Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 417 PM EST Sun Feb 20 2022 Valid 00Z Mon Feb 21 2022 - 00Z Thu Feb 24 2022 ...Western U.S.... Days 1-3... Widespread precipitation will affect much of the Western United States through the middle of the upcoming week. A steady stream of shortwave energy emanating from the Arctic will support a deepening trough, with cold air and the threat for snow spreading south through the western U.S. this period. Models show a well-defined shortwave carving out a deepening trough across the northwestern U.S., supporting snow spreading south from the Pacific Northwest and northern Rockies into the Great Basin and central Rockies. For the 24 hr period ending 00Z Tuesday, WPC probabilities show the greatest potential for heavy snow accumulations is along the Oregon Cascades, where accumulations of 8 inches or more are likely. While widespread heavy amounts are not expected, falling snow levels will support at least some light accumulations across the lowlands of western Washington and Oregon. From Monday night into Tuesday, as the trough continues to amplify the potential for heavy snow will shift farther south into the Sierra with some threat for accumulations of 8 inches or more across the higher elevations of the northern Sierra by late Tuesday. Meanwhile, southwesterly flow ahead of the trough will support deepening moisture and the increasing potential for heavy snow across the central Rockies, with WPC probabilities showing the greatest potential for heavy snow -- 8 inches or more -- across the Elk and San Juan mountains. East of the Divide, upslope flow on the backside of a front dropping south through the High Plains will support snow spreading south along the Front Range, where WPC probabilities indicate that accumulations of 4 inches or more are likely. On Wednesday night and continuing into Thursday, the upper trough is expected to continue amplifying with a deep trough digging into the Southwest by late Wednesday. This will bring the potential for heavy snow into the Southern California ranges, the southern Utah mountains, and the Mogollon Rim in Arizona. Locally heavy accumulations of 8 inches or more are possible across these areas. Meanwhile, heavy snow is likely to continue across the western Colorado ranges, especially across the San Juan and Elk mountains, where additional accumulations of a foot or more are most likely. ...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. Models continue to signal a prolonged, multi-phase winter storm producing heavy snow from southwestern Montana to the U.P. of Michigan and significant icing from the mid Mississippi Valley to northern Lower Michigan. The initial phase is expected to unfold beginning this evening as increasing low-to-mid level frontogenesis, overlapped by favorable upper jet forcing, is forecast to support a band of moderate to potentially heavy snow extending from southwestern Montana to along the North and South Dakota border. This band will continue to shift east overnight before reaching across central Minnesota into northwestern Wisconsin and the U.P. of Michigan on Monday. Consensus of the models over the past few runs has been to shift the axis of heavier amounts further north and this continued with today's 12Z runs. This is reflected in the WPC probabilities, along with an upward trend in heavier amounts within the band. Most recent WPC guidance for the 24 hr period ending Tue 00Z shows high probabilities -- greater than 70 percent -- for snow accumulations of 4 inches or more across southern South Dakota and the northern tier counties of North Dakota, through central Minnesota, and into northwestern Wisconsin and the western U.P. of Michigan. Along this axis locally heavier totals are likely, especially along the eastern North and South Dakota border into central Minnesota, where WPC guidance shows high probabilities for accumulations of 8 inches or more. This initial band will continue to translate east across the northern Great Lakes and southeastern Canada late Monday into Tuesday before reaching into northern New York and New England as a mixture of snow, sleet, and freezing rain, with generally light accumulations expected on Tuesday. Meanwhile, the second phase of this storm is expected to develop Monday night into early Tuesday across the Plains. A developing deformation band associated with a shortwave ejecting out of the longwave trough in the West is forecast to move across northern Nebraska and South Dakota Monday night and early Tuesday before lifting through the upper Mississippi Valley into the Great Lakes during the day on Tuesday. General consensus of the latest models indicate that additional heavy snow amounts for the 24 hr period ending 00Z Wednesday are most likely from the northern South Dakota-Minnesota border across central Minnesota into northwestern Wisconsin and the western U.P. of Michigan. Farther to the southeast, the models have backed away from their signal for heavier accumulations across southeastern Minnesota into central Wisconsin; however, with the ECMWF remaining a heavier outlier this has become an area of greater uncertainty. South of the heavier snow axis, a wintry mix including freezing rain is expected. Overall, the models have trended north with the axis of heavier ice accumulations across southern Wisconsin and Lower Michigan. Model consensus has also increased as well, with the latest WPC probabilities indicating ice accumulations of 0.10 inch or more are likely across parts of southern Wisconsin and central Lower Michigan. On Wednesday the potential for heavy snow and significant icing will decrease across the western Great Lakes as the associated shortwave lifts northeast of the region. Farther east, as a surface low approaches from the west snow and ice are expected to continue, with additional light snow and ice accumulations expected across far northern New York and New England. ...Southern Plains... Days 3... As the upper trough in the West continues to amplify and move east, light to moderate precipitation is expected to develop farther east across portions of the southern Plains. Beginning as early as Wednesday afternoon, low level southerly winds feeding moisture into a low level boundary will support precipitation developing from North Texas through the Ozarks. Shallow arctic air will likely support a wintry mix, with accumulating freezing rain expected across this region by late Wednesday. Pereira 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. A mixture of snow, sleet, and freezing rain may come in two waves to parts of the region. - Snow will become heavy, with snowfall rates likely exceeding 1"/hr at times. Increasing wind may lead to significant blowing and drifting of snow. Near blizzard conditions are possible over parts of North and South Dakota. 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 over the two-day period. - Significant ice accumulations are possible into the Upper Midwest and Great Lakes starting Monday evening into Tuesday. Some areas may receive a quarter inch of ice. - Very cold air is expected for much of this week with temperatures near record low values and dangerously cold wind chills in the Plains.