Bitterroots and the ranges of northwest Wyoming. A second and stronger jet streak is progged to reach the Pacific northwest by Tuesday morning. Moist and confluent low level flow under the jet produces bands of enhanced moisture and lift from the northern Washington Cascades and then over the ranges of Idaho and northwest Montana on Tuesday. This will likely result in an additional round of accumulating snow for the higher elevations of the Cascades and northern Rockies. By Tuesday night into Wednesday, the well defined upper level jet with divergence aloft crosses from southern British Columbia through northeast Washington, Idaho, western MT, and Wyoming, with heavier snow for the Bitterroots and the mountains of northwest Wyoming on Wednesday, with WPC probabilities being moderate for at least 4 inches of snow for these areas. Colorado and New Mexico ----------------------- Heavy snow is likely for the Front Range of the Colorado Rockies and extending southward into northern New Mexico during the Day 2 period Tuesday night and continuing into Wednesday morning. A pronounced southern stream shortwave is progged to dive southeastward from the Pacific Northwest (currently over the northeast Pacific) and reach northern New Mexico by 12Z Wednesday morning, preceded by significant mid-level height falls. There has been a more amplified trend with this shortwave trough over the past several model cycles. This will likely be combined with upper level divergence situated over low level frontogenesis, providing forcing for deep layer ascent. A strong cold front is forecast to move through this region early on Tuesday with surface cyclogenesis ensuing over the central plains by Tuesday afternoon. Cool and moist low level northeast winds developing north of the boundary, with PWs near 0.5 inch over eastern Colorado, will create a favorable upslope flow over the Palmer Divide and the foothills, and extending across the Front Range to include the Sangre de Cristo range. These are all favorable parameters for early season snow, and numerous GEFS and EC ensemble members are indicating the potential for 8 to locally 12 inches of snow. WPC probabilities are moderate for 8+ inches of snow, and low to moderate for 12 inches. The probability for significant icing (0.25 inches or more) is less than 10 percent for all three days. D. Hamrick