Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 445 AM EST Mon Feb 21 2022 Valid 12Z Mon Feb 21 2022 - 12Z Thu Feb 24 2022 ...High Plains through the Great Lakes and far Northern New England... Days 1-2... Two rounds of heavy wintry precipitation will impact this region through mid-week. The first is a developing area of heavy snowfall associated with low/mid level fgen produced via the elevated front behind a southward sinking surface boundary overlapped with the RRQ of an upper jet streak shifting off to the northeast into Canada. Modest isentropic ascent at 285K on low-level southerly flow will advect ample moisture northward noted by mixing ratios of 3g/kg, supporting a surge in PWs to more than +1 standard deviation above the climo mean. At the same time, arctic air flooding southward will deepen the DGZ, and where this ascent maximizes into the crystal growth zone, heavy snowfall rates of more than 1"/hr are likely. A band of snowfall should translate eastward today from near the SD/ND border through central/northern MN and into the U.P. of MI where WPC probabilities for more than 4 inches of snow are above 90%, and locally up to 10" is possible as high SLRs approach 20:1. This leading polar jet streak will continue to arc away to the northeast by Tuesday morning, but upper diffluence will remain impressive as the weakening RRQ begins to couple with an intensifying LFQ of a subtropical jet streak arriving from the southwest. This secondary jet will strengthen in response to upstream amplification of an upper trough digging through the Intermountain West. The deep layer ascent through height falls and upper diffluence will overlap with the low-level baroclinic gradient to drive surface cyclogenesis in the lee of the CO Rockies early on Tuesday. This low will then race northeastward towards the Great Lakes Tuesday evening and then into the Canadian Maritimes on Wednesday. As this low develops and strengthens in the High Plains, easterly flow north of the low will combine with E/NE winds behind the cold front to intensify upslope ascent into the Central Rockies, with heavy snow likely in the terrain. Moderate snow is also likely along the I-25 corridor and into the High Plains east of Cheyenne and Denver where WPC probabilities are moderate to high for at least 2 inches each day, with storm total snow of 4-6" or locally higher possible. Further to the northeast, even more impressive moist advection drives the theta-e ridge northward into the Upper Midwest and Great Lakes, serving to re-introduce heavy snow to the MN/WI/MI area. The guidance is again suggesting an intense deformation axis developing across SE MN and pivoting northward with CSI potential and snowfall rates of 1"/hr or more thanks to high SLRs. There remains some spread in the placement and intensity of this axis, but WPC probabilities are high for an additional 4+ inches on D2 from near the Twin Cities through the U.P. of Michigan, with locally an additional 8 inches possible leading to storm total snowfall of 12-16" in parts of MN/WI/MI. Further to the south, from late D1 through D2 a warm nose on the increasing LLJ will produce some mixed precip of freezing rain and sleet. Forecast soundings are somewhat dry aloft suggesting more scattered coverage of precipitation in more showery-type activity with some low-probabilities for instability lifting northward into WI and the L.P. of MI. Although precip may be generally light, surface wet bulbs temps in the mid to upper 20s and relatively light precip rates could lead to efficient accretion of freezing rain. WPC probabilities for freezing rain accretion have increased, and now feature a greater than 90% chance for 0.1", with locally up to 1/3" freezing rain possible, especially in lower Michigan. Across the northern L.P., more sleet is suggested by the HREFv3 and forecast soundings, but light ZR is also likely during D2. Finally, as the surface low continues to the northeast Tuesday night into Wednesday /end of D2/ and the jet streak lifts away from the area, additional moderate snow and light icing is likely into Northern New England. WPC probabilities for 4+ inches of snow are 50-70% for northern Maine, and as high as 30% for 0.1" of ice in the White Mountains. ...Western U.S.... Days 1-3... Widespread precipitation will affect much of the Western United States through the middle of the upcoming week as an upper trough continually becomes reinforced by several shortwave impulses, and amplifies into a positively tilted longwave trough across much of the West. Two distinct shortwaves, one moving into the Pacific Northwest this morning, and a second dropping along the OR/CA coast on Tuesday before shifting into the Great Basin Wednesday, will act together to carve out an anomalously deep trough centered over Nevada by D3. Each of these shortwaves will produce waves of PVA and height falls, with associated jet streaks driving rounds of modest divergence. As this trough sharpens and deepens, a surface cold front will dig across the region causing snow levels to plummet to the surface, while also driving enhanced instability through steepening lapse rates beneath the associated cold pool. This setup will produce waves of moderate to heavy snow across nearly the entire West, with a slow southward push of the heaviest accumulations through mid-week. For D1, WPC probabilities for more than 6 inches of snow are highest in the OR Cascades, the Shasta/Trinity ranges of northern CA, and the Sierra where they eclipse 70%, and locally 12 inches is likely. Additionally, heavy snow exceeding 6 inches is supporting by WPC probabilities o more than 70% in the Central Rockies including the San Juans, Snowy Range, CO Rockies, and into the Uintas and Wasatch of Utah where vorticity lobes, persistent mid-level divergence, and at least modest upslope on E/NE winds around a surface low and cold front banked against the terrain will overlap. By D2, the heaviest snowfall along the coast will shift into the Sierra and the San Bernadino range where WPC probabilities for more than 6 inches reach 70%. However, persistent mid-level divergence directly downstream of the amplifying trough combined with mid-level WAA will drive significant snowfall accumulations of more than 12 inches into the San Juans and southern Wasatch, with additional heavy snowfall in the surrounding terrain including thee Mogollon Rim. With snow levels falling through the period, light accumulations are also expected in the valleys and in the lower elevations surrounding San Francisco, CA. By D3, the trough amplifies to its deepest anomaly producing heavy snow once again in the Mogollon Rim, San Juans, and Sangre De Cristos where WPC probabilities are once again high for more than 6 inches. 3-day snowfall in the Central Rockies could be quite impressive, exceeding 2 ft in many of the ranges, with more than 4 ft possible in the San Juans. ...Southern Plains into Mid-Mississippi Valley... Day 3... An arctic cold front front will drop all the way to the Mexican border with TX Wednesday night, surging sub-freezing surface air well into Texas and towards the Gulf Coast. As this occurs, upstream amplification of a positively tilted longwave trough will drive an intensifying jet streak over the Plains Midwest. This will produce an overlap of deep layer ascent through mid-level divergence east of the trough axis, and upper jet diffluence within an impressive RRQ. At the same time, moist advection out of the Gulf of Mexico will become impressive as the LLJ intensifies out of the S/SW, driving robust isentropic ascent at 290-295K atop the surface front. Together, this will produce an expanding area of precipitation from central TX through the Ozarks and into the Mid-Mississippi Valley, with freezing rain and sleet likely to become widespread as a warm nose surges atop sub-freezing surface temperatures. There continues to be spread in the low-level thermal fields as to whether the precipitation will be more freezing rain or sleet, but the consensus is for a developing significant freezing rain event, and WPC probabilities D3 are already above 60% for 0.1" of accretion, with a narrow corridor of greater than 40% for 0.25", highest in the Ozarks. Weiss Key Messages for Feb 21-22 Winter Storm: - A significant and long duration winter storm will bring two waves of snow, sleet, and freezing rain to areas of the Plains into the Great Lakes. - Snow will become heavy today and tonight, with another round of heavy snow likely on Tuesday. Snowfall rates will likely exceed 1"/hr at times, which when combined with increasing wind may produce near blizzard conditions with considerable blowing and drifting. Travel is discouraged. - Total snowfall amounts will likely exceed 6 inches from the Dakotas into the western Great Lakes. More than 12 inches is possible in some areas. - Sleet and freezing rain will likely overspread the Upper Midwest and Great Lakes Monday evening through Wednesday morning. Significant ice accumulations of more than one-quarter inch are possible. - Very cold air is expected for much of this week with temperatures near record low values and dangerously cold wind chills in the Plains.