Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 313 AM EST Thu Jan 16 2025 Valid 12Z Thu Jan 16 2025 - 12Z Sun Jan 19 2025 ...Great Lakes & Central Appalachians... Days 1... A potent 500mb trough diving south through the Great Lakes this morning will foster healthy upper level divergence over the Northeast and Central Appalachians today while also directing plume of 700-300mb layer-averaged moisture through these regions. In addition, the brief shot of low-level CAA will trigger some lake-effect snow showers today with the Chautauqua Ridge sporting moderate-to-high chances (50-70%) for snowfall totals >4" through Thursday evening. The Central Appalachians are most favored for heavy snowfall totals given their favorable position beneath the diffluent left-exit region of a 100kt 500mb jet streak and upwards of 40kt westerly winds at 850mb aiding in healthy upslope snowfall. Snow looks to fall heaviest beginning late morning and lasting through the afternoon with rates topping 0.5"/hr in some cases. In fact, given the steepening lapse rates Thursday afternoon over the Mid-Atlantic, residents in the region should be on the lookout for possible snow squalls given the favorable time of day and more than adequate upper-level divergence aloft. Snow in the Central Appalachians looks to continue Thursday night and finally tapers off by Friday morning. WPC probabilities depict high chances (>70%) for snowfall totals >6" from the Laurel Highlands of southern Pennsylvania and near the MD/WV border to the Allegheny Highlands in West Virginia. Some of the taller peaks of eastern West Virginia could see totals around a foot of snow by Friday morning. Day 3... By Friday night, a deepening area of low pressure will escort a strong Arctic front south that will deliver a frigid air-mass that is the coldest and most dangerous of the season this weekend and into next week (see Key Messages linked below). The Arctic front will turn on the LES machine over Lake Superior by early Friday morning while periods of snow develop along and in wake of the Arctic frontal passage over the Great Lakes Saturday afternoon, then over the eastern Great Lakes and central Appalachians Saturday evening. LES bands look to form over parts of central and western New York Saturday night that likely persist through the remainder of the weekend. Through 12Z Sunday, WPC probabilities show moderate-to-high chances (50-70%) for snowfall >4" and low chances (10-30%) for snowfall >6" in the Tug Hill Plateau, but expect these probabilities and snowfall totals to rise as the LES bands that linger in the medium range enter the short range over the next 24-48 hours. ...Rockies and High Plains... Days 2-3... The impending snowfall in these region, starting late Thursday night in the northern Rockies/High Plains, is driven by a combination of both upper level disturbances and the arrival of a bitterly cold Arctic air-mass that is destined to be the coldest air-mass of the season to infiltrate the Lower 48. This exceptional cold front (a "blue norther") out ahead of will push south Thursday night at the same time as 500mb PVA ahead of an approaching upper trough occurs over the northern Rockies/High Plains. Snow showers will breakout across Montana early Friday morning and make its way south throughout the day into Wyoming, then into Colorado by Friday night. Southern Montana and into Wyoming, in particular, sport notably higher snow squall parameters given the stronger surface- based heating that will cause steeper low-level lapse rates. Snow squalls would cause rapid reductions in visibility due to a combination of heavy snow rates and whipping wind gusts, as well as rapid accumulations on roads as temperatures plummet well below freezing. Motorists in these areas should keep a close eye on potential snow squalls as it can cause dangerous travel conditions in a matter of seconds. As the front plunges south Friday evening, the snow potential shifts south into Colorado with periods of snow getting started near the start of the evening rush hour in the Denver/Boulder metro area. While the snow squall potential may not be as high, the easterly upslope-enhancement of snowfall rates will make for occasional periods of heavy snow Friday evening. Snowfall rates will weaken as the front works its way south of the Palmer Divide Friday night and down the spine of the Sangre De Cristo early Saturday morning with some snow along the Front Range lingering into the day on Saturday as a dome of ~1050mb high pressure builds in from the Canadian Prairies. WPC probabilities are keying in on the Front Range as the focus for the heaviest snowfall with moderate-to-high chances (50-70%) for snowfall >4" from the Laramie Range on south to parts of the Sangre De Cristo. The I-25 corridor from Fort Collins on south to the Colorado Springs area Friday have similar probabilities for >4" of snowfall with even low chances (10-30%) for greater than 6" between Friday evening and Saturday morning in spots. Some of the higher peaks (>10,000ft) have moderate chances (40-60%) for snowfall >8" through Saturday morning. The WSSI is currently depicting Minor Impacts for just about all of these referenced areas, suggesting the potential for winter weather conditions that would require enhanced caution while driving in these affected areas. Mullinax ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png