Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 340 PM EST Wed Feb 23 2022 Valid 00Z Thu Feb 24 2022 - 00Z Sun Feb 27 2022 ...Southwest U.S.... Day 1... An anomalous longwave trough/closed upper low will unravel this evening as it shifts eastward and becomes absorbed into the westerlies. As it does so, 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. An associated cold front will allow for lowering snow levels to around 1000-2000 ft in the southern/central Rockies, and 2000-3000 ft across central/southern AZ/NM. Above these levels, this intense ascent will drive heavy snowfall that began this afternoon, which could feature intense rates as well as instability climbs beneath the associated cold pool. WPC probabilities 00Z are moderately high for additional 8+ inches this evening along the San Juans and Sangre de Cristos, of northern NM and southern CO. The best ascent and moisture will shift eastward by late tonight. ...Southern Plains through the Mid-Mississippi Valley and Great Lakes... Days 1-2... A long duration and multifaceted storm will bring moderate to heavy wintry precipitation to a large portion of the central CONUS. An anomalously deep 500mb trough, forecast to produce heights in the bottom 5th percentile across the Southwest tonight 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 NM 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 wintry precipitation that is currently located over southern MO and northern AR is expected to weaken tonight along the nose of a potent LLJ out of the Gulf of Mexico. This will likely be in response of the LLJ turning parallel to the surface front, avoiding major overrunning until the next surge of moisture enters by early Thursday. The regeneration of precipitation, with a much larger expanse, is forecast to be initially light, with freezing rain and sleet once again developing across northern TX and OK by Thursday morning /D2. This precipitation 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 through D2, and where the robust WAA drives the warm nose above 0C, a corridor of moderate to heavy freezing rain and sleet is likely. For D1, the greatest risk for more than 0.1" of freezing rain will extend across a far region from far north-central TX, through SE OK, N/Central Arkansas, S MO, and S IL. An embedded area of a quarter in inch or more of ice is likely (WPC probs 50-70%) throughout northern and NE AR. Here, ice accretion could lead to scattered tree damage and power outages. North of the mixed zone, light to moderate snow is possible (amounts generally under 5 inches) across the Midwest and Great Lakes into Day 2. WPC probabilities for at least 4 inches of snow are 30-50% from northern IL to northern IN and southwestern MI. Locally heavier snow is expected over Chicago due to lake enhancement in easterly flow off of Lake Michigan - local 6 inches are likely there. ...Ohio Valley, Mid-Atlantic, and New England... Day 2-3... 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 extreme 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 (>70%) for more than 8 inches from the Adirondacks through much of VT, NH, and northern MA. Isolated amounts of 12" or more are also possible as shown by the WSE 90th percentile for this region as an additional axis of 850 mb FGEN lifts into northern MA following the formation of a secondary low pressure system off of southern New England on Friday. The biggest question mark for the Northeast and New England is how far north the mid-level warm nose extends. For example, most guidance keeps the mixing line south of Albany and Boston; however the 12z NAM 3km extends this line much farther north. Given the anomalously warm air advecting from the Southeast, this warmer aloft solution is a possibility and would significantly reduce snowfall amounts along the southern tier of highest snowfall probabilities. Farther south, warm air will win out aloft and lead to widespread freezing rain in the Ohio Valley, central Appalachians, and northern Mid-Atlantic. WPC probabilities for at least a quarter inch of ice are high (>50%) across the Laurel Highlands and Allegheny Mountains of SW PA and far W MD (a well known prolonged icing area in cold air damming). Low probabilities of a quarter inch of freezing rain extends across southern PA (to the western Philly suburbs) and far northern MD as ice accretion most likely occurs before sunrise on Friday. FZRA is also possible along a narrow corridor north of the warm front in the Ohio Valley from southern/central IN through central OH. Snell 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. - Significant ice accumulations greater than 0.25†are likely from the Red River Valley of Texas through the Ozarks and southeast Missouri. Locally, damaging ice of 0.5†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 1â€/hr at times. Dangerous travel is likely.