Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 407 PM EDT Sat Apr 27 2019 Valid 00Z Sun Apr 28 2019 - 00Z Wed May 01 2019 ...Midwest to Northern Mid-Atlantic... Day 1... A clipper low will continue to shift east from the Midwest and across the northern Mid-Atlantic tonight into Sunday. Low level frontogenesis will continue along the north side of the low with enough dynamics to create a band of locally heavy snow despite the surface low pressure slowly filling from the minimum this afternoon of 999mb along the IA/MO border. Dry advection from the northeast will continue lower near-surface wet bulb temperatures, so as precipitation falls additional column cooling will occur. Furthermore, we are within two months of the most intense sunshine, so the snow will stick more starting this evening after having difficulty today. The swath of snow shifts east from Chicago early this evening, crosses southern MI and then along the NY/PA border. Moderate probabilities for four inches are in these areas for Day 1. ...Northern Rockies to Lake Superior... Days 1-2... Low pressure shifts southeast from the WA/BC border and into MT tonight before shifting east to the Arrowhead of MN through Monday. Snow elevations drop below 2000ft over MT tonight and northern ND/MN on Sunday. A deepening surface low over the northern Plains and a 1030mb high shifting south from Alberta Sunday will allow a tight pressure gradient, hence the blizzard warnings. Moderate probabilities for six inches are along the northern MT border and the northwestern quad of ND for Day 1 with moderate probabilities for four inches over the Pembina Escarpment of northeastern ND and the Arrowhead of MN on Day 2. ...Central Rockies and Central High Plains... Days 2-3... A closed low moves across southern CA Monday and open as it moves across the southern Rockies on Tuesday. A northern stream shortwave trough shifts southeast from the Pac NW on Tuesday setting up a large trough over the Intermountain West that is a major weathermaker into the middle of next week. A plume from the tropical Pacific and an open Gulf of Mexico provides ample moisture to the system with PWAT anomalies to +2 to +3 standard deviations above the mean across the Central and Southern Rockies Monday. At the same time, a 140kt jet streak over the northern Plains will place CO in the right entrance region, all while a cold front drops southward and banks against the Front Range producing easterly low-level flow. All the ingredients appear to be in place for a significant snow event late Sunday and through Monday for the Rockies into the upper plains as this robust lift takes place in an anomalously moist environment. Snow levels will have a tight gradient from NE to SW, with moderately high probabilities for 12 inches on Day 2.5 in terrain above 6000 ft in the San Juans and Colorado Rockies, with upslope enhancement favoring the Front Range just west of Denver for the highest totals. Further east into the Plains, the jet streak will likely produce a swath of snow across northeast CO and southern WY where there are moderate Day 2.5 WPC probabilities for 6 inches with moderate Day 3 probabilities for four inches over the NE Panhandle. The probability for significant icing is less than 10 percent all 3 days. Jackson