Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 410 PM EST Sat Feb 19 2022 Valid 00Z Sun Feb 20 2022 - 00Z Wed Feb 23 2022 ...Western U.S.... Days 1-3... A series of shortwaves moving through a broad and slowly amplifying trough will support periods of winter weather and cold air spreading south across the western U.S. this period. Ahead of an amplifying shortwave dropping southeast from the Gulf of Alaska, heavy, orographically-focused precipitation is expected to develop across the Northwest into the northern Rockies beginning this evening and continuing into Sunday. Deepening moisture along with favorable upper jet forcing will support heavy mountain snow developing across western Washington to the northern Rockies. WPC probabilities indicate that accumulations of a foot or more are likely across the high elevations of the Washington Cascades, the Blue Mountains, and the northern Idaho and northwestern Montana ranges. As snow levels drop, significant snow accumulations will become increasingly likely at the pass level, with WPC probabilities indicating high potential for accumulations of 8 inches or more at both Snoqualmie and Stevens passes in the Washington Cascades. As the shortwave continues to dig south, heavier snows are expected to shift south into the Oregon Cascades late Sunday into Monday, with heavy accumulations becoming likely by early Monday. Even though additional heavy accumulations will become increasingly unlikely, the lingering trough will support snow showers farther north across Washington. And as cold air continues to spill into the region, the threat for light accumulations across the lowlands will become more likely. By late Monday and continuing into Monday, snows will begin to spread south along the northern Sierra and spilling east into the Great Basin. While some locally heavier amounts are possible across the higher peaks of the Sierra, guidance is showing a limited threat heavy amounts across California and the Great Basin through late Tuesday. A more significant threat is forecast to develop farther east across the central Rockies, where deepening moisture supported by southwesterly flow ahead of the amplifying trough, along with favorable upper forcing, will support snows developing across the western Colorado ranges. Meanwhile upslope flow on the backside of a front plunging south through the High Plains will encourage snows developing along the Front Range. By late Tuesday, widespread accumulations of 4 inches or more are expected, with the potential for 8 inches or across the higher terrain. ...Northern Plains through the Great Lakes... Days 1-3... A series of shortwaves ejecting east from the previously noted longwave trough over the western U.S. is expected to support periods of unsettled weather, including significant snow and ice across portions of the region. A leading wave, currently dropping into the Pacific Northwest, is expected to race across the northern Plains and Upper Midwest on Sunday. Warm advection precipitation ahead of the system, followed by additional precipitation along the trailing cold front will contribute to a narrow swath of generally light amounts from northwestern Montana to northern Minnesota. While widespread heavy snow accumulations are not expected, strong winds are likely -- resulting in widespread blowing snow and reduced visibilities, especially from eastern North Dakota across the Red River into northwestern Minnesota. By late Sunday the next system is expected to develop and spread east from the High Plains. As the upper trough in the West continues to amplify and energy ejects east, increasing low-to-mid level frontogenesis overlapped by favorable upper jet forcing will support a narrow stripe of moderate to potentially heavy snows developing across southwestern Montana, northern South Dakota, and southern North Dakota Sunday night before shifting east through central Minnesota into northern Wisconsin on Monday. For the 24-hr period ending late Monday, the latest WPC probabilities for significant snow accumulations have shifted a little farther north than the previous run but remain centered just along and south of the North and South Dakota border into central Minnesota. Across this area, WPC probabilities indicate that widespread accumulations of at least 4 inches are likely, with some potential for localized amounts exceeding 8 inches. As this organized band of heavier snow continues to shift east across the northern Great Lakes region Monday night, a more amplified shortwave will begin to lift out of the trough in the West, ejecting east of the Rockies into the northern Plains Monday night into Tuesday. This is expected to support the redevelopment of organized heavier snow centered across South Dakota and northern Nebraska Monday night into early Tuesday, before shifting back into the Upper Great Lakes later in the day. Closer to the associated surface low tracking northeast from the mid Mississippi Valley to the Great Lakes, a wintry mix is expected with accumulating ice likely. For the 24-hr period ending late Tuesday, WPC probabilities indicate that snow accumulations of 4 inches or more are likely from southern Minnesota to the U.P. of Michigan, with some potential for up to 8 inches or more in some locations. South of the heavier snow amounts, ice accumulations of 0.10 inch or more are most likely from along the Illinois-Wisconsin border through central Lower Michigan. Pereira Key Messages for Feb 21-22 Winter Storm: - A significant winter storm will impact areas of the Plains into the Great Lakes starting Sunday. A mixture of snow, sleet, and freezing rain may create hazardous travel conditions. - Snow will expand across the Plains and Upper Midwest Monday, becoming heavy at times. Increasing wind may create blowing and drifting snow, making travel difficult. Total snowfall amounts may exceed six inches from northern South Dakota eastward. - By Tuesday, an area of sleet and freezing rain will lift into the Great Lakes region with significant ice accumulations possible. - Very cold air will follow in the wake of the system with temperatures near record low values Tuesday and Wednesday in the Plains.