Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 410 PM EDT Sun Oct 28 2018 Valid 00Z Mon Oct 29 2018 - 00Z Thu Nov 01 2018 ...Days 1, 2 and 3 for the WA/OR Cascades and Northern Rockies... On day 1 (Sunday night-Mon), An upper level shortwave progresses across the Pacific northwest to the Northern Rockies. Steep lapse rates follow with the passage of the cold front, with snow levels falling in the Washington Cascades and Oregon Cascades. Upslope enhancement is expected owing to the steady onshore low level flow from the Pacific. As the moisture plume reaches the northern Rockies by Sunday night and Monday morning, several inches of snow is likely from the Absaroka Mountains to the Bitterroots with decent lift within the dendritic growth zone and left exit jet dynamics in place. WPC probabilities are moderate for 4 inches of snow for the higher mountains of the Cascades and Salmon River Mountains/Boise Mountains to the Absaroka Range. On day 2 (Monday night and into Tuesday), this northern stream upper trough departs the Rockies and crosses the northern plains. The jet crossing into CO leads to a period of light snow for the front range of central CO. 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 during the Day 2 period Monday night into early Tue. On day 3 (Tue night into Wed), the well defined upper level jet with embedded upper divergence maxima crosses from southern British Columbia through northeast WA, ID, western MT to adjacent WY. The duration of high 700 mb RH is limited with early low values but another moisture surge and increase in 700 mb lift moves across WA into ID, western MT and adjacent northwest WY. Light to moderate snow should develop, with potential for several inches in favored upslope areas. ...Day 3 for 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 3 period Tuesday night and into Wednesday. A strong cold front is forecast to move through this region on Tuesday with surface cyclogenesis near the CO/NM border Tuesday. Cool and moist low level east 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/Front Range, including the Sangre de Cristo mountains. A pronounced southern stream upper level shortwave crosses CO and NM, preceded by a coupled upper jet maxima that results in enhanced upper divergence across southeast CO and northern NM Tue night into early Wed. The combined lift from low level frontogenesis and upslope flow and upper divergence favors heavy snow, and numerous GEFS and EC ensemble members are indicating the potential for 8 to 12 inches of snow. WPC probabilities are moderate to high for 8+ inches of snow, and low to moderate for 12 inches Tue night to Wed. The probability for significant icing (0.25 inches or more) is less than 10 percent for all three days. Petersen