FOUS11 KWBC 070734 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 334 AM EDT Mon Apr 7 2025 Valid 12Z Mon Apr 07 2025 - 12Z Thu Apr 10 2025 ...Northern Great Lakes... Day 1... A progressive wave of low pressure tracking across the northern Great Lakes this morning will direct its strong cold front through the region with 850mb temps as cold as -16C (or below the 2.5 climatological percentile per ECMWF SAT) over Lakes Superior and Michigan. Brisk NWrly winds racing over Lakes Superior and Michigan and steepening low-level lapse rates will prompt the development of lake-effect snow bands over Michigan's U.P. and over the northern half of Michigan's Mitten. Lake effect snow streamers should taper off by Tuesday morning as a large dome of high pressure building in from the west. WPC probabilities show low chances (10-30%) for additional snowfall totals >2" across the eastern tier of Michigan's U.P. and in the northwestern section of Michigan's Mitten. ...Interior Northeast... Days 1-2... While some light snow may fall from northern Pennsylvania on east through the higher terrain of the interior Northeast, most snow will struggle to accumulate below 1,000ft in elevation. This initial round of snow looks to conclude by Monday afternoon. Meanwhile, the same area of low pressure that is responsible for the snow across the northern Great Lakes will direct its strong cold front east across western Pennsylvania Monday evening and towards the I-95 corridor by Tuesday morning. As the front passes through Monday night, there is the potential for snow squalls from the upper Ohio Valley on east through northern Pennsylvania, Upstate New York, and into central New England. Coverage and intensity of any potential snow squalls would have been worse had the cold front passed through during the daytime hours (peak surface based heating would steepen low-level lapse rates). However, the frontal passage coming at night should help to limit their coverage and severity with squalls being very localized. By early Tuesday morning, snow on the northern side of a departing storm system off the coast may reach the Maine coastline, while cyclonic flow on the backside of low pressure tracking through the St. Lawrence River Valley in Quebec results in lake effect snow band development throughout the day. In addition, a low-level TROWAL with modest 850mb WAA over southern Ontario will foster a narrow deformation zone of snow that tracks across northern New York Tuesday morning. Lastly, the heaviest snow is likely to unfold from the White Mountains on north through northern Maine thanks to an initial front-end thump of snow via WAA. Then as the 850mb low tracks through the region, the aforementioned deformation zone will work its way through the region through Tuesday afternoon. Lingering upslope flow on the western flank of the storm is also likely to linger through Tuesday evening before finally tapering off by Wednesday morning. WPC probabilities show moderate-to-high chances (50-70%) for snowfall totals >6" in the peaks of the Green Mountains of northern Vermont, the Whites of northern New Hampshire and far western Maine, and through much of northern Maine. There are some low- chance probabilities (10-30%) for localized amounts >12" in northern Maine, but this is most likely to occur in the highest terrain. Farther west, there are low-to-moderate chance probabilities (30-70%) for >4" of snow in the Tug Hill and Adirondacks. The WSSI shows Minor Impacts in the Tug Hill, parts of the Adirondacks, Green, Whites, and northern Maine. Potential impacts are likely to include snow covered roads and reduced visibilities. ...Pacific and Interior Northwest... Days 1-2... A pair of shortwave troughs rotating beneath the base of a longwave trough in the northeast Pacific will impose upper-level divergence aloft and direct plumes of Pacific moisture at the Northwestern U.S. mountain ranges. Snow will increase in intensity over the Olympics and Cascade Ranges Monday afternoon and continue into the day on Tuesday. Tuesday morning is most likely to see the heaviest snowfall as the nose of a >400 kg/m/s IVT is aimed directly at the Pacific Northwest coast. Meanwhile, some Pacific moisture will stream as far inland as the Northern Rockies, leading to some measurable snow in mountain ranges such as the Blue, Sawtooth, Bitterroots, Absaroka, Teton, and Lewis Range. Snow rates are forecast to diminish starting Tuesday night and should conclude some time Wednesday morning as upper-level ridging strengthens over the western U.S.. With a lack of a sufficiently cold/dry air-mass in place, look for most heavy snow to ensue within higher/more remote peaks of the northwestern U.S. mountain ranges. Snowfall accumulations >6" are most likely in the Olympics above 4,000ft, the Washington Cascades above 5,000ft, and the volcanic peaks of the Oregon Cascades. Notable passes in Washington that could see as much as 3-6" of snow are Snoqualmie and Stevens Passes, which could make for some hazardous travel conditions for those commuting over the Cascades. The snowfall in the Northern Rockies will be light-to-moderate with snowfall of 1-4" most common. Some peaks of the Lewis Range, the Sawtooth, Blue, and Tetons may eclipse 6" by the time the snow concludes early Wednesday morning. WPC probabilities for ice accumulations above 0.1" throughout the CONUS are below 10%. Mullinax $$