Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 447 AM EST Sun Feb 20 2022 Valid 12Z Sun Feb 20 2022 - 12Z Wed Feb 23 2022 ...Western U.S.... Days 1-3... Widespread precipitation will affect much of the Western United States through the middle of the upcoming week. Mid-level confluent flow will gradually amplify into broad trough enveloping much of the west, with a large positively tilted longwave trough centered over the Great Basin progged by Wednesday morning. This trough will amplify in response to a series of shortwaves digging southward along the Pacific Coast, and each successive impulse will bring increased ascent through height falls and PVA, while also driving more impressive instability as the cold pool aloft deepens. At the same time, a surface cold front will slowly drop southward across much of the West, providing low-level convergence, but also leading to pronounced upslope into the terrain of the Rockies where the front gets wedged, and driving lowering snow levels through the period. For D1, the heaviest snow is likely in the WA/OR Cascades, as well as in the Northern Rockies and adjacent terrain including the Blue Mountains and Absarokas. Increasing moisture within the mid-level onshore flow and embedded within Pacific jet energy will manifest as heavy snowfall where ascent is most impressive. WPC probabilities for 6+ inches are above 80% in the Cascades and higher terrain of the other ranges, with locally well over 1 foot possible, especially in the Cascades. With the cold front sinking southward during this time, upslope flow into the Absarokas and Northern Rockies will aid in producing heavy snowfall there as well. Additionally, lowering snow levels will allow for snowfall to become increasingly impactful across many of the Western Mountain Passes beginning late D1. By D2, the heaviest precipitation begins to split into two primary axes. The first will persist across the Cascades where a second shortwave will dig southward within pinched mid-level flow, while low-level flow upslopes into the region. With snow levels continuing to fall beneath the upper trough, another day of heavy snow is expected from near Mt Rainier southward towards Mt. Shasta, and WPC probabilities are high for at least 6 inches, with locally an additional foot possible in the highest terrain. Additionally on D2, a wave of low pressure is expected to develop in the lee of the Colorado Rockies, with low-level convergence along the cold front and upper divergence within an increasingly coupled upper jet structure leading to surface pressure falls. Increasingly impressive deep layer ascent through divergence will combined with enhanced E/NE upslope flow behind the front and north of the developing surface low to increase moisture and spread heavy snow into the High Plains of WY and CO and westward into the Laramies, Snowies, CO Rockies including the Front Range, Park Range, and San Juans, and as far west as the Wasatch in UT. WPC probabilities are high for 6+ inches, with locally more than 1 foot likely. East into the High Plains and I-25 corridor, several inches of snowfall is forecast. During D3, the heaviest snow will again be split into two regions across the West, with heavy snow likely in the Sierra where height falls and PVA through a potent vorticity lobe combine with onshore low-level flow upsloping into the terrain. WPC probabilities for more than 6 inches are high in the Sierra, with light snow accumulations possible even into the lower elevations around San Francisco as snow levels crash to just a few hundred feet. More impressively D3, impressive WAA and jet level dynamics combine over the Four Corners with upslope flow from the E/NE aiding ascent. Significant moisture and persistent ascent should create a long period of at least moderate snowfall across the Four Corners rangers, including the San Juans, Sangre De Cristos, and the Mogollon Rim. WPC probabilities D3 are high for more than 6 inches, with well over 1 foot likely in the San Juans. ...Northern Plains through the Great Lakes and far Northern New England... Days 1-3... Two waves of heavy snowfall are likely through mid-week, with freezing rain and sleet also likely. A strong cold front dropping out Canada will shift southward while a polar jet streak shifts eastward into eastern Canada. Confluent mid-level flow east of a shortwave across the Pacific Northwest will drive enhanced moisture into the Northern Plains D1, upon which intensifying fgen through the elevated baroclinic gradient behind the surface front combined with the RRQ of the upper jet streak will drive a corridor of heavy snowfall from west to east. The guidance has trended a bit northward with this band tonight due to a more northward progression of the upper jet, but a cooling column, deepening DGZ, and impressive 285K isentropic ascent should produce heavy snowfall with high SLRs. Additionally there is some potential for CSI banding within this swath, but confidence is somewhat limited in that development due to the placement of the most robust ascent being slightly above the DGZ. Still, heavy snow is likely D1 along the ND/SD border where WPC probabilities are moderate for 6 inches. This band will intensify into D2 as the DGZ deepens to more than 100mb and isentropic ascent maximizes into the column. Additionally, overlapped synoptic ascent and better CSI potential where theta-e lapse rates begin to fold suggest snowfall rates could exceed 1"/hr late D1 into D2 from eastern SD through central MN. This band will likely translate rapidly to the northeast limiting the duration of heavy snow and capping the maximum potential accumulation, but WPC probabilities D2 are high for 6 inches from near Aberdeen, SD to just north of the Twin Cities and up into the Bayfields Peninsula of WI. Locally, up to 12 inches of snow is possible. Also on D2 and then extending into D3, a more impressive precipitation event should occur, beginning in the lee of the CO Rockies. A wave of low pressure is likely to develop and move northeast, deepening in response to height falls and an increasingly coupled jet structure across the Central Plains. As this low shifts northeastward, warm and moist advection ahead of the wave will drive PWs as high as +2 standard deviations above the climo mean as the theta-e ridge lifts into the Upper Midwest. Deep layer ascent will help expand the precipitation shield from SW to NE, while a potent deformation axis may arc just NW of the low center to enhance ascent within the very moist column. All of this ascent may occur within an extremely deep DGZ and very cold column supporting high SLRs. SLRs are currently forecast to reach as high as 17-20:1, near the Baxter 75th percentile, but coordination with the affected WFOs, and analysis of the NBMv4.1 and NAM Cobb SLRs suggest SLRs may exceed 20:1 at times. Uncertainty persists with the placement of this deformation axis, as well as its intensity, and how impressive the "fluff factor" may be due to the limiting factor of impressive moist advection. The current WPC probabilities for more than 6 inches are moderate and broad reflecting the spread in the guidance by D3, but do reach as high as 70% focused in SE MN and into the U.P. of MI where some LES enhancement is also possible. The swath of snow will also likely extend well northeast within the departing polar jet streak to produce heavy snowfall across far northern ME on D3, where WPC probabilities for more than 6 inches reach 50%. Additionally, as the warm advection surges northward D2-D3, a warm nose will likely drive 850mb temps above 0C from central IA through southern WI and into MI. There is considerable spread in the intensity and latitudinal gain of this warm nose, but it is increasingly likely that a swath of moderate to heavy icing will occur. If some of the cooler guidance verifies, the cold nose may remain strong enough for more IP than ZR, but the preference is for some of the warmer guidance due to the recent northward shift, and WPC probabilities are 50-70% for 0.1" across southern WI, including Milwaukee, and the central L.P. of MI. Weiss Key Messages for Feb 21-22 Winter Storm: - A significant and long duration winter storm will impact areas of the Plains into the Great Lakes starting this evening. A mixture of snow, sleet, and freezing rain is likely. - Snow will become heavy, with snowfall rates likely exceeding 1"/hr at times. Increasing wind may lead to significant blowing and drifting of snow, making travel difficult. - Total snowfall amounts will likely exceed 6 inches from the Northern High Plains through the Western Great Lakes. More than 12 inches is possible in some areas. - Monday night into Tuesday, an area of sleet and freezing rain is likely to lift into the Upper Midwest and Great Lakes. Significant ice accumulations are possible. - Very cold air will follow in the wake of the system with temperatures near record low values Tuesday and Wednesday in the Plains.