Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 401 AM EDT Sat Mar 18 2023 Valid 12Z Sat Mar 18 2023 - 12Z Tue Mar 21 2023 ...Great Lakes & Interior Northeast... Days 1-2... Lake effect snow and snow squalls are expected to plague portions of the Great Lakes and interior Northeast this weekend. An anomalously strong upper low will rotate across the northern Great Lakes and into Ontario before shearing off into the Canadian Maritimes on Sunday. Beneath this feature, an occluded surface low will track northeast into Canada, while a secondary cold front and surface trough rotate cyclonically around it and through the Great Lakes and into the Northeast. These secondary features will be driven by vorticity lobes noted in mid-level fields shedding around the primary closed low as it moves away from the region this weekend. This secondary cold front and surface trough will both act to increase mesoscale ascent through low-level convergence, but more importantly produce enhanced CAA with 850mb temps crashing to around -20C. As this CAA spreads across the still warm lake waters, it will result in steepening low-level lapse rates thanks to sfc-850mb deltaT of 25C and inversion depths reaching 10,000 ft according to regional soundings. This will support heavy LES in the typical favored W/NW snow belts south of Lake Superior, and then east of Lakes Michigan, Erie, and Ontario. Snowfall rates will almost certainly exceed 1"/hr at times D1 into D2, with the heaviest snow shifting eastward with time and focusing into the Tug Hill Plateau and Chautauqua Ridge primarily on D2 before waning late Sunday night. WPC probabilities for more than 4 inches are high on D1 for the eastern U.P., the northwestern L.P., along the Chautauqua Ridge, and into the Tug Hill Plateau, with moderate probabilities continuing east of Lake Ontario through D1.5. Locally more than 8 inches is expected in many of these LES bands, with more than 12 inches possible into the Tug Hill. Additionally, as the secondary cold front surges eastward, it will drive a modest overlap of 0-2km fgen, SBCAPE, and sufficient moisture to produce scattered convective snow showers and snow squalls. The greatest risk for these snow squalls appears to be from the eastern Great Lakes through the interior northeast Saturday aftn and eve, with a secondary but less significant potential reaching through Northern New England on Sunday. While snowfall totals within any of these squalls will be limited, brief heavy snow rates and gusty winds will combine to produce rapidly changing conditions due to low visibility, and dangerous travel. ...Southern Rockies & West Texas... Day 1... A modest mid-level shortwave trough will dive southeast across the Four Corners on Saturday, interacting with a southward advancing surface cold front through the Southern Plains. The overlap of downstream divergent flow from the mid-level trough and at least modestly sloped fgen across portions of West Texas and the Southern Rockies will result in broad ascent across the region, with moisture increasing through the column in response to Pacific jet energy and return flow up the Rio Grande Valley from the Gulf of Mexico. Guidance has continued to shift the axis of heaviest precipitation southward tonight, but there is still likely to be a region of moderate to at times heavy snow near the Big Bend/Davis Mountains/Stockton Plateau of Texas on D1, aided by upslope flow and continued cooling low-level temperatures. With the greatest coverage of precipitation occurring during the daylight hours on Saturday, it will likely require more impressive rates of snowfall to create significant accumulations, and the WPC snowband tool suggests the heaviest rates will be along or south of the international border. This has led to a subtle reduction in snowfall accumulation potential, but WPC probabilities are still 10-20% for 4 inches across this area. ...The West... Days 2-3... An amplified trough over the northern Pacific will amplify into a broad negative tilt and shed waves of vorticity and associated shortwaves towards the United States beginning Sunday. Multiple shortwaves troughs are progged to lift across the West the latter half of the forecast period, while the core of the closed low hangs back over the ocean through Monday. These shortwaves will eject eastward embedded within otherwise confluent mid-level flow, which will be zonally oriented onshore and beneath an intensifying Pacific jet which is progged to surge to as much as 170kts late Monday. The overlap of this deep layer easterly flow will drive a moderate atmospheric river into CA and then across much of the west in a decaying manner, especially D3, noted by CW3E probabilities for IVT exceeding 500 kg/m-s. Within this moistening column, deep layer synoptic ascent through height falls/PVA and within the diffluent LFQ of this jet streak will result in increasingly widespread coverage of heavy snowfall across the west, with snow levels generally around 3000-4000 ft, but briefly rising to as high as 7000 ft across CA and the Great Basin. On D2, WPC probabilities for more than 6 inches are confined to the Sierra and Shasta/Trinity/Siskiyou region, but expand dramatically on D3 to cover most of the terrain from the Sierra northward through the WA Cascades and as far east as the CO Rockies and San Juans. 2-day snowfall will likely eclipse 2 feet in the Sierra, and 1-2 feet in some of the higher terrain of the Wasatch and San Juans. ...Key Messages for March 16-18 Winter Storm... --Bands of heavy lake effect snow will expand downwind to the south and east of all the Great Lakes today, continuing east of Lakes Erie and Ontario into Sunday. --Snow will be heavy at times with rates in excess of 1"/hr. Where these bands are most vigorous, they will be accompanied by low visibility, blowing snow, and rapidly changing road conditions leading to hazardous travel. --Additional snowfall will likely exceed six inches in favored locations across the U.P. of Michigan, northwestern Lower Michigan, and southwestern Upstate NY. Locally 12 inches of snow is possible in the NY Tug Hill Plateau. --Snow squalls will likely develop along a cold front from the eastern Great Lakes through the interior Northeast. Brief heavy snow rates and gusty winds within any snow squalls will produce hazardous travel. Weiss