Subj : North Plains Winter Storm To : All From : Dumas Walker Date : Thu Mar 21 2024 08:18:00 FOUS11 KWBC 210819 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 419 AM EDT Thu Mar 21 2024 Valid 12Z Thu Mar 21 2024 - 12Z Sun Mar 24 2024 ....Pacific Mountain Ranges... Days 2-3... A large, barreling upper low off the West Coast on Friday will begin directing a healthy fetch of 850-700mb moisture at the Pacific Northwest and northern California. NAEFS is indicating an IVT of 300-400 kg/m/s (>90th climatological percentile) will steadily work its way into California on Friday. This IVT will be the primary driver in producing heavy snowfall in the Salmon/Siskiyou Mountains of northern California, as well as along the spine of the Sierra Nevada. These mountain ranges also reside beneath the diffluent left-exit region of 180kt 250mb jet streak located off the Southern California coast Friday night. NAEFS is showing 200mb winds Friday afternoon and into Saturday that are above the observed 200mb wind speeds within the CFSR climatology database for this time of year. This shows that along with the usual strong upslope ascent, that impressive synoptically-forced ascent will be present as well. Snow levels will initially start out between 4,000-5,000ft Friday night but drop to as low as 3,000ft by Saturday as the upper trough moves into the Northwest. However, snow totals that likely surpass 12" will be at elevations >5,000ft Saturday and through Sunday morning. Farther north, while the moisture transport is not as prevalent, the Pacific Northwest lies directly ahead of the approaching upper low, which will still support prolonged synoptically-forced ascent aloft and lowering snow levels. Snow levels will be a near carbon-copy of the scenario farther south in the California Ranges, but the Cascades Range and Olympics >5,000ft will have the best odds of seeing cumulative snowfall totals top 12 inches for this event. As the upper low moves ashore and over the Northwest on Saturday, the axis of Pacific moisture will be directed farther inland as well, leading to periods of snow for elevations >6,000ft in the Great Basin, the Blue Mountains of eastern Oregon, the Boise/Sawtooth of Idaho, the Tetons and Absaroka, and event as far inland as the Wasatch and Colorado Rockies by Saturday night. WPC PWPF has identified the Sierra Nevada as the only mountain range sporting high chances (>70%) for snowfall totals >12". Once the snowfall threshold dips to 6", however, just about every aforementioned mountain range above sports moderate-to-high chances (50-80%) for >6" of snowfall l;ate Friday night and through Saturday night as well. ....Northern Rockies & Northern High Plains... Days 2-3... Even in wake of the storm system that tracked into the Upper Midwest Thursday night, the large dome of high pressure over the Canadian Prairies and lower pressure in the West continues to support a prolonged low-level easterly flow regime into the western Montana Rockies, as well as far south as the Absaroka and Big Horns of northern Wyoming. Another 700mb FGEN band will strengthen throughout the day Friday and into Friday night as 700mb WAA increase over the northern High Plains. In addition, much of Montana lies beneath the divergent right-entrance region of a 100kt 250mb jet streak. This is a recipe for periods of snow on Friday, however snowfall rates will not come down heavily enough during the day on friday to do much more than Minor Impacts. Snow will be able to accumulate better Friday night once the sun sets. By Saturday morning, the approaching upper low positioned over the Pacific Northwest will spawn a new surface low somewhere over eastern Idaho Saturday morning that then tracks into northern Wyoming by Saturday evening. This new surface low grows to become rather anomalous, eventually dropping below the 1st climatological percentile according to NAEFS over central Wyoming Saturday evening. To the north of this surface low, easterly 850mb winds will increase in strength from central Montana to the Dakotas to the point where 850mb zonal wind anomalies also dip below the 1st climatological percentile. These 850mb winds will also direct a surge of 850mb moisture flux that originates from a LLJ that originates as far south as the Lower Rio Grande Valley. With a strengthening low pressure system emerging into the northern High Plains Saturday night and cold Canadian High Pressure anchored to the north, all the ingredients are coming together for a potential significant winter storm from the northern Rockies to the Northern Plains this weekend. This far out, however, there remains uncertainty in storm track, duration, precipitation type, and of course, snowfall totals. Between 12Z Friday - 12Z Saturday, WPC PWPF shows the Lewis Range as having low chances (10-30%) for >6" of snow with elevations >6,000ft most favored. By 12Z Saturday - 12Z Sunday, the low-to- moderate chance swath (30-50%) for >6" of snowfall grows substantially to include most of the northern Rockies in elevations >6,000ft, through central and eastern Montana, and into western North Dakota. Of course, the storm system will continue to produce heavy snow and blustery winds into Sunday, so those potentially in the path of this impending winter storm will want to check in with their local WFO and our WPC medium range suite of products of the latest regarding this winter storm for Sunday and into next Monday. Mullinax ***Key Messages for Northern Plains Winter Storm March 23 - March 26*** -- Winter storm becoming likely: Confidence continues to increase that a large storm system will produce a swath of heavy snow, freezing rain, and sleet across portions of the Northern Plains and Upper Midwest into early next week. -- Widespread heavy snow: Heavy snow is expected to overspread eastern Montana late Saturday, then expand into the Northern Plains and Upper Midwest by Sunday night, producing significant accumulations. Snow should exit the area during Tuesday. -- Significant impacts due to snow, ice, and wind: A combination of heavy snow and gusty winds, as well as freezing rain south of the heavy snow, will likely produce areas of hazardous travel and impacts to infrastructure. -- Forecast changes anticipated: Uncertainty remains with the timing and location of the storm track, which will affect where the most significant impacts will occur. Keep up to date with the latest forecast as this storm evolves. $$ --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175) .