Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 416 AM EST Tue Feb 22 2022 Valid 12Z Tue Feb 22 2022 - 12Z Fri Feb 25 2022 ...Upper Midwest through far Northern New England... Day 1... A surface low deepening in response to mid-level height falls and a modestly coupled jet structure will strengthen across the Upper Midwest this morning and then race eastward to exit into the Canadian Maritimes Wednesday morning. While this feature will be fast moving, moisture within the leading WAA is likely to be impressive with PWs surging to +2 standard deviations above the climo mean. This moisture will be wrung out as wintry precipitation in the form of snow across central MN, northern WI, and the U.P. of MI through this evening, with sleet and freezing rain likely across southern WI and the L.P. of MI. An elevated baroclinic gradient north of the surface cold front will act in tandem with the RRQ of the departing jet streak to produce some enhanced fgen, into which a deformation axis will stretch SW to NE to enhance snowfall. The HREF continues to indicate snowfall rates exceeding 1"/hr this aftn and evening within this deformation, enhanced by high SLRs in the cold column. WPC probabilities for more than 4 inches continue to be high from near the Twin Cities, through Duluth, over the Bayfields Peninsula, and into the Keweenaw Peninsula where some LES enhancement will also occur. Local maxima above 8 inches is likely, and event-total snowfall may reach 2 feet in some places. South of the heavy snow, the robust WAA will drive a warm nose >0C northward as far as Alpena, MI. This will produce a region of sleet and freezing rain, and while guidance still features some discrepancy in the latitudinal gain of freezing rain, the trend has been slightly further north/warmer suggesting more ZR than IP. Significant accumulations of ice are likely across the L.P. where WPC probabilities for 0.1" are above 50%, and locally more than 0.25" are possible. Lighter accumulations of ice are likely into WI near Milwaukee as well. Further to the northeast, as the wave races into New England and then Canada, a stripe of heavy snow is expected, generally confined to far northern Maine where WPC probabilities for 4+ inches are above 50%. Southwest of there, some modest icing is forecast near the White Mountains of NH into western ME where WPC probabilities for 0.1" of ice are 30-40%. ...Western U.S.... Days 1-2... A large and amplified trough will dig across the Intermountain West through Wednesday as vorticity lobes periodically drop through the base to reinforce the trough. At the same time, Pacific jet energy will rotate around the trough as well, eventually merging into a large scale subtropical jet streak originating near Baja California and shifting northeast into the Plains. This jet streak is progged to exceed 150kts late Wednesday, leaving impressive RRQ diffluence for ascent across the Four Corners. Mid-level WAA downstream of the amplifying trough will work in tandem with the jet streak to enhance moist advection into the region, upon which nearly continuous large scale ascent will occur to drive heavy snowfall. Beneath this trough, a surface cold front will dig southward causing snow levels to plummet to the surface, while additionally steepening mid-level lapse rates to drive instability and better snow rates, especially where upslope flow aids in omega. Widespread heavy snow is likely in the terrain, generally above 1500 ft, from the Sierra and San Bernadino ranges, eastward through the Mogollon Rim and into the Wasatch, San Juans, and CO Rockies D1. WPC probabilities for more than 8 inches are highest in the Wasatch, San Juans, and atop the Kaibab Plateau. Locally in excess of 18 inches is possible in these ranges. Additionally, more than 12 inches is likely in the southern Sierra and highest San Gabriels/San Bernadinos. By D2, the trough amplifies further but moves little. However, the focus of precipitation will shift away from CA such that the Four Corners receive the brunt of the heavy snowfall. Once again, WPC probabilities for 8+ inches of snow are high in the San Juans and Mogollon Rim, with moderate probabilities extending into the Sangre De Cristos. 2-day snowfall will likely approach 4 feet in the San Juans. ...Southern Plains through the Mid-Mississippi Valley, the Ohio Valley, and into the Northeast.. Days 2-3... A large scale winter weather event will begin as some sleet and freezing rain across the Southern Plains and Lower/Mid Mississippi Valley Wednesday evening, and then broaden across the region and expand with all winter-weather p-types into the Northeast by late D3 /Friday morning/. The primary driver of this winter weather event will be an amplified, positively tilted trough, ejecting from the Four Corners region Thursday morning, and this feature will advect eastward, potentially developing into a negatively tilted trough but with less depth by Friday morning over New England. Above this shortwave, an impressive subtropical jet streak is forecast to arc from the Desert Southwest all the way across the country into New England, with some splitting of energy leading to a coupled jet structure over the Mid-Atlantic D3. This synoptic lift will be aided by intense moist advection out of the Gulf of Mexico becoming robust 285-295K isentropic ascent atop an arctic cold front. The overlap of this moisture and deep layer ascent will lead to widespread heavy wintry precipitation across a large swath of the CONUS. As isentropic ascent maximizes and vorticity lobes swing eastward from the parent longwave trough over the West, waves of sleet and freezing rain are likely due to surface temperatures below 0C, but a pronounced 850mb warm nose reaching as high as +8C. The guidance still features some latitudinal spread in the placement of the sleet/freezing rain boundary, but there is increasing confidence in a corridor of more than 0.25" of freezing rain from near the Red River Valley of OK/TX northeast through the Ozarks on D2, and then extending northeast through the Ohio Valley and into the Laurel Highlands D3. The greatest risk for more than 0.25" of freezing rain is likely across the Ozarks where locally more than 0.5" of freezing rain is possible. Surrounding this region of heavy icing, a larger swath of greater than 0.1" of ice accretion is forecast from central Texas into the Mid-Atlantic, where even the I-95 corridor from Washington, D.C. to New York City has a threat for some light icing on D3. While the column is likely too warm to support any significant snow on D2 with this system, a shortwave moving through the Great Lakes will help pull moisture northward into a colder airmass from the northern Ohio Valley and Great Lakes into the Northeast. This will also be during the time of greatest surface pressure deepening, and guidance is beginning to suggest a significant snow event is possible for northern PA, Upstate NY, and New England, especially for D4 just beyond this forecast period. However, on D3, WPC probabilities do indicate a 20-40% chance for 4+ inches of snow from near Chicago, IL northeast through Albany, NY and parts of central New England. Weiss Key Messages for Feb 21-22 Winter Storm over the Upper Midwest: - Periods of moderate to heavy snow will continue today across Minnesota, Wisconsin, and Michigan. Additional snowfall may exceed 6 inches, bringing event-total snowfall to 1-2 ft in some areas. - Moderate to heavy snow combined with strong winds gusting over 30 mph will create near blizzard conditions at times with considerable blowing and drifting snow. Travel is discouraged. - Periods of sleet and freezing rain will lift across Wisconsin and Michigan through this evening. Significant ice accumulation exceeding one-quarter inch is possible which will create hazardous travel and isolated power outages. - Very cold air is expected for much of this week with temperatures near record low values and dangerously cold wind chills in the Plains. Key Messages for Feb 23-25 Winter Storm from Southern Plains to Northeast: - A storm is forecast to develop over the Southern Plains on Wednesday, move toward the Ohio Valley on Thursday, and then into the Northeast on Friday. Snow, sleet, and freezing rain are all likely. - Freezing rain and sleet are forecast to expand over the Southern Plains and Mid-Mississippi Valley starting Wednesday and continue into the Ohio Valley Thursday. Significant ice accumulations are likely, with damage ice possible in the Ozarks. Modest snowfall accumulations are also possible. - As the storm moves northeast on Friday, snow is likely across the eastern Great Lake, northern Mid-Atlantic, and much of New England. Heavy snow accumulations are possible. Mixed precipitation including sleet and freezing rain is likely south of the heavy snow. - Hazardous travel conditions and impacts to infrastructure are becoming more likely. - Details in the track and timing of the storm, in addition to precipitation type and amounts, remain uncertain, but will be refined as the forecast evolves.