Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 340 AM EST Tue Nov 29 2022 Valid 12Z Tue Nov 29 2022 - 12Z Fri Dec 02 2022 ...Pacific Northwest, California, and Northern Rockies... Days 1-3... The next in a series of active weather systems will begin to affect the Pacific Northwest later tonight into Wednesday. A potent shortwave trough seen in the current water vapor imagery off of British Columbia will eventually close off at 500 mb as it drops toward northwest Washington. That feature gradually opens up into a positively titled trough over the West but then is followed by another strong closed low approaching the Northwest by Friday. For today, the greatest snow accumulations are focused on the Washington Olympics and Cascades, but snow levels as low as a few hundred feet will support lower elevation snows, potentially impacting the Seattle metro. The latest guidance remains a bit uncertain on the depth and degree of cold air with some guidance trending warmer compared to the 12Z cycle, while the GFS remains a colder solution. The latest WPC probabilities are 30 to 50 percent for at least 1" for Seattle metro. At higher elevations and places with direct exposure to Pacific moisture a heavy snow event for the mountain areas is expected to begin late today in western WA and shift south down the coast through the Cascades through Wednesday night, continuing in northern California through Thursday. Moisture increasingly reaches inland areas such as eastern Washington and Idaho as the southwesterly jet downstream of the trough axis strengthens Wednesday and Thursday. WPC snow probabilities for 8 or more inches are high for the Olympics and Washington Cascades and moderate for the ranges of north-central and northeast Washington and the Oregon Cascades. The latest WPC snow probabilities are also high for 8 or more inches over the Oregon and California Cascades and the northern Sierra Nevada and moderately high for the Sawtooth and Bitterroot Ranges of ID/MT. Storm total snowfall for the event through Day 3 (12Z Friday) could exceed 3 feet in places for the highest terrain areas. Please the latest WPC Winter Key Messages below for more details and impacts on this winter storm. ...Central Rockies, Central Plains through the Upper Midwest... Day 1... Current water vapor imagery this morning showed a digging trough over the central Rockies lee cyclogenesis takes shape over eastern Colorado. This low is forecast to track northeast as the trough amplifies over the Plains. The low track hasn't changed too much over the last couple of forecast cycles and the most recent model guidance shows high confidence. The increasing upper divergence and low level frontogenesis northwest of the surface low track will bring a stripe of accumulating heavy snowfall across portions of Nebraska through the Upper Peninsula of Michigan. The 00Z guidance trended wetter with its QPF axis from far northeast NE, northwest IA, southeast SD through much of MN into northwest WI and U.P. of Michigan. A narrow stripe of heavy snow with accumulations likely over 6" appears likely from southwest MN northeast through the U.P. of Michigan, including MSP metro, as depicted by the latest WPC probabilities for 6" now over 50-60 percent. Accumulations 8-12" appear possible across northwest WI into U.P. Michigan where the frontogenetical forcing will be strongest for a longer duration with some enhancement due to the northerly flow of Lake Superior as well. ...Great Lakes... Days 2-3... In the wake of the strong low pressure and cold front passing through the region Wednesday, an impressive 1030+ mb high settles over the mid Mississippi Valley. The resulting pressure gradient and westerly flow will set up a favorable period of lake effect snow Wednesday into Thursday. The most recent WPC probabilities show high probabilities for at least 4 inches in the favored snow belt areas in the western U.P. and northwestern L.P., with high probabilities for 6 inches off Lake Erie around the Chautauqua region in western New York and in the Tug Hill off Lake Ontario. For Days 1-3, the probability of icing exceeding 0.1 inch is less than 10%. Taylor ***Key Messages for Nov 30 to Dec 2 Winter Storm Affecting Western U.S. from Wednesday to Friday*** -Widespread travel disruptions are expected, especially at higher elevations and in mountain passes, across the Western U.S. from Wednesday to 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 Northwest, including the Cascades, will be affected from Wednesday into early Thursday. Very 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 Wednesday Night into Thursday Night. 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. -The Intermountain West and Northern Rockies will be affected by heavy snow from Thursday into Friday, with a continuation of travel impacts.