Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 513 AM EDT Thu Apr 15 2021 Valid 12Z Thu Apr 15 2021 - 12Z Sun Apr 18 2021 ...Northeast... Days 2-3... A late season Nor'easter will bring rain changing to heavy snow across primarily interior portions of New York and New England Thursday night and continuing into Friday night. A closed mid level low crosses southern New York and southern New England through Friday and then moves offshore by Saturday. Surface low development tracks north to southeastern New England, where movement is slow early Friday before the system moves offshore later Friday. Rain develops across the Northeast as Gulf-sourced moisture is lifted up the Eastern Seaboard. Rapid height falls and associated cooling aloft allow a changeover to snow for higher elevations of the Adirondacks/Catskills, Green Mountains,and northern Berkshires late tonight. Heavy snow is most likely in the terrain of the Berkshires, Adirondacks, Green, Monadnock, and White Mountains. Day 2 snow probabilities are moderate for 6 or more inches in these ranges. Probabilities are low in lower elevations of southern Maine, southern New Hampshire, and central MA but cooling within mesoscale bands may produce enhanced snowfall. Event totals of 1-2 feet are possible in the southern Green Mountains and into the White Mountains of NH. Day 2 cyclone tracks differ with the NAM on the southeast edge of the cluster of solutions, and the 00z ECMWF slower, which produces more QPF and snow potential in the Monadnocks of NH to the Worcester Hills of MA and adjacent areas. Significant QPF and temperature profiles and differences in the low level jet structure and resultant strongest convergence/lift still make this an uncertain forecast. The event ends on Day 3 as the cyclone departs east of New England. ...Central to Southern Rockies to Central High Plains... Days 1-3... Long duration heavy snowfall progresses south from the WY Rockies into Friday as deep layer ascent within a moist airmass moves across eastern Wyoming and northeast Colorado and into the Plains of western Nebraska to adjacent southwest South Dakota. A well defined upper divergence maxima combines with low level frontogenesis and theta-e advection/700 mb convergence to produce the lift. Snow is expected in the mountains with a changeover from rain to snow expected on the High Plains toady. The highest probabilities for 8 inches of snow are where the stronger 300 mb divergence is forecast near the NE/CO border, and also in the front range of northern CO. The mid-level low tracks east of the Rockies across the central Plains Friday, with ascent ending where it cold enough for snow. Continued confluent flow south of the upper trough supports both deep moisture and steep lapse rates in the ranges of CO on day 2. Persistent 700 mb convergence in the ranges of central Co aids in producing lift, and resultant snow. Probabilities are moderate for four inches and low for 8 inches with modest convergence following light winds though the sfc-600 mb layer. The upper trough helps develop a wave that approaches the Four Corners on Saturday, with upper divergence/700 mb convergence maxima downstream crossing the San Juans of southwest CO and northern NM, reaching the Sangre DeCristo Mountains by Saturday night. Deep layer moisture remains in place so even though the sfc-600 mb winds are light, the lift in the moist airmass is sufficient to produce periods of snow in these mountain ranges, with several inches expected. or the San Juans of CO/NM with moderate snow probabilities for 4 inches and low probabilities for 8 inches on Day 3. For Days 1-3, the probability of significant icing (0.25 inch or greater) is less than 10 percent. Petersen