Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 348 AM EDT Fri Apr 26 2019 Valid 12Z Fri Apr 26 2019 - 12Z Mon Apr 29 2019 ...Northern Plains to the Great Lakes... Days 1-3... A brief but intense snow event is becoming more likely with heavy snow spreading from the western Dakotas through southern Minnesota and into the L.P. of Michigan today through Saturday. Small but potent shortwave will combine with an increasingly coupled upper jet structure to drive cyclogenesis from Nebraska to Indiana Saturday, before shifting rapidly off the Mid-Atlantic coast Sunday. North of this low, several ingredients combine to produce a stripe of heavy precipitation despite a short duration event. Ascent will be intense through both synoptic and mesoscale processes, with coupled jet diffluence, height falls, isentropic lift, a TROWAL, and intense frontogenetical circulation all contributing to the robust omega. Although the low-level thermal profile will initially be marginal, it is likely that dynamic cooling will quickly transition rain to snow, with rapid accumulation likely as noted by moderate to high HREF probabilities for 1-2"/hr snows. These rates are enough to overcome late April sun angle, and unfavorable time of day, especially across IA/WI which will experience the heaviest snow Saturday afternoon. While guidance still features some discrepancy in placement of the low and heaviest snow axis, there is better agreement and this leads to increasing confidence in placement of the heaviest snow. This is reflected in a moderate risk for 8" of snow in the WPC probabilities from far eastern SD southeast towards Lake Michigan at the WI/IL border. Note that the WPC superensemble has a large spread in accumulations, and some locations may receive over 12" where the intense snowfall rates persist the longest, despite a relatively short duration event overall. A second intense shortwave will follow quickly behind the first, dropping into MT/ND Sunday into Monday. Renewed surface cyclogenesis is likely, and with cold air in place across the northern tier CONUS, heavy snow may spread southward into North Dakota by the end of day 3. Forcing looks to be robust with this system as well as a potent jet max places the LFQ diffluence in collocation with strong PVA to produce heavy snow, and WPC probabilities for 4 inches are high on day 3 across northern ND. ...Northern and Central Rockies... Days 1-3... Modest ascent within the diffluent portion of a jet streak shifting eastward into the Plains today and Saturday will tap Pacific moisture to produce moderate snow in the terrain from the Washington Cascades into the Northern Rockies and southward into the Colorado Rockies. This forcing is expected to be transient as the jet shifts quickly to the east, and WPC probabilities feature a moderate risk for 4 inches on the windward slopes along the Rocky Mountain chain where upslope enhancement is favored on westerly 700mb flow. The secondary shortwave noted above on day 3 will drive a surface low into Montana late Sunday and Sunday night. Strong ascent due to jet level diffluence, height falls, and PVA will support precipitation spreading across the region Saturday night through Sunday night. Broad cyclonic flow south of this feature will produce snow above 9000 ft in the Colorado Rockies, especially on Sunday, but the heaviest snow is likely in the mountains of WY/ID/MT where snow levels are much lower and forcing is more robust. Here, WPC probabilities moderate for 6 inches, with snow levels falling to 2000-3000 ft behind the eastward shifting low on Sunday. The probability for significant icing is less than 10 percent all 3 days. Weiss