Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 338 AM EST Wed Nov 30 2022 Valid 12Z Wed Nov 30 2022 - 12Z Sat Dec 03 2022 ...Pacific Northwest, California, and Northern Rockies... Days 1-3... A parent upper-level low off the coast of British Columbia this morning will act as an anchor point for lobes of potent vorticity to dive southward from the eastern Gulf of Alaska to the Pacific Northwest over the next few days. The closed low will sag southward today and eventually take on a positive tilt as it shears open over the Pacific Northwest Thursday. An atmospheric river will focus the greatest QPF and mountain snows today initially over southern Oregon then reach northern California tonight. Underneath the mid/upper level troughing, lighter but still widespread precipitation is expected over the OR and WA Cascades and moisture spilling eastward into ID/MT will bring the threat for heavy snow there too. Snow levels across much of the southern Cascades and Sierra Nevada are expected to oscillate around 4000 ft for much of the event. WPC snow probabilities for 8 or more inches are high for the OR Cascades and northern California ranges today, as well as high for the Sawtooth and Bitterroot Ranges of ID/MT. Further north, 8 inch probabilities are moderate for the WA Cascades. By Day 2, the 8 inch probabilities are quite high for the Sierra Nevada and portions of the central ID ranges as well as eastward around Yellowstone. After a brief lull in activity late in the day 2 and early day 3 period, another closed low approaches the Pacific Northwest Friday into Saturday, bringing another round of precipitation. Probabilities for heavy snow are high again for the mountain areas of the WA Olympics, Cascades and into the northern California ranges. Cold air remaining in place could bring another possible lower elevation snow to portions of western OR and WA valleys. Please the latest WPC Winter Key Messages below for more details and impacts on this winter storm. ...Great Lakes... Day 1... A strong low pressure lifting north of Lake Superior will continue to race northward today with a progressive cold front sweeping the region through this evening. In the wake of this strong system, a tight pressure gradient develops across the Great Lakes thanks to a 1030+ mb high settling over the mid Mississippi Valley. This will result in a short but intense favorable period of lake effect snow through early Thursday. The LES bands will first develop off Lake Superior and Michigan this morning then begin over the eastern Lakes later tonight. Probabilities for 4 inches are more are moderate to locally high downwind of the favored snow belt areas with this wind regime, particularly across the Chautauqua region in western New York and parts of the Tug Hill with localized 6-12 inch totals for the day 1 period. For Days 1-3, the probability of icing exceeding 0.1 inch is less than 10%. ...Northern Plains... Day 3... A positively titled trough quickly swinging through the western U.S. late Thursday will reach the northern Rockies by Friday. Favorable forcing for ascent downstream will induce cyclogenesis over the Plains that then quickly tracks northeast toward the Upper Midwest through Friday night. Northwest of the low track, a band of accumulating snow is increasingly likely with the latest model guidance trending stronger/deeper with the low and the resulting low level frontogenesis. The WPC probabilities for 4 inches or more have increased this cycle, with a moderate signal (40-50 percent) showing up across eastern North Dakota and northern Minnesota. There is already an isolated signal for 6-8"+ showing up in the probabilities. The pressure gradient is likely to be very tight as well across the region with a 1040+ mb high back over the Rockies and the low deepening. The strong winds and accumulating snow could result in reduced visibility and hazardous travel conditions for the region. Taylor ***Key Messages for Nov 29 to Dec 2 Winter Storm Affecting Western U.S. into Friday*** - Widespread travel disruptions, especially at higher elevations and in mountain passes, spread across much of the Western U.S. through Friday. - Check the forecast if you plan to travel by road through The West. If you must travel, especially at elevations above 2500 feet, be prepared for rapidly changing conditions and have winter driving supplies. - The Pacific Northwest, including the Cascades, will continue to be affected today. Low snow levels will lead to impacts in many of the mountain passes and some lower elevation communities. - Northern California mountains, including the Sierra Nevada, will be affected from tonight into Friday. At the peak of the storm, very heavy snow rates in excess of 3 inches per hour will be possible, leading to dangerous or impossible travel and some avalanches. - Heavy snow impacts to travel will spread south and east across the Intermountain West and Northern Rockies through Friday.