Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 321 PM EST Wed Nov 30 2022 Valid 00Z Thu Dec 01 2022 - 00Z Sun Dec 04 2022 ...Pacific Northwest, California, Central and Northern Rockies... Days 1-3... A closed upper-level low with a depth of about 522 dam is forecast to enter western WA tonight from off the coast of British Columbia and will open up into a sharp longwave trough as it swings into the Great Basin on Thursday, then tighten as it crosses the Rockies on Friday in response to a strong upper-level ridge remaining in place over the southern Plains. An atmospheric river will slowly slide southward and focus the greatest QPF and mountain snows tonight initially over southern Oregon and northern California, before impacting more of the Sierra Nevada and northern Rockies on Thursday. 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. As the trough and associate cold front pushes into UT and the central Rockies, heavy snowfall is possible across the higher terrain along with gusty winds. WPC snow probabilities for 8 or more inches are high for the OR Cascades, northern California ranges, and Sierra Nevada through Thursday, as well as high for the Sawtooth and Bitterroot Ranges of ID/MT. By Day 2, the 8 inch probabilities remain quite high for the southern Sierra Nevada, as well as from the greater Yellowstone region to the Wasatch Mountains of UT. Moderate probabilities exist on Friday across the highest terrain of central and north-central CO. After a brief lull in activity across the Northwest on Day 2, another closed low approaches the Pacific Northwest late Friday into Saturday, bringing another round of precipitation. Probabilities for heavy snow are high again for the mountain areas of the WA Olympics, with lower probabilities in the Cascades and over the northern California ranges. Guidance has lowered QPF amounts for this area as the orientation of the upper low directs more of the precipitation to along the coast or just offshore. However, cold air remaining in place could allow for another possible lower elevation snow to portions of western OR and WA valleys with whatever moisture does make it far enough inland.. 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 tonight deeper into Ontario, Canada with a progressive cold front sweeping off the East Coast. 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. LES is already occurring downwind of Lake Superior, Michigan, and Erie, with further development of LES bands downwind of Lake Ontario 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. ...Northern Plains... Days 2-3... A positively titled trough quickly swinging through the western U.S. late Thursday (the same system discussed in the first paragraph) 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 preferred model guidance continuing with a stronger/deeper surface low and resulting low level frontogenesis. Additionally, very cold arctic air in place underneath the best forcing will allow for snow ratios around 15:1 or locally higher. However, it is worth noting this level of cold could create a DGZ that falls below the better omega, limiting the potential for even higher ratios. Additionally, gusty winds on the backside of the tracking surface low could lead to low visibility and hazardous travel conditions where the heaviest snow bands set up. The WPC probabilities for 4 inches or more of snow depict a moderate signal (40-50 percent) across far north-central MN, with 20 percent values extending to eastern North Dakota. For Days 1-3, the probability of icing exceeding 0.1 inch is less than 10%. Snell ***Key Messages for Nov 29 to Dec 2 Winter Storm Affecting Western U.S. into Friday*** - Mountain travel disruptions spread southeast across much of the Western U.S. through Friday. - Check the forecast if you plan to travel by road across the Cascades or Sierra Nevada. 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 have light to moderate snow with low snow levels tonight and Thursday which will lead to continued impacts from lower elevations to mountain passes. - Northern California mountains, including the Sierra Nevada, will have major impacts tonight through Thursday night. At the peak, very heavy snow rates in excess of 3 inches per hour are forecast, leading to dangerous or impossible travel and avalanches. - Heavy snow and moderate impacts to travel will spread southeast across the Intermountain West and Northern Rockies through Friday.