Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 330 PM EST Mon Jan 6 2025 Valid 00Z Tue Jan 07 2025 - 00Z Fri Jan 10 2025 ...Mid-Atlantic... Day 1... Trailing mid-level shortwave behind the outgoing major winter storm will pass quickly through the Mid-Atlantic early this evening, spurring another inch or two of snow east of the mountains. Over central WV, some upslope enhancement will help squeeze out perhaps a few inches, with WPC probabilities for an additional 4 inches of snow after 00Z around 30-50%. Snow will taper off there by Tuesday afternoon. ...Great Lakes and Northeast... Days 1-3... A large, sprawling, and multi-pronged circulation over southeastern Canada will send mid-level shortwaves into the eastern Great Lakes and Northeast over the next few days, maintaining high chances of at least light snow over the higher elevations (NY and northern VT/NH) and also downwind of the Great Lakes in general owing to the broad cyclonic flow. For D1, a compact closed low near Montreal this evening will swing through northern NY/VT overnight with some terrain enhancement over the northern Adirondacks and into the Green Mountains. WPC probabilities for at least 4 inches of snow tonight through Tuesday are 50-80% there. By D2, with already cold 850mb temps below -12C or so, a reinforcing shot of cold Canadian air (via another short wave out of Canada) will steepen lapse rates and reinvigorate lake effect snow off the western Great Lakes, especially along the western shore of Lower Michigan, and also off Lakes Erie/Ontario on NW flow. WPC probabilities for at least 4 inches of snow D2 are highest south of Muskegon right along the lake short, but will be dependent on the fetch and trajectory of the band. For D3, one last shortwave will push through the eastern Great Lakes and New England, maintaining additional light snow off Lake Ontario on NW flow and additional snow for the northern Adirondacks and Green Mountains. WPC probabilities for at least an additional 4 inches of snow are moderate (40-70%) over these areas. ...Intermountain West, Rockies, and Southwest... Days 1-3... Broad height falls moving through the Great Basin tonight will sharpen into a closed low over the lower CO River Valley late Tue. With high pressure nosing down out of the northern Plains, upslope snow will expand across the CO Front Range via an easterly low level flow along with a relatively low DGZ. However, the snow should not last too long as the upper low will likely move into northwestern Mexico by the end of the period. This will help to drag the snowfall southward as well through the Mogollon Rim, Sangre de Cristos, Sacramento Mountains, etc. Models continue to vary on QPF (and thus snow) over parts of the Southwest but at least a large footprint of light snow is expected. For the D1-3 period, WPC probabilities for at least 4 inches of snow are >50% over the Front Range into the CO Rockies, San Juans, and higher elevations in AZ/NM above 8000ft or so. Farther north, a shortwave moving out of Canada through Montana on Wednesday will touch off some light to locally moderate snow over much of the state that will push into Wyoming and the Black Hills on Thursday. WPC probabilities for at least 4 inches of snow are highest (>50%) over the Little Belts, Gallatin Range, and into the Bighorns. ...Southern Plains... Day 3... As the aforementioned upper low closes off over the Southwest Thursday and continues to dig into northwestern Mexico Friday it will become anomalously deep (below the 0.5th climate percentile per the NAEFS ESAT). Meanwhile, a separate shortwave rounding the far eastern Pacific ridge is forecast to dive southward through Utah and interact with this feature and spawn strong southwest flow containing rich moisture and prominent upper divergence from an increasingly buckled jet. Given the strong high situated to the north over the Central Plains and a cold low-level airmass in place, snow and mixed precipitation is expected over much of western and north-central TX late Wed and through Thursday, expanding eastward to the Ozarks by the end of the period. Thermal uncertainty is high (as expected) due to the upstream players (timing, depth, etc.) and the NBM ptype probabilities show a zone of maximum uncertainty from near the Big Bend northeastward to around TexArKana. All ptypes are in play -- snow/sleet/freezing rain and plain rain to the south -- and each will likely not be stagnant in location. For now, the highest probabilities of snow lie on the northwest side of the precipitation shield in the deeper colder air from the Permian Basin across Northwest TX into North TX (near and northwest of the D-FW Metroplex). There, WPC probabilities for at least 4 inches of snow are around 30-50%. To the southeast, within the zone of maximum uncertainty, probabilitiesfor accumulating snowfall decrease while probabilities for ice increase. Areas from near Austin northeastwardthrough the Piney Woods into the Hill Parishes of LA show the highest chance of at least 0.1 inches of ice accumulation (30-60%). This system has prompted the issuance of a new set of Key Messages which are linked at the bottom of the discussion. Fracasso/Snell ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png