Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Wed Oct 17 2018 Valid 12Z Wed Oct 17 2018 - 12Z Sat Oct 20 2018 Day 1... ...Southern and Central Rockies... A closed mid-level low across AZ will begin to open into a positively tilted trough and gradually lift northeast into Utah by the end of Day 1. Lift associated with this feature due to jet level diffluence and weak mid-level vorticity will interact with moisture being advected into the Rockies on S/SE 700mb flow. Upslope enhancement is likely, especially into the San Juans of Colorado and the Wasatch of Utah, with the best moisture/lift combination expected across the former. Snow levels will remain high, above 7000 feet, and while some discrepancy still exists in the guidance into where the strongest lift will occur, it appears the San Juans will be favored for the heaviest snow where upslope is most intense. Across these mountains, WPC probabilities show a high risk for 4 inches of snow or more, with 8 inches or more possible in the highest terrain. Elsewhere, WPC probabilities are moderate for 4 inches. ...Great Lakes... Upper trough centered across the Hudson Bay will push eastward through Day 1 while a shortwave digs across the Great Lakes. This shortwave will initiate cold air advection across the Lakes, producing enough instability to create lake effect snows downwind. Unidirectional winds from the W/NW will support lake effect precipitation SE of most of the Lakes, but the heaviest precipitation is likely east of Lake Ontario where moisture from Lake Huron will combine with the fetch, and be enhanced by upslope into the Adirondacks and Tug Hill Plateau. Temperatures will be marginal however, so snow is expected primarily above 1000 feet, and WPC probabilities are only mentionable for 2 inches of snow. ...New Mexico... Shallow cold air will remain across New Mexico early Wednesday before warm advection erodes the shallow cold layer. Overrunning precipitation will fall into a sub-freezing surface layer evident on area soundings to produce some light freezing rain accretion across the Sacramento Mountains. A few hundredths of an inch are possible before the column warms too much to support freezing rain during the afternoon. Day 2... ...Mountains of Utah and Colorado... Mid-level positively tilted trough will fill and eject eastward into the Plains during Day 2. Moisture and lift will combine early across the Mountains to produce snow showers, before dry advection and warming temperatures bring an end to the snow. The longest duration of snowfall will be across the San Juans where WPC probabilities are moderate for 4 inches of snow. Elsewhere across the high terrain of Utah, including the Wasatch Range, a few inches of snow are possible above 9000 feet. 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