Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 414 PM EST Tue Jan 04 2022 Valid 00Z Wed Jan 05 2022 - 00Z Sat Jan 08 2022 ...Pacific Northwest to the Central Rockies... Days 1-3... Strong upper high northwest of Alaska will favor troughing to its south out of the Gulf of Alaska into western British Columbia and the Pacific Northwest as a series of jet streaks pass through. By the end of the week, troughing from the Gulf of Alaska as well as the northern Pacific in the mid-latitudes will increase broader troughing into much of the Northwest with two attendant surface systems moving into the region. Working in tandem, these systems will promote a surge in moisture into western Washington by late Wed/early Thu with precipitable water values over 1 inch on the coast which is +2 to +3 sigma. This will shift inland across Idaho on Thursday and into Wyoming/Utah as well where snowfall will be significant in higher elevations. Temperatures will moderate as milder air draws northeastward, but initially cold surface temperatures in lowland/valley locations east of the Cascades will support some freezing rain (also perhaps west of the Cascades before the column warms completely). Heavy snow is forecast for the Cascades with significant accumulating snow across the passes Thursday. Snow levels will rise late Thu into Fri over western WA and OR over 5000ft before trending down as the upper trough moves in. WPC probabilities of at least 12 inches of snow are moderate to high each day over the Washington Cascades and mountainous Idaho/western Montana. ...Northern Plains and Great Lakes... Days 1-2... Shortwave trough energy originating from Alberta and a piece of sheared vorticity from the Pacific Northwest works together to eventually carve out a closed 500 mb low over the western Great Lakes Wednesday. At the surface, low pressure amplifying over the northern Plains is forecast to track northeast across southern Minnesota into Wisconsin overnight. A corridor of frontogenetical forcing within the very cold air will support moderate to locally heavy snowfall across the northern Plains to Upper Midwest late tonight into early Wednesday and then as the system passes east of the Lakes, the very favorable wind direction and cold air will bring a prolonged, potentially significant, lake effect snow event to the Great Lakes. SLRs approaching 20:1 will help drive higher snow totals in excess of a foot locally for the favored areas of Lake Superior, Michigan, and perhaps Erie/Ontario snow belt areas. The latest WPC probabilities for 6 inches are moderate to locally high for the Superior/Michigan/Erie/Ontario snow belt areas. ...Central High Plains... Day 2... Increasing divergence along the left-exit region of an upper jet extending southeast from the northern Rockies, overlapping strong low-to-mid level frontogenesis, is expected to support the increasing potential for heavy snow development from southeastern Wyoming into the Nebraska Panhandle on Wednesday. WPC probabilities continue to increase, indicating a high threat for accumulations of 4-inches or more centered across this region. The threat for heavy snow is less certain farther east, but a stripe of at least light accumulations is expected across Kansas and Nebraska into Missouri late Wednesday into Thursday. ...Tennessee and Ohio Valleys into Mid-Atlantic/Northeast... Day 2-3... Shortwave energy crossing the central U.S. in the quasi-zonal flow begins to sharpen and dig as it approaches the Tennessee and lower Ohio valleys Thursday. An area of low pressure over the Deep South is then expected to strengthen as it lifts northeast through the southern Appalachians then off the Mid-Atlantic coast Friday morning. Then on Friday, as the associated shortwave begins to assume a negative tilt and lift northeast, the surface low is expected to rapidly intensify and track along the Northeast coast. Modest moisture being tapped from the Gulf will help drive widespread precipitation across the lower Ohio Valley through much of the central/southern Appalachians into the Mid-Atlantic and Northeast. As noted in previous discussions, there remains considerable uncertainty in the eventual low pressure track and therefore the amount/depth of cold air in place and its potential impacts. However, there remains a significant signal for several inches of snow for portions of eastern Kentucky into West Virginia through by early Friday. The latest WPC probabilities for 4 inches remain high across eastern Kentucky through much of West Virginia with a broad area of moderate probabilities around it from central Kentucky into the Mid-Atlantic. As the system intensifies, the potential for significant snows is expected to increase from the northern Mid-Atlantic to New England, including the I-95 corridor. Latest WPC probabilities show moderate probabilities for snowfall accumulations of 4 inches or more from the Baltimore metro through eastern Maine. Fracasso/Pereira/Taylor ~~~ Key Messages for the Tennessee and Ohio Valleys to the Northeast Winter Storm ~~~ -There is potential for another storm to impact portions of the Tennessee and lower Ohio valleys, central Appalachians, Mid-Atlantic, and Northeast Thursday through Friday. -Potential exists for several inches of snow and widespread hazardous travel conditions. -Mixed precipitation, including freezing rain, is possible south of the heavier snow in the Tennessee Valley. -There remains a lot of uncertainty in the track of the storm and potential snow/ice amounts, but an impactful event is becoming increasingly likely.