Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 402 PM EST Wed Jan 17 2024 Valid 00Z Thu Jan 18 2024 - 00Z Sun Jan 21 2024 ...Prolonged Great Lake Effect Snow... Days 1-3... A large trough elongated west to east south of the Hudson Bay will maintain broad lowered heights across the Great Lakes. Although this trough will be rather weak D1, a cold front surging eastward through Thursday will enhance CAA across the lakes, which will drive steepening low-level lapse rates to combine with the lowered heights aloft to enhance instability and reinvigorate LES in the favored W/NW bands. This will drive inversion heights up to as high as 10,000 ft, highest east of Lake Ontario, and regional forecast soundings indicate a favorable setup for heavy lake effect snow (LES) on Thursday. The most intense LES D1 is likely east of lakes Ontario and Erie into the Tug Hill Plateau and Chautauqua Ridge, respectively, as single bands drop slowly southward through the day. Additional heavy LES is likely in portions of the U.P. southeast of Lake Superior. WPC probabilities for more than 6 inches of snow are above 80% east of Lakes Ontario and Erie, with more than 1 foot likely near Buffalo, NY, and more than 2 feet possible in the Tug Hill Plateau. The pattern shifts beginning D2 as a more pronounced shortwave sheds around the primary gyre and drops out of Manitoba Thursday night into the western Great Lakes before pivoting southeast into the Mid-Atlantic states by Saturday aftn. This will produce more impressive CAA characterized by 850mb temps reaching close to -25C over Lake Superior (but moderating southeast of there) with a shift to a more northerly low-level wind direction. This strong CAA will generate heavy LES D2 and D3, but the focus will transition to the more typical southerly lake bands, with the most intense snowfall likely occurring near Cleveland, OH, and along the south shore of Lake Michigan in northern IN. Additional heavy snow is also possible across the Huron and Porcupine Mountains in the U.P. of MI. For D2, WPC probabilities for more than 6 inches of LES are 20-40% across parts of the U.P. and near Cleveland, OH. Higher snowfall is likely south of Lake Michigan near South Bend, IN where the favorable flow along the long fetch of Lake Michigan has a high chance for producing 6+ inches, with up to 12 inches possible, and this snow band may persist into D3 as well. ...The West... Days 1-3... The remnants of this morning's compact mid-level low which moved onshore the OR coast will continue to weaken and shed vorticity off to the east through the Central Rockies this evening as the flow remains quite progressive. Immediately behind it, another shortwave, this one with less amplitude, will advect onshore WA and then race eastward into the Northern Rockies and eventually the Northern Plains by Thursday night in response to pinched flow upstream of a broad trough amplifying over the eastern CONUS. This second shortwave, while weaker than the first, will be accompanied by a potent and more favorably placed upper jet streak to drive robust ascent across the Pacific Northwest D1, shifting into the Northern and Central Rockies D2. The overlap of this intensifying jet streak with the height falls/PVA associated with the shortwaves should result in expanding precipitation from west to east through D2. At the surface, a cold Canadian high pressure originating over Alberta will drop southward and become anchored over MT by Friday morning. This will result in impressively cold air across the interior Northwest, with a slow moving cold/nearly stationary front, abutting the terrain Thursday into Friday to produce low-level convergence, some modest fgen, and upslope flow to further enhanced ascent. The result of this setup will be two waves of precipitation. The first an area of snow starting over WA/OR this evening and spreading rapidly east-southeast to the High Plains by the end of D1. Forcing and moisture decrease with time to the east, but heavy snow is still expected, especially above 5000 ft. WPC probabilities are high (70+%) for more than 6 inches on D1 from the Cascades through the Sawtooth/Salmon River ranges, into the higher terrain around Yellowstone NP and into the northern Colorado Rockies. Locally 1-2 feet is likely. By D2, the focus shifts primarily to the Cascades associated with the second shortwave and accompanying IVT, especially early in the day, with WPC probabilities indicating a 50-80% chance for an additional 6 inches or more. A longwave ridge will build across the west late D2 into D3, shutting off precipitation, at least briefly, before a more pronounced shortwave approaches the Pacific Coast on D3. Although the trough axis accompanying this feature is progged to remain offshore by the end of the forecast period, impressive downstream divergence and waves of PVA should advect better moisture onshore resulting in another round of precipitation. Snow levels may rise above 7000 ft within this next round of WAA, but WPC probabilities indicate a 20-40% chance of snowfall exceeding 6 inches occurring in the Sierra. ...Northern/Central Plains... Day 1... Elongated but cyclonic flow across the eastern half of the CONUS will allow shortwaves to periodically drop out of western Canada and race southeast within the generally fast flow. One of these shortwaves and its associated vorticity maxima will leave British Columbia tonight and then surge southeast reaching SD Thursday afternoon before continuing on into the Ohio Valley Friday morning. This feature is expected to strengthen, albeit modestly, during this evolution, and the overlap of its height falls and PVA with a potent jet streak aloft should result in a stripe of moderate snowfall across the Northern/Central Plains on Thursday. Total forcing is modest, but some enhancement is likely as downstream WAA along a warm front drives fgen, which may be enhanced by the favorably placed upper level jet streak. Moisture will increase on the WAA/isentropic upglide, and within an anomalously cold column which contains a deep DGZ, this will result in a fast moving swath of moderate snow. The transient nature of this feature will limit snow accumulations, but the fluffy SLR leading to rapid accumulation could still produce as much as 4 inches of snow as reflected by WPC probabilities which are above 80% for 2 inches, but less than 5% for 4 inches. ...Ohio Valley through Southern New England... Days 2-3... A fast moving shortwave will dig out of British Columbia, traverse the Central Plains, and then begin to amplify as it advects into the Ohio Valley Friday morning. This feature will be embedded within pinched flow south of an amplifying trough centered over the northern Great Lakes, so amplitude gain is likely to be modest, and it should remain progressive as it shifts across the Mid-Atlantic states and offshore Friday night. However, a secondary impulses will follow immediately in the wake of this first shortwave, moving from IA Friday night to VA Saturday evening, and this second impulse will allow forcing to persist longer from the Ohio Valley into the Mid-Atlantic and southern New England. Although both of these shortwaves will be of modest intensity, height falls and PVA will combine with a zonally oriented jet streak reaching a potent 170kts over the lower Ohio Valley on Friday. Downstream of this jet, the overlap of its diffluent LFQ and the lead shortwave could result in a modest surface wave developing over the TN VLY before racing E/NE off the Atlantic coast well south of New England by Saturday morning. There remains some spread in the ensemble clusters as to the position of this surface wave, but overall this still appears to be a modest snowfall event as slightly anomalous moisture combines with the robust but transient ascent to result in a swath of snowfall. The risk for at least moderate accumulations is supported by two primary factors. One, the column is quite cold ahead of the impulse, and while some warming is likely on the associated WAA, this will occur into a deep DGZ (DGZ depth probabilities for 100mb from the SREF of 30-50%) to support high SLR and efficient fluffy accumulations. Additionally, the secondary shortwave trailing the lead may result in a weak inverted trough developing to the W/NW of the surface low, which could allow for ascent to linger/expand to the west even as the low pulls away. Despite all that, total accumulations from the Ohio Valley through southern New England should be generally 4 inches or less as reflected by WPC probabilities that are 10-30% for 4+ inches although locally higher totals are possible. The heaviest snowfall from this event is likely in the WV Appalachians where upslope flow will contribute additional ascent to wring out moisture, and here, WPC probabilities Friday and Saturday are above 80% for 4 inches, and locally as much as 12 inches could fall. Weiss ***Key Messages for Northwest Winter Storm*** --Additional Wintry Precip Thursday & Friday As the region contends with heavy snow and treacherous ice accumulations today, a second storm system will direct additional heavy precipitation across the Pacific Northwest Thursday into Friday. --Continued Ice Threat for Columbia Basin/Gorge Light freezing rain continues today east from Portland and into the Columbia Basin through the Gorge. Additional heavy freezing rain is likely to develop Thursday and continue into Friday for portions of the Cascades and Lower Columbia Basin. Significant accumulations of ice will likely result in dangerous travel and scattered power outages. --Heavy Snow Tonight and again Thursday Periods of heavy snow rates exceeding 1"/hr are expected for parts of the Northern Rockies and Northern high Plains tonight. Additional rounds of heavy snow rates above 1"/hr will spread into the Cascades late Thursday. Total snowfall of 1-3 feet is likely in higher terrain. --Mountain Pass Travel Heavy snow tonight and Thursday will transition to mixed precipitation, including freezing rain, at many of the mountain passes. This could produce dangerous travel through Friday, especially across the Cascade Passes.