Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 305 PM EST Tue Jan 28 2025 Valid 00Z Wed Jan 29 2025 - 00Z Sat Feb 01 2025 ...Great Lakes, Northeast, Mid-Atlantic... Days 1-3... ...Snow squalls racing across the Great Lakes tonight, then through the Northeast on Wednesday may lead to hazardous travel... A progressive area of low pressure will track through the Upper Great Lakes this evening with both its warm front and cold front featuring prominent roles in the development of heavy snow over the next 24-36 hours. Regarding the warm front, modest 925-700mb WAA aloft will overrun the sufficiently cold air-mass in place from western NY on east through the rest of Upstate NY this evening. This favorable WAA configuration aloft will support isentropic ascent as far east as the New England coastline by early Wednesday morning, while at the same time, the region resides beneath the diffluent left-exit region of a 140kt 250mb jet streak. The areas favored to see the heaviest snowfall through Wednesday are the Tug Hill and Adirondacks which not only reside beneath the best synoptic and mesoscale atmospheric drivers aloft, but WSWrly 40-50kt 850mb winds will also favorably upslope into these areas, thus maximizing snowfall rates would support >1"/hr snowfall rates tonight. Look for the White Mountains, Green Mountains, and Berkshires to also witness periods of heavy snow tonight and through Wednesday via the same processes as their NY neighboring mountain ranges. WPC probabilities show moderate-to-high (50-70%) chances for >12" of snow through Wednesday evening with similar probabilistic chances exist over the Adirondacks for >8" of snow. The taller peaks of the Berkshires, Green, and White Mountains sport moderate chances (40-60%) for snowfall totals >6" through Wednesday evening. The improving FGEN along the New England coast Wednesday morning, combined with the aforementioned left-exit region of the 250mb jet streak overhead, will help spawn a new coastal low off the MA Capes. Easterly flow north of the low will tap into Atlantic moisture that in turn allows for periods of snow throughout much of southern Maine and Downeast Maine. The progressive nature of the storm system will keep periods of heavy snowfall short in duration, but there are low chances (10-30%) for snowfall totals >4" along the immediate Maine coastline. Meanwhile, on the western flank of the low traversing the Great Lakes tonight, an Arctic front will spark snow squalls over northern MN that will race southeastward this evening through the Great Lakes. By Wednesday morning, the front will head for the eastern Great Lakes. Out ahead of the front, daytime heating will help to steepen low-level lapse rates while 850mb winds, ranging between 40-50kts, are above the 90th climatological percentile. Areas most at risk for snow squalls stretch from northeast OH and much of northern PA through Upstate NY and into central New England. At the same time, periods of LES are forecast across Michigan's U.P. and the northern tip of Michigan's Mitten. Even as the snow squall threat winds down Wednesday evening, LES will continue in northeast PA and west-central NY as brisk low-level NWrly winds continue. LES bands should finally taper off by Thursday morning as NWrly flow weakens. Day 1 WPC probabilities show moderate chances (40-60%) for >4" of snowfall in the central and eastern Michigan U.P., as well as localized northern areas of Michigan's Mitten. For more on the potential snow squall impacts, please visit our latest Key Messages below. Following a brief reprieve Wednesday night and into Thursday morning, the next round of unsettled weather arrives Thursday night as a closed 500mb low over the Central Plains directs a fetch of precipitation at the Mid-Atlantic. There remains modest uncertainty in timing of the precipitation's arrival, how far north QPF can reach into New England, and how much of a sub-freezing air-mass remains as the storm's precipitation shield arrives on Friday. That said, the mountainous terrain of the Northeast likely sports the best chances for measurable snowfall late Friday, while valleys and coastal areas will not be as favored to see heavy snow given the lack of a sufficiently cold air-mass by the time the storm arrives on Friday. ...Southwest through the Four Corners... Days 1-2... A large closed upper-low located over the Southwest is rather anomalous for the time of year, sporting 500mb heights as low as the 2.5 climatological percentile near the northern Gulf of California coast. Temperatures are also rather cold by this time of year's standards with 500-700mb temps generally below the 2.5 climatological percentile according to NAEFS over much of AZ tonight and over western NM during the day on Wednesday. There is just enough 700-300mb moisture aloft, combined with favorable 250-500mb diffluence aloft ahead of the closed low and modest upslope ascent to support heavy snow in mountain ranges such as the San Juans, the Mogollon Rim, the Gila Mountains, and as far east as the Sangre De Cristo. The heaviest snowfall will reside in the San Juans tonight through Wednesday, then late Wednesday into Thursday in the Sangre De Cristo and as far north as the Palmer Divide as easterly upslope flow helps with dynamic cooling of the atmospheric column and supports heavier snow rates. Lower elevation snowfall will be harder to come by given the lack of a cold continental polar (cP) air-mass, but minor accumulations (<3") are possible in the central High Plains. The tallest peaks of the San Juans and Sangre De Cristos sport moderate-to-high chances (50-70%) for >8" of snowfall with localized areas likely surpassing a foot by the time the event concludes Thursday afternoon. The Palmer Divide does sport low-to-moderate chances (30-50%) for snowfall totals >4", but at elevations <6,000ft, probabilities are generally <20%. Still, minor accumulations of 1-3" could result in slick travel conditions for travelers along the I-25 corridor on Thursday. ...Pacific Northwest... Day 3... Multi-vort mid-level trough will approach the PacNW early Friday with downstream moisture advection (IVT >90th percentile) and rising snow levels (from ~2500-4000ft up to 3500-5500ft north to south through the WA/OR Cascades). This will favor snow in the higher elevations, mostly above pass level, especially as the WAA drives in higher QPF. Guidance differs on how much snow levels will rise, with a spread of 2500ft or so (near and higher than Snoqualmie). For the higher elevations above 4000ft, WPC probabilities for at least 8 inches of snow are high (>70%) through 00Z Sat (with additional precipitation thereafter). A heavy and wet snow can be expected around the oscillating snow level, which is shown in the WSSI-P snow load parameter. For the Days 1-3 period, the probability of significant icing greater than 0.10" is less than 10 percent. Mullinax/Fracasso ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png