Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 419 PM EST Wed Jan 05 2022 Valid 00Z Thu Jan 06 2022 - 00Z Sun Jan 09 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 today/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. For Days 1 and 2, WPC probabilities of at least 12 inches of snow are moderate to high across the Washington Cascades and mountainous Idaho/western Montana. By late Friday and continuing through Saturday, the potential for heavy snow is expected to diminish as the upper trough shifts east and an upper ridge begins to move onshore. ...Northern Plains and Great Lakes... Days 1-2... As an upper low transitions to an open wave and lifts north/northeast of the Great Lakes, northerly to northwesterly winds will increase setting up a favorable long duration potentially significant lake effect regime. The much colder air and maximum lift well within the DGZ supports SLRs 20:1 or even a bit higher in the more intense snow bands. As a result, total accumulations in excess of 8 inches are expected off the Lakes, particularly for the favored snow belt areas where isolated totals of a foot or more are possible. Snows are expected to diminish on Friday as a shortwave ridge moves east across the region. ...Tennessee and Ohio Valleys into Mid-Atlantic/Northeast... Days 1-2... Shortwave energy crossing the central U.S. in the quasi-zonal flow begins to sharpen and dig as it approaches the TN/OH Valley Thursday as an area of low pressure develops off the southeastward moving frontal boundary. This low is expected to track through the southern Appalachians before exiting off the Mid-Atlantic coast early 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 northeast, potentially affecting the Northeast/New England region. While uncertainty remains, the consensus of the 12Z deterministic model cycle continued to show a progressive system with an offshore track that would limit the potential for heavy snow well inland across the Northeast. However, there continues to be some ensemble support for a stronger/closer system so over the next several days the details will ironed out. For the Mid-South to the central Appalachians, the latest WPC probabilities show the greatest chances for at least 4 inches from central Kentucky and Middle Tennessee through West Virginia, especially for the favored terrain/upslope areas, while those probabilities drop off to the slight/moderate range for the Mid-Atlantic. In addition to the orographic ascent, models are indicating a period of strong forcing supported by low-to-mid level frontogenesis overlapped by a coupled upper jet that may support banded heavy snow (1+ inch/hr rates) developing and translating east across Kentucky and Tennessee into the Appalachians on Thursday. South of the heavier snow, models continue to present a significant signal for mixed precipitation and the potential for accumulating freezing rain across portions of the Tennessee Valley. Across the Northeast, given the continued uncertainty, for southern New England and coastal Maine, the highest probabilities remain centered near the coast. Pereira/Taylor ~~~ Key Messages for the Tennessee and Ohio Valleys to the Northeast Winter Storm ~~~ - A winter storm is likely to impact areas from the Mid-South to the Northeast Thursday and Friday. - Several inches of heavy snow and a zone of mixed precipitation, including freezing rain, is likely to cause travel disruptions and hazardous to potentially dangerous road conditions from the Mid-South to the southern and central Appalachians Thursday and Thursday night. - Accumulating snow, heavy in places, is likely as the system moves through the Mid-Atlantic and Northeast Thursday night into Friday. - There remains uncertainty in the track and potential snow amounts, particularly for the Northeast and New England.