Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 444 AM EDT Wed May 08 2019 Valid 12Z Wed May 08 2019 - 12Z Sat May 11 2019 ...The West... Days 1-3... A northern stream trough is now over UT with a reinforcing trough making a more positively tilted longwave trough as it digs to CA through Thursday. This first shortwave is accompanied by a departing jet max leaving right entrance region diffluence across the Rockies, while a front banked against the Front Range both increases low-level frontogenesis but also serves to drive easterly upslope flow into WY and CO. These features together will continue to produce heavy snow south from WY and down the CO Rockies through today. Snow levels will decrease under the trough 5000 ft over WY and 8000 ft near the Four Corners. Above these elevations, heavy snow is likely with the highest 12 inch probabilities today over the CO Rockies. Enhanced mesoscale ascent combined with upslope flow due to the orthogonal nature of the low-level winds, will produce an environment favorable for persistent heavy snow Thursday into Friday, especially in the Colorado Rockies, Sangre De Cristos, and San Juans. Snow levels will be quite high, 7000 to 8000 ft, but above these levels WPC probabilities are low to moderate for 12 inches in all of these ranges, and some locations in the San Juans may approach 30 inches by Saturday morning. Additionally on Friday, snow will increase across the highest terrain of the Sierra Nevada in California as heights fall beneath the upper trough and moist advection intensifies. WPC probabilities here reflect a moderate risk for 8 inches above 9000 ft on day 2. ...Upper Midwest... Day 1... Southern stream energy is rounding into a northern stream trough over the southern high plains with a surface trough gathering strength as it heads toward the cold air in place around Lake Superior. This particularly late season storm will need to rely on elevation and nocturnal timing for snow accumulation. Hi-res and NAM guidance are too strong with dynamic cooling with a stripe of snow from SD to western Lake Superior. Low-level thermal profiles are marginal, but robust ascent through isentropic lift, left exit region jet level diffluence, and increasing 700-600mb frontogenesis will at times allow the column to dynamically cool enough to produce snow. Preference was with the ECMWF which is back a bit farther east. That said the elevation of the Arrowhead warrants the highest probabilities for four inches (which are moderate). Will also note the potential for lake enhanced snow on the back side of the low late Wednesday night into Thursday where an additional inch south of Lake Superior is possible. The probability of significant icing is less than 10 percent for Days 1-3. Jackson