Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 222 PM EDT Tue Apr 8 2025 Valid 00Z Wed Apr 09 2025 - 00Z Sat Apr 12 2025 ...Northern New England & Maine... Day 1... Low pressure tracking through the St. Lawrence River Valley will direct its strong cold front east across the remainder of New England this afternoon as a compact occluded low slides eastward across Maine this evening. This will prompt a healthy precipitation shield across northern New England and Maine until early Wednesday, when the system weakens and accelerates east over Nova Scotia. The heaviest snowfall rates (1-2"/hr) are expected through about 06Z Wednesday as a combination of favorable upslope northwesterly flow across northern New England and an inverted low-to- mid level trough develops across central Maine. Additional snowfall amounts of 3-6" are expected across the northern Greens of VT and Whites of NH/ME as well as a stripe through central ME towards the Interior Downeast. WPC probs for an additional >4" are 30-60% for these areas. ...Great Lakes & Interior Northeast... Days 2-3... A complex forecast for much of the Great Lakes from Wednesday into early Thursday, which could lead to a swath of light to locally moderate snowfall. As a 500mb shortwave trough approaches from the west on Wednesday, it will generate modest upper-level divergence at the same time as 850mb FGEN ensues over the Lower Great Lakes. There remains some subtle differences with the evolution of this trough and the interaction of several shortwaves, which could strengthen or weaken the 850mb FGEN. Even if it does snow as with the stronger solutions, marginal boundary layer and surface temperatures will make it tough for snow to accumulate efficiently. WPC probabilities are nonexistent for >4", but have increased for >2" across the southern MI L.P. to 10-30%. Additionally, persistent light precipitation on the northern periphery of the diving shortwaves could produce 1-4" snowfall amounts along the North Shores of Lake Superior. By Thursday night, an amplifying upper trough over the Ohio Valley will develop low pressure over the Carolinas. It will direct a plume of moisture northward into the Northeast that could be sufficiently cold enough to support mountain snow in parts of the Interior Northeast. There remains a fair amount of uncertainty regarding the QPF and if the boundary layer is also sufficiently cold enough to support snow. At the moment, WPC probabilities due show low chances (10-40%) for snowfall >2" in parts of the highest elevations of the Adirondacks, Green, and White Mountains. ...Northern Rockies, Cascades, and Olympics... Days 1 & 3... A series of shortwave troughs rotating east at the base of a longwave trough over the northeast Pacific will impose upper-level divergence and direct plumes of Pacific moisture at the Northwestern U.S. mountain ranges through Friday with a brief break Wednesday night. Snow across the Olympics and Cascades will continue tonight as IVT weakens and snow levels fall below 4,000ft. Meanwhile, Pacific moisture will also spill over into the Northern Rockies that results in measurable in mountains such as the Blue, Sawtooth, Bitterroots, Absaroka, Teton, and Lewis Range. Snow should taper off throughout the northwestern U.S. Wednesday afternoon, but look for another round of high- elevation (generally above 5,000ft) snow in the Olympics and Cascades Thursday night as another Pacific disturbance tracks north towards British Columbia. With a lack of a sufficiently cold/dry air-mass in place, look for most snowfall accumulations >6" to be confined to the higher/more remote peaks of the northwestern U.S. mountain ranges. Snowfall accumulations >6" are most likely at elevations >4,000ft in the Olympics and Cascades. Notable passes that could see measurable total snowfall include Snoqualmie and Stevens, where hazardous travel are possible. The snowfall in the Northern Rockies will be light-to-moderate with 1-4" of snowfall most common. Some peaks of the Lewis Range and Absaroka may eclipse 6" by the time snow concludes on Wednesday. WPC probabilities for ice accumulations above 0.1" throughout the CONUS are less than 10%. Snell/Mullinax