Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 430 AM EST Wed Feb 23 2022 Valid 12Z Wed Feb 23 2022 - 12Z Sat Feb 26 2022 ...Southwest U.S.... Days 1-2... An anomalous longwave trough will deepen over the Great Basin this morning before finally beginning to shift eastward in response to slow weakening and absorption of the primary energy into the westerlies. As this trough begins to eject eastward, the powerful downstream sub-tropical jet streak will continue to intensify, reaching above 170 kts over New England tonight. This jet streak will hang back its favorable RRQ for diffluence/ascent over the Southwest, which will combine with impressive height falls and PVA to drive deep layer ascent across the Four Corners region. A wave of low pressure at the surface beneath the upper trough will advect southeast towards the NM international border, with an associated cold front lowering snow levels to around 1000 ft in the Sierra, around 2000 ft in the Rockies, and 3000-4000 ft across central/southern AZ/NM. Above these levels, this intense ascent will drive heavy snowfall, which could feature intense rates as well as instability climbs beneath the associated cold pool. WPC probabilities on D1 are high for 8+ inches along the Mogollon Rim, the San Juans, and other ranges of northern AZ and southern CO. Locally, more than 2 ft of snow is likely on D1. Although the best ascent and moisture will shunt eastward the first half of D2, additional moderate to heavy snow is likely across the San Juans, Sangre De Cristos, and White Mountains where WPC probabilities are again high for an additional 8+ inches of snow. ...Southern Plains through the Mid-Mississippi Valley, Ohio Valley, Mid-Atlantic, and New England... Days 1-3... A long duration and multifaceted storm will bring moderate to heavy wintry precipitation to a large portion of the eastern and central CONUS. An anomalously deep 500mb trough, forecast to produce heights in the bottom 5th percentile across the Southwest according to NAEFS ensemble tables, will slowly begin to fill and eject eastward across the country tonight. Downstream of this trough, a power sub-tropical jet streak will arc from southern CA through New England, peaking above 170 kts to produce impressive RRQ diffluence aloft for ascent. As this trough begins to shift east Thursday, the primary jet streak is progged to split over the Central Plains/Mid-Mississippi Valley, leading to an increasingly coupled jet structure shifting eastward through late week. The overlap of a surface cold front, height falls/PVA ahead of the filling trough, and this coupled jet will lead to surface pressure falls and a strengthening area of low pressure is likely to move quickly from the Lower MS VLY through the Mid-Atlantic and off the New England coast late Friday. Ahead of this wave, low-level flow will increasingly back out of the Gulf of Mexico driving IVT above the 95th percentile into the eastern CONUS, which will help expand multiple waves of heavy precipitation across the region. The first wave of precipitation is expected to develop this evening across the Southern Plains and Mid-Mississippi Valley along the nose of the increasing LLJ out of the Gulf of Mexico and in response to this feature rising isentropically atop cold Canadian high pressure. The guidance has shifted southward just slightly overnight in response to the strength of the northern high, and it is likely this evolution will lead to a wide swath of sleet and freezing rain. Some of this freezing rain is likely to be heavy, especially in eastern OK and along the Ozarks where WPC probabilities are greater than 70% for at least 0.25" of freezing rain. Locally, in excess of 0.5" is likely which will lead to widespread impacts including power outages. This first wave of precipitation is likely to wane as a weak vort swings northeast and the LLJ veers to parallel the surface front. However, regeneration of precipitation, with a much larger expanse, is forecast to develop shortly after, with freezing rain and sleet once again developing across northern TX and OK by Thursday morning /D2./ This precipitation will initially be light but is expected to increase in both coverage and intensity as PWs surge towards +2 standard deviations above the climo mean, and the most intense synoptic ascent moves overhead. This will also be when the surface low is progged to develop and shift northeast, and the combination of enhanced low-level moist advection and impressive upper diffluence should spread precipitation northward while also intensifying the precip. The cold surface high will remain in place across eastern Canada behind the surface cold front, and is likely to be reinforced by mid-level confluent flow downstream of the approaching trough axis. The guidance has trended a little colder tonight for D2 and D3, and where the robust WAA drives the warm nose above 0C, a corridor of moderate to heavy freezing rain and sleet is likely. For D2, the greatest risk for more than 0.1" of freezing rain will again be across the Ozarks, with a secondary maxima from the eastern OH Valley into the Mid-Atlantic, with more than 0.25" likely in the Laurel Highlands. Late D2 into D3 the freezing rain threat will wane, but light accumulations of freezing rain are possible Thursday night into Friday morning as far east as the I-95 corridor from Washington, D.C. to New York City. North of the mixed zone, there is increasing confidence in a swath of moderate snow across the Great Lakes, with heavy snow likely in Upstate New York and New England. For the Great Lakes, the northern jet streak will tug moisture northward, aided by a weak northern stream shortwave embedded within the flow. This should produce some enhanced ascent with modest fgen leading to a stripe of snow which WPC probabilities indicate has a 20-30% chance of exceeding 4 inches. Near Chicago, some E/NE flow across Lake Michigan could produce locally heavier snow through LES enhancement. As the surface low shifts northeast on Friday, intense WAA will lead to a stripe of strong mid-level fgen beneath some neutrally symmetric instability with SEPV below 0.25. This suggests intense snowfall rates developing within the warm-advection "thump" from northern PA through Upstate New York and much of southern and central New England. The DGZ will be somewhat narrow which will limit snowfall rates, but it is likely at least several hours of 1"/hr snowfall will occur in this region. The system will exit rather quickly by Friday night, but WPC probabilities are high for more than 6 inches from the Catskills through much of New England except the south coast and far northern Maine. Isolated amounts of 12" or more are possible as shown by the WSE 90th percentile. Weiss Key Messages for Feb 23-25 Winter Storm from Southern Plains through the Northeast: - A significant winter storm will bring a variety of winter hazards from the Southern Plains through the Ohio Valley today through Thursday night, and the Northeast Thursday night through Friday night. - Significant ice accumulations greater than 0.25 inches are likely from the Red River Valley of Texas through the Ozarks and southeast Missouri. Locally, damaging ice of 0.5 inches or more is possible which could lead to scattered power outages, tree damage, and dangerous travel. - Another area of freezing rain is likely from the eastern Ohio Valley through the Mid-Atlantic states. Significant ice accumulations are possible across south-central Pennsylvania and western Maryland, with hazardous travel possible. - Heavy snow is expected across Upstate New York and New England on Friday when snowfall rates will likely eclipse 1"/hr at times. Dangerous travel is likely.