Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 340 PM EST Mon Dec 30 2019 Valid 00Z Tue Dec 31 2019 - 00Z Fri Jan 03 2020 ...Great Lakes... Days 1-2... A deep upper-level/surface low crossing the Great Lakes tonight through Tue is forecast to produce a large area of light to moderate snow, with embedded heavy snows in favored areas of lake effect enhancement. The heaviest snows, likely exceeding 6 inches, are forecast across portions of the Upper Peninsula of Michigan downwind of Lake Superior, and across western Lower Michigan, downwind of Lake Michigan. As the upper low moves east on Tue, expect lake effect snows to ramp up and continue into Tue night downwind of Lake Erie and Lake Ontario, where amounts exceeding 6 inches are possible (snow totals may exceed a foot across portions of the Tug Hill Plateau). Snow across the Great Lakes should quickly wind down on Wed as the upper low moves northeastward away from the region, and low-level winds back to a more southerly direction. The probability of 0.25 inches of icing is less than 10 percent Days 1-3. ...Northeast... Days 1-2... Warm advection precipitation across the Northeast this afternoon will become enhanced tonight as a developing coastal low moves along the New England coast, nearing Boston or Cape Cod by Tue morning and paralleling the coast of Maine during the day Tue. Prior to coastal low development, pockets of accumulating freezing rain are still possible across portions of the Catskills, Berkshires, Adirondacks, and Green Mountains early tonight. The freezing rain threat should quickly wane by late evening, however. Latest model guidance shows the potential for a decent band of frontogenetic forcing across interior northern New England overnight, particularly from northeastern New Hampshire across much of central and northern Maine. Impressive upper dynamics also work in favor of a band of enhanced snow, with the region lying in the left exit region of an approaching upper-level jet streak and strongly diffluent flow aloft due to a slight negative tilt to the approaching upper trough. Snow accumulations exceeding 8 inches are expected to be fairly widespread across the aforementioned areas in this band of heavy snow, a few locations may even approach a foot before snow begins to taper off late Tue into early Wed. The probability of 0.25 inches of icing is less than 10 percent Days 1-3. ...Pacific Northwest through the Central Rockies... Days 1-3... Confluent flow setting up over the northern Pacific will drive high PWATs into the Northwest, while mid-level moisture pooling and convergence on the nose of the 700 mb jet/in the left exit region of a 170 kt 300 mb jet support onshore flow and precipitation across the ranges of WA/ID/northwest MT starting late tonight and continuing thru Wed. Heavy snow is likely in the northern WA Cascades at higher elevations, with a few feet expected. By Wed-Thu as an upper shortwave and associated cold front move inland, the best moisture and lift should move southeast across the OR Cascades inland through southern ID and western WY, followed by northern NV, northern UT, and then northwest CO. Several inches are expected across the impacted ranges, with higher amounts expected due to longer duration snow in the WA Cascades and Bitteroots of ID. Probabilities are high for 12 inches in the highest terrain of the Cascades and Bitterroots through day 2 and then in the WY Tetons on Day 3, with lower probabilities forecast across the other ranges. The probability of 0.25 inches of icing is less than 10 percent Days 1-3. Ryan