Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 323 AM EST Thu Jan 18 2024 Valid 12Z Thu Jan 18 2024 - 12Z Sun Jan 21 2024 ...Prolonged Great Lake Effect Snow... Days 1-3... Through Saturday night, a large/deep upper trough will settle over the Great Lakes region with a reinforcing cold front sweeping across the region later today and tonight. This will bring a renewed surge of cold air advection over the Lakes and help to reinvigorate the lake effect snow bands in the favored locations. Though for today, the favored W/SW winds ahead of the front will bring several inches of new snow downwind of Lake Erie and Lake Ontario mainly for the Tug Hill Plateau and Chautauqua Ridge. Then tonight into Friday, west/northwest winds developing and a very cold air mass will allow for intense snow bands to develop off Superior and Michigan. In fact, across far northwest Indiana in the favored snow band region, localized intense snow rates are likely Friday/Friday night where total amounts may locally exceed 12-18 inches. Elsewhere the WPC probabilities for 6 inches are moderate (40-60%) across northeast Ohio, locally in the U.P. of Michigan and downwind of Lake Erie and Lake Ontario. ...The West... Days 1-2... Moderate to locally heavy snow will continue early this morning across portions of the Northern Rockies as shortwave trough energy moves southeast through the region and a narrow but intense jet streak positions itself over the Pacific Northwest and Intermountain West, putting this area in the favored left exit region. The bulk of the heaviest snow will be winding down before 12Z but light snow and additional accumulations over the Rockies will persist this morning. Meanwhile, another storm system will quickly fill in behind, bringing another round of wintry precipitation to the region beginning later today. Pieces of shortwave trough energy lifting northeast ahead of the main shortwave, will advect onshore Washington and then race eastward across the Northern Rockies and eventually the Northern Plains by tonight. This system will have the benefit of a strong/favored upper jet streak diving through the Rockies along with a very cold air mass seeping westward into the Oregon Columbia River Basin/Gorge. This system is expected to bring heavy snowfall today/tonight to the WA Cascades where additional 1-2 feet will be possible. In the lower levels, cold air in place and overrunning precipitation will bring the threat of ice accumulation to the Columbia River Basin/Gorge where ice accumulations may total in excess of 0.10" (WPC ice probabilities are locally 50-70%) and a few locations may reach 0.25" of ice accumulation through the end of day 1 (0.25" ice probabilities are near 40 percent). This could create a dangerous/significant ice accumulation situation. A longwave ridge will build across the west Day 2, shutting off precipitation mostly, at least briefly, before a more pronounced shortwave approaches the Pacific Coast during Day 3. 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 30-50% chance of snowfall exceeding 6 inches occurring in the Sierra. ...Northern/Central Plains... Day 1... A stripe of moderate snowfall is expected across portions of the Plains into the Midwest as an embedded shortwave trough within the larger fast cyclonic flow tracks from British Columbia and the Northern Rockies this morning to the Midwest by tonight. A fairly strong shortwave trough and a potent jet streak providing favorable forcing for ascent will work with modest amounts of moisture to produce a stripe o 0.05-0.20" of liquid. The airmass is quite cold and there is decent overlap of lift within the DGZ which is expected to produce higher than climatology snow to liquid ratios. In fact, where the best forcing/lift sets up, SLRs above 20:1 (as high as 25:1) may be observed and the event should bring very efficient and fluffy snowfall accumulations on the order of 1-3" as shown by the latest WPC snowfall probabilities which are moderate/high (50-80%) for 2" but fall to under 5 percent for 4". ...Ohio Valley through Southern New England... Days 1-2... A strong and fast moving shortwave trough currently moving into the northern Rockies this morning will track across the Plains today before amplifying as it moves into the Ohio Valley by Friday morning. A developing jet streak on the periphery of the longwave trough will increase to over 150 kts at 250 mb, positioned the region in the favored left exit region. A surface low is expected to organize over the Ohio Valley then intensify as it races off the Mid-Atlantic and Northeast coast Friday into Friday evening. The combination of the height falls, large scale forcing for ascent, and the developing surface wave should provide enough forcing/lift to work with the available moisture to produce widespread light to moderate amounts of precipitation across the region. The trend in the latest guidance is for slightly stronger forcing along portions of the Mid-Atlantic coast (eastern PA through NJ). The moderate snowfall is supported by both the deep DGZ in place due to the very cold airmass that remains in the region and it should remain quite cold ahead of the approaching system despite some warm air advection today into tonight. This will support slightly higher than climatology snow to liquid ratios (generally 13-16:1) and will produce efficient/fluffy snow. Secondly, as the system pulls away Friday, an inverted trough developing west/northwest of the low center will enhance/linger forcing for ascent back across portions of the Mid-Atlantic and may produce localized/narrow higher snow amounts. This is where the latest WPC snow probabilities for 4 inches are up to 30-40 percent, though the heaviest snowfall from this event is expected in the WV Appalachians where upslope flow will contribute additional ascent to wring out moisture, and here, WPC probabilities are above 90% for 4 inches, and locally as much as 8-12 inches could fall. Taylor ***Key Messages for Jan. 17-19 Northwest Winter Storm*** -Additional Wintry Precipitation Today through Friday. Another strong storm system is expected to direct a round of heavy precipitation across the Pacific Northwest today through Friday. -Continued Ice Threat for Columbia Basin/Gorge. Heavy freezing rain is expected to develop again today and continue into Friday for portions of the Cascades and Lower Columbia Basin. Significant accumulations of ice (localized ice amounts > 0.20â€) will likely result in dangerous travel and scattered power outages. -Heavy Snow Develops Again Late Today. Heavy snow will overspread the Cascades late today into tonight. Expect snow rates to peak above 1â€/hr at times and total snowfall accumulations of 1-2 feet are expected for the Washington Cascades. -Mountain Pass Dangerous Travel. Heavy snow today will transition to mixed precipitation, including freezing rain, at many of the mountain passes. This will produce dangerous travel through Friday, especially across the Cascade Passes.