Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 445 PM EDT Thu Apr 15 2021 Valid 00Z Fri Apr 16 2021 - 00Z Mon Apr 19 2021 ...Northeast... Days 1-2... A late season Nor'easter will bring rain changing to heavy snow across primarily interior portions of New York and New England tonight and Friday, pushing across Maine Friday night. A closed mid-level low currently near Lake Erie shifts east across PA/southern New England through Friday before occluding/stalling near the Gulf of Maine Friday night and shifting east Saturday morning. Rapid height falls with the cold core low allow a changeover to snow for higher elevations of the Adirondacks/Catskills, Green Mountains,and northern Berkshires by this evening with dynamic snow bands persisting north of the low over central New England late tonight through Friday morning. Heavy snow is expected in the terrain of the Berkshires, Adirondacks, Green, Monadnock, and White Mountains through Day 1 with the potential for 1 to 1.5 ft above about 3000ft (and high probabilities for 6 or more inches above about 1500ft). However, localized heavy snow is likely down to valley level starting late tonight and should result in some localized areas of 3-6 inches despite snow probabilities clinging to the terrain. The heavy snow focus shifts into Maine through Friday with some potential for the wrap around band of the occluded low to extend to the Maine coast down to central Mass, though there remains uncertainty with the Day 2 low track position and associated snow bands with the 12Z ECMWF/NAM farther east than the 12Z GFS/CMC/UKMET. ...Central Rockies and Central High Plains... Day 1... A positively tilted upper trough continues to bring heavy snow to the WY and CO WY Rockies and spreads heavy snow across the central High Plains into Friday as deep layer ascent within a moist airmass ahead of the trough moves east from CO. A well defined upper divergence maxima combines with low level frontogenesis and theta-e advection/700 mb convergence to produce the lift. Snow spreading east over WY into SD this afternoon will shift south over the Neb Panhandle and northeast CO (to the KS border) in this area of great convergence north of the surface low over ern NM/the TX Panhandle. The highest Day 1 probabilities for 8 inches of snow 50 to 60 percent under the stronger 300 mb divergence across the NE/CO border, and also on the eastern slopes of the Front Range of northern CO. ...Southern Rockies and Wasatch... Days 2/3... The upper trough helps develop a wave that approaches the Four Corners on Friday night, with upper divergence/700 mb convergence maxima downstream crossing the San Juans of southwest CO and northern NM, reaching the Sangre De Cristo 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. Day 1.5 snow probabilities for 6 or more inches are moderate in the southern Wasatch of UT, and moderate to high for the San Juans and Sangre De Christos of CO/NM centered on Days 2/2.5. ...Montana... Day 3... A shortwave trough with Arctic origins drops south over Alberta on Sunday, reaching Montana by Sunday evening. Rapid height falls provide ample lift with a continental airmass resulting in little moisture. Therefore, heavy snow should be limited to upslope areas of the MT Rockies including around and south of Glacier NP. Day 3 snow probabilities are low for 4 or more inches in these ranges, though more snow is expected across the eastern ranges of the northern Rockies through Sunday night, so stay tuned for further updates. For Days 1-3, the probability of significant icing (0.25 inch or greater) is less than 10 percent. Jackson