Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 341 PM EST Fri Feb 18 2022 Valid 00Z Sat Feb 19 2022 - 00Z Tue Feb 22 2022 ...Upper Midwest through the Northeast... Days 1-2... Broad cyclonic flow is forecast to persist across the eastern U.S. into the weekend, with another embedded shortwave dropping southeast from central Canada into the Upper Midwest/Great Lakes regions this evening. Warm advection precipitation ahead of the system is expected to produce generally modest snowfall accumulations from the Great Lakes overnight and into the Northeast on Saturday. In the wake of the system, a powerful cold front will drop southeast, with lake effect snows developing behind the front. Strong northwesterly flow will support wind-driven snows across the U.P., the northern L.P., and along the eastern shore of Lake Michigan trough early Saturday before a shortwave ridge begins to move across the region later in the day. Farther east, deep westerly flow from the late morning into the early evening is expected to support single-band heavy snow developing from Lake Ontario into the Tug Hull Region and western Adirondack foothills. WPC probabilities indicate accumulations of 6 inches or more are likely across the Tug Hill by Saturday evening. There remains an impressive signal for snow squalls along and immediately behind this strong cold front as it races across the area. The high-res guidance continue to indicate convective elements within the simulated reflectivity from the Great Lakes into the Ohio Valley overnight, before reaching into the Mid-Atlantic and Northeast during the day on Saturday. While the exact timing and coverage of these squalls are uncertain, overlapping 0-2 km fgen and theta-e lapse rates of less than 0C/km are progged to occur directly behind the front as instability climbs above 100 J/kg. This is a favorable setup for scattered to widespread snow squalls with snowfall rates of greater than 1"/hr and winds of more than 25 kts. Given their rapid advance, total accumulation from these snow squalls is likely to be less than 1 inch, but hazardous travel can be expected during their passage. ...Pacific Northwest... Days 2-3... Confluent but nearly zonal flow into the Pacific Northwest will gradually amplify as a series of modest shortwaves dig down from the Gulf of Alaska merging into a positively tilted longwave trough across the area by Sunday night. These shortwaves together will act to amplify the mid-level flow and will be accompanied by waves of upper diffluence maxima to enhance synoptic ascent. The overlap of mid-level confluence with upper level jet energy emerging from the Pacific will increase column moisture, and PWs are progged to reach +1 standard deviation above the climo mean Saturday. This moisture will be wrung out by the deep layer ascent to manifest as heavy snow by late Saturday across the Washington Cascades and Northern Rockies, and then gradually shift southward overnight into Sunday into the Oregon Cascades, Blue Mountains of Oregon, and portions of the Northern Rockies. Snow levels to begin the period will be generally 2000-3000 ft, but a southward surging cold front beneath the amplifying trough will help lower snow levels to 1000-2000 ft by Sunday. This will allow accumulating snow to fall into the foothills and some elevated valleys on Sunday, with light accumulations reaching as far east as many of the High Plains of Montana as well. For the 48 hr period ending late Monday, WPC probabilities indicate that accumulations of 6 inches or more are likely along the Washington and Oregon Cascades, the Blue Mountains, and portions of the Northern Rockies from northern Idaho and northwestern Montana to northwestern Wyoming. Locally heavier amounts of a 12 inches or more is most likely across the higher elevations of the the Washington Cascades. ...Northern Plains... Day 3... An amplifying longwave trough across the Pacific Northwest will support downstream divergence across the High Plains and Northern Plains on Monday. An intensifying jet streak downstream of this trough will leave favorable right-entrance region dynamics positioned across the Northern Plains. This is expected to overlap the best low-to-mid level frontogenesis developing behind a strong surface cold front digging southward across the area. This should produce impressive ascent within a relatively narrow corridor centered along and south of the North and South Dakota border stretching eastward quickly by late into Minnesota. The column is likely to be extremely cold behind the front, so fluffy SLRs are forecast, and as moisture increases from the Pacific, heavy snow should be the result. While guidance still features some uncertainty into the placement of this axis, it is likely that a swath of heavy snow exceeding 6 inches will occur, and currently the highest WPC probabilities for that are across far northern South Dakota. Pereira/Weiss Key Messages for the Feb 18-19 Snow Squalls: - A powerful arctic cold front will race through the Great Lakes region tonight and into the Northeast early Saturday. - Along and behind the front, narrow bands of snow squalls will move rapidly from northwest to southeast. These bands will be accompanied by heavy snowfall rates of 1"/hr or more, gusty winds which may exceed 50 mph, and extremely limited visibility. - The combination of brief heavy snow and wind will likely create dangerous travel conditions as the front moves through. - Temperatures will drop quickly behind the front leading to a rapid refreeze of wet roadways. - The front and accompanying intense snow squalls will spread into parts of the Mid-Atlantic and Northeast on Saturday.