Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 407 AM EST Fri Feb 18 2022 Valid 12Z Fri Feb 18 2022 - 12Z Mon Feb 21 2022 ...Upper Midwest through the Northeast... Days 1-2... Three distinct areas of impactful winter weather will affect this region during the next few days. A low pressure system will eject rapidly northeast across New England and into the Canadian Maritimes this aftn. Pronounced ascent through height falls and low-level WAA will maintain moderate to heavy precipitation across northern New England through the first half of the day, with rain, sleet, freezing rain, and snow all expected. Although cold advection will gradually turn all precip over to snow as the low departs, this will also be accompanied by rapid dry advection, and snowfall is likely to wind down before sunset across Maine. Where snow is the primary p-type, moderate accumulations exceeding 4 inches are likely, and WPC probabilities indicate a better than 40% chance of this only in far northern Maine. As this lead low departs, a potent shortwave will follow quickly on its heels as it dives out of Alberta within pinched mid-level flow to reach Lake Superior this evening. This shortwave will be joined by modest LFQ diffluence at the leading edge of a polar jet streak moving into the region, and the overlap of these features will lead to a modest wave of low pressure, which will move eastward as a clipper-type system across the Great Lakes today and then over New England on Saturday. This low will help drive a powerful cold front southeastward as well, with WAA out ahead of the low/front quickly replaced by impressive CAA in its wake. WAA ahead of this feature will likely spread modest snowfall across the Arrowhead of MN this morning, and then into the U.P. and L.P. of MI this evening, and across the Northeast and New England on Saturday. However, accumulations within the WAA snow are likely to be less than 4 inches. However, as CAA commences behind the low, a strong signal for LES develops in much of the guidance with steepening lapse rates leading to deepening inversions, a lowering DGZ, and unidirectional flow across the Lakes in a N/NW direction. This should produce heavy LES on D1 across much of the U.P., the NW portion of the L.P. near Traverse City, and along the eastern shore of Lake Michigan. WPC probabilities on D1 are above 70% for 4+ inches, with the heaviest likely across the eastern U.P. between Manistique and Mackinaw City where more than 6 inches is likely in some locations. As this wave progresses into D2, heavy LES will likely develop downwind of both Lake Erie and Lake Ontario with several inches of accumulation likely. However, channeled moisture surging into the Tug Hill Plateau D2 could create some robust snowfall rates and heavy accumulations, which WPC probabilities indicate have a 20-30% chance of exceeding 8 inches. Lastly, there continues to be an impressive signal for snow squalls along and immediately behind this strong cold front as it races across the area. The high-res guidance all indicate convective elements within the simulated reflectivity across the Midwest and Great Lakes D1, reaching into the Mid-Atlantic and Northeast D2. While the exact timing and coverage of these squalls are uncertain, overlapping 0-2km 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. The HREF indicates low probabilities for blizzard conditions within these snow squalls as well. While confidence is highest across MN/WI/IA today, there is likely to be at least some continuation of these snow squalls across the L.P. tonight, and then into PA/NY and New England on Saturday. Total accumulation from these snow squalls is likely to be less than 1 inch, but hazardous travel is likely during their occurrence. ...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 on D2 across the WA Cascades and Northern Rockies, shifting southward D3 into the OR Cascades, Blue Mountains of OR, Absarokas, Sawtooth, and ranges of NW WY/southern MT. 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 D3. This will allow accumulating snow to fall into the foothills and some elevated valleys by D3, with light accumulations reaching as far east as many of the High Plains of MT as well. WPC probabilities D2 are high for 6+ inches in the WA Cascades and Northern Rockies near Glacier NP, and high on D3 in the OR Cascades and parts of the Absarokas/Blue Mountains. Locally more than 12 inches of snow is likely in parts of the Absarokas, Tetons, and Northern Rockies, with more than 2 ft likely in the higher terrain of the WA Cascades. ...Northern Plains... Day 3... An amplifying longwave trough across the Pacific Northwest will leave downstream divergence across the High Plains and Northern Plains on Monday. An intensifying jet streak downstream of this trough will leave the favorable RRQ for diffluence atop the Northern Plains, coincident with the best low and 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 in the vicinity of the border of ND and SD stretching eastward quickly by late D3 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 SD. Weiss Key Messages for the Feb 18-19 Snow Squalls: - A powerful cold front will drop across the Northern Plains and Upper Midwest today and tonight. - Along and behind this front, narrow bands of snow squalls are likely which 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. - This combination of heavy snow and wind will likely create dangerous travel and slippery roads, and may impact the Friday evening rush hours around Duluth, Minneapolis, and Green Bay. - Temperatures are likely to drop quickly behind the front leading to a rapid refreeze of wet roadways. - This front and accompanying intense snow squalls may spread across Lower Michigan Friday night, and into parts of the Mid-Atlantic and Northeast on Saturday.