Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 323 PM EST Thu Jan 16 2025 Valid 00Z Fri Jan 17 2025 - 00Z Mon Jan 20 2025 ...Great Lakes & Central Appalachians... Days 1 & 3... Day 1... Continued snowfall across the Buffalo South towns down into the proxy of the Chautauqua Ridge will be ongoing for another few hours with multi-hour accumulations approaching 4+" within the most persistent bands downwind of Lake Erie. This setup will cease with the loss of sufficient forcing, bringing an end to the threat by the beginning of the 00z Friday time frame. Over the Central Appalachians, the previously identified favorable forecast for heavy snowfall remains steadfast as the overall synoptic pattern maintained continuity within the 12z suite of NWP. Heavy snowfall totals are anticipated 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. This has been highlighted within the latest hi- res window with the recent 18z RAP/HRRR output indicating a healthy uptick in snowfall beginning ~20z, lingering through portions of the overnight period before decaying prior to sunrise as dry air advection and decreased mid-level forcing ushers an end to the potential. Snow looks to fall heaviest between now and 06z Friday with rates topping 0.5"/hr in some cases. Steepening lapse rates have been analyzed via Mesoanalysis over the Mid- Atlantic over the past few hours along and east of the Appalachian front, likely providing residents in the region to be on the lookout for possible snow squalls given the favorable time of day and more than adequate upper-level divergence aloft. With the heaviest snowfall still relegated to maintain the Central Appalachians, WPC probabilities continue to depict locally elevated chances (>40%) for additional snowfall totals >4" near the MD/WV border down through the Highlands in West Virginia. Some of the taller peaks of eastern West Virginia could see snow totals between 8-12" by Friday morning from the cumulative nature of the event, especially within Tucker, Randolph, and western Pocahontas counties. 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. Across the Central Appalachians, a secondary round of heavy snow within the 72 hour period will take shape as the Arctic front pushes across the terrain with prevailing westerly flow advecting bitter cold air within the primary cold air advection regime post- Arctic front. A period of upslope snowfall will begin Saturday behind expected fropa, weakening as we step into early Sunday laying the ground work for the next round of expected snowfall as surface cyclogenesis is forecast across the Southeast thanks to an amplifying trailing wave tracking towards the Mid Atlantic coast. Ensemble spread has tightened as of the recent 12z suite, however there is still some discrepancy on the magnitude of the snowfall potential within the region. A lot is contingent on the amplification regime of the trailing wave with most of the ensembles now trending towards a more robust surface reflection and regional ascent pattern across the entire Mid Atlantic extending west into the Appalachian Front. Latest probabilities have increased substantially run-to-run given the trends with the >4" signal now within the moderate-high category (40-70%), even carrying a small region of (40-60%) for >6" within the favored areas of West Virginia, primarily the zones capable of both the upslope component combined with the synoptic impacts anticipated over the course of Sunday. ...Rockies and High Plains... Days 2-3... There has been little change with the impending snowfall in these regions, 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 (25-50%) 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. ...Mid Atlantic and Northeast... Day 3... A trailing wave along the southern periphery of the Arctic front will exit out of the southeastern U.S with an expectation for the wave to amplify as it approaches the Southern Mid Atlantic with attendant surface cyclogenesis across the Deep South, moving northeast. Recent ensemble and deterministic output via the 12z suite of NWP has become increasingly aggressive with the signal for a low end moderate event for heavy snow prospects, mainly from southwestern VA up through the Central Mid Atlantic, eventually bleeding into the periods following as the pattern shifts northeast up through the Northern Mid Atlantic (PA/NY) and Southern New England. There has been some discrepancy within the suite on the exact placement of the heaviest precip potential with a split on two camps favoring a more amplified setup (UKMET/CMC) and a less amplified evolution (ECMWF) with the GFS now solidly in the middle of the two camps. ECMWF AIFS ML output has been very steady over the course of the last 3+ days of output with the latest run showing great consistency when assessing run-to-run variability. This lends credence to a slightly better confidence with regards to snowfall potential across the area with the probabilities of >4" going from close to 0% overnight towards (10-30%) within a span of one run. It's a period to monitor as heavy snowfall threat would impact the population centers of the DMV up through Philadelphia and points northeast in the periods beyond. Mullinax/Kleebauer