Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 402 PM EDT Wed May 08 2019 Valid 00Z Thu May 09 2019 - 00Z Sun May 12 2019 ...The West... Days 1-3... A longwave trough across the west will become reinforced through Friday while taking on a deeply positive tilt as it closes off near California. In this alignment, moist advection will feature prominently as mid-level SW flow continuously transports Pacific moisture into the region as noted by anomalously high PWATs through the forecast period. At the same time, synoptic ascent will remain robust due to right entrance region diffluence and periods of PVA as pieces of vort energy swing through the trough and into the Four Corners region. While the synoptic ascent will be significant, it will also be aided by mesoscale lift due to frontogenesis along a boundary banked along the Front Range, and upslope flow as low-level winds become orthogonal to the Southern Colorado ranges, and later, the California Sierra. Snow levels will remain high in the Sierra through Saturday, but will lower to 6000-9000 ft in UT/CO/NM. This will generally confine the heaviest snowfall to the highest peaks of the San Juans, Sangre De Cristos, Colorado Rockies, and Sierra, where multiple days of heavy snow will produce widespread 1-2 feet, with over 2 feet possible in the Sierra, and more than 30 inches possible in the San Juans. On any given day, probabilities for 12 inches are moderate, highest on day 1 in the Colorado Rockies, and D2/D3 the San Juans and Sierra. ...Upper Midwest... Day 1... Weak low pressure beneath a shearing out shortwave will lift northeast into the eastern U.P. of MI by Thursday morning. This low will be accompanied by significant moist advection, and robust synoptic ascent through isentropic lift and exit region diffluence. While the low-level thermal profiles are initially too warm for snowfall, guidance has come into good agreement that cooling due to wet bulb effects and dynamics through strong 700-600mb frontogenesis will gradually transition the rain over to snow. Impressively, a stripe of frontogenesis collocated with some heavier rainfall currently on the regional radar mosaic will remain nearly stationary as it gets strung out to the northeast, likely producing a heavier band of snow along the northern Lake Shore of Superior. This will combine with intensifying mid-level deformation NW of the 700mb low center, and aided by moist advection on low level flow off the Lake. The HREF mean and WPC super-ensemble both suggest a maximum snow swath from near Duluth and into the Iron Ranges of the Arrowhead of MN, as well as in far NW WI where WPC probabilities now feature a moderate risk for 6 inches. Some of this snow in far NW WI, including the Bayfields Peninsula is likely to occur on the back side of the system as deformation and some subtle lake enhancement is possible later on day 1. Elsewhere, a few inches of snow are possible across the western U.P. of MI, with WPC probabilities for 4 inches exceeding 10 percent only within the higher terrain of the Keweenaw Peninsula. The probability of significant icing is less than 10 percent for Days 1-3. Weiss