Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 347 PM EDT Tue Oct 23 2018 Valid 00Z Wed Oct 24 2018 - 00Z Sat Oct 27 2018 Day 1... ...Northern New England... Significant early season winter storm will bring periods of heavy snow to New Hampshire and Maine tonight and Wednesday. Upper low diving through the Great Lakes will induce cyclogenesis at the surface which will then deepen while lifting northeast towards New Brunswick, Canada. Guidance has become more progressive with this upper feature, leading to a later northern push of the surface low as the upper trough tilts negatively, and most guidance has shifted south and east with its heaviest snowfall axis. Despite modest upper support and late strengthening of the surface low, strong mesoscale features will likely produce periods of heavy snowfall very late tonight into Wednesday afternoon. An overlap of strong mid-level frontogenesis and intense 700mb deformation NW of the mid-level center into the dendritic growth zone suggests a band of heavy accumulation is likely. A trowal noted at 600mb will also overlap this other robust forcing, further enhancing snowfall potential. Boundary layer temperatures will be marginal keeping snow-to-liquid ratios modest, but dynamic cooling of the column and snowfall rates of up to 1"/hr should overcome the otherwise unfavorable environment for snow accumulation. WPC probabilities show a high risk for 4 inches of snow across central Maine and into the White Mountains of New Hampshire, with 8 inches or more possible in isolated locations across central Maine. Depending on how the low pivots to the north on Wednesday, there is the potential this deformation band will shift south towards the coastal plain and an inch or two of snow is possible nearer to the coast. WPC probabilities are low for this evolution however. ...Central Rockies... A broad 500-700mb trough will advect eastward across the Four Corners region tonight and Wednesday. Ahead of this feature, moist southerly 700mb flow will combine with weak PVA and height falls to produce lift across much of the Colorado Rockies, as well as the mountains of Wyoming and New Mexico. Although the column is warm, the highest terrain will likely receive moderate snowfall, and WPC probabilities are high for 4 inches of snow across the San Juans, Rockies, and Sangre De Cristos, with 8 inches or more possible. Elsewhere above 9000 feet, a few inches of snow is possible in the Wind River and Teton Range of Wyoming. The probability of significant icing (0.25 inches or more) is less than 10 percent. Day 2... ...Northern Rockies... Mid-level shortwave moving eastward across southern Canada will be accompanied by a jet max pushing through the northern Rockies. Moisture will pool ahead of this feature, and diffluence within the left exit region will drive enough lift to produce snow showers across the high terrain of Wyoming and Montana. WPC probabilities show a slight risk for 4 inches of snow across the Big Horn and Absaroka Ranges. The probability of significant icing (0.25 inches or more) is less than 10 percent. Day 3... The probability of significant snow (4 inches or more) is less than 10 percent. The probability of significant icing (0.25 inches or more) is less than 10 percent. Weiss