Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 300 AM EST Sun Feb 25 2024 Valid 12Z Sun Feb 25 2024 - 12Z Wed Feb 28 2024 ...Pacific Northwest and California to the northern and central Rockies... Days 1-3... A strong winter storm and cold front will begin to impact the northwestern U.S. later today, before progressing further southeast Monday and Tuesday -- producing widespread heavy mountain snow and dangerous travel conditions across the region. A dynamic shortwave trough, currently positioned over the Gulf of Alaska, is forecast to dive southeast -- carving out an amplified trough across British Columbia into the Pacific Northwest later today and through the overnight. Increasing onshore flow ahead of the wave will support heavy snow developing initially over the Olympics, northern Cascades, and the northern Rockies, before spreading south along with a sharp cold front that will begin to plunge southeast across the Northwest by this evening. Snow is expected to quickly accumulate, with guidance continuing to show snowfall rates of 1-2 in/hr developing over the northern Cascades, and portions of the northern Idaho and the northwestern Montana Rockies by the afternoon, with these rates continuing as the snow edges south during the evening and overnight hours. By early Monday, WPC PWPF continues to indicate that accumulations of a foot or more will be common across the northern Cascades and the northern Idaho and northwestern Montana ranges -- likely impacting travel as snow levels drop below pass level. The upper trough will continue to amplify and dig southeast -- pushing its associated cold front into Northern California, the Great Basin, and the central Rockies by late Monday. This will bring areas of snow through the Oregon Cascades and into the Sierra Nevada, as well as across the remainder of the northern and into central Rockies. WPC PWPF shows that widespread snow accumulations of 6 inches or more are likely across these areas on Monday into early Tuesday. Supported by strong onshore flow and favorable upper forcing, portions of the Oregon Cascades are likely to see over a foot of snow during this period. The WPC PWPF also indicates that amounts of a foot more are also likely along the Tetons, as right-entrance upper jet forcing helps to increase ascent across western Wyoming late Monday. As it plunges southeast across the region, snow squalls are likely along the path of the cold front, especially over the northern Great Basin and Rockies on Monday. Brief, but intense snowfall, along with strong gusty winds may rapidly reduce visibility and contribute to dangerous travel conditions. While amounts will be less than the previous day, persistent onshore flow, along with trailing energy dropping into the base of the broader scale trough, will support additional snowfall from western Washington State to the northern Rockies. Snow levels, which are already expected to be below 500 ft across much of the region early Monday, will dip even further, bringing the potential for accumulating snow to the coast and onto the valley floors from Washington State through the Intermountain West Monday into Tuesday. On Tuesday, drier conditions will spread from west to east across California and the Great Basin as the upper trough begins to pivot east and is followed by a shortwave ridge that will move across the West beginning late in the day. However, heavy snow will continue into late Tuesday for portions the central Rockies. Additional accumulations of 8 inches or more are likely for many of the western Colorado ranges and into far north-central New Mexico along the San Juans before ending Wednesday morning. By early Wednesday, the ridge in the Northwest will begin to give way to the next approaching upper trough, with warm advection precipitation and the threat for heavy snow returning to the Olympics and the northern Cascades. Although snow levels will be rising across western Washington, the Cascade passes are likely to be impacted by several more inches of snow late Tuesday into Wednesday. ...Northern Plains to the northern Great Lakes... Days 2-3... On Monday, light snow will spread east of the Rockies, with a band of potentially heavier snow setting up across eastern North Dakota into northern Minnesota Monday night into Tuesday. While the general model consensus does not indicate widespread heavy amounts at this point, there is good signal for a potential narrow band of heavy snow, supported by an upper jet couplet and low level frontogenesis, setting up across the region. This initial band is expected to be followed by a second band, setting up a little further to the east across northern Wisconsin and the U.P. of Michigan as a secondary wave developing along the trailing cold front, becomes the primary low and lifts across the region Tuesday night. WPC PWPF shows high probabilities for accumulations of 4 inches or more extending from northwestern Wisconsin into the western U.P. Pereira *** Key Messages for Significant Western U.S. Storm *** --A strong winter storm and cold front will move into the Northwest later today and progress southeastward on Monday into the Northern Rockies. --Heavy mountain snow over the Cascades will impact the passes by late tonight, where there is >80% chance of at least a foot of snow above 1500 ft through early Tuesday. --Snowfall will become heavy at times with rates of 1-2"/hr along with strong winds, creating areas of blowing and drifting snow as well as significantly reduced visibility. --Snow squalls are likely along the path of the cold front on Monday over the northern Great Basin/Rockies, which could create a rapid drop in visibility and a flash freeze on roads leading to dangerous travel. --Much colder air behind the strong cold front will drop temperatures into the teens and colder by Tuesday morning.