Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 315 PM EST Tue Nov 29 2022 Valid 00Z Wed Nov 30 2022 - 00Z Sat Dec 03 2022 ...Pacific Northwest, California, and Northern Rockies... Days 1-3... A parent upper-level low near Victoria Island in far north-central Canada 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 next upper-level feature to enter the Northwest and produce widespread heavy snowfall throughout a majority of the West and Rockies will impact the northern Cascades and other parts of northern WA through the overnight hours. This feature will eventually close off at 500 mb as it drops toward northwest Washington. This system 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. Through Wednesday night, the greatest snow accumulations are focused across north-central WA, the WA Olympics and WA/OR Cascades, but snow levels as low as a few hundred feet to start this afternoon will support lower elevation snows. In fact, snow has already occurred throughout the Seattle metro based on latest observations. Snow levels should gradually increase to around 1000-2000 ft as warmer Pacific air infiltrates northeastward tonight. At higher elevations and places with direct exposure to Pacific moisture, a heavy snow event for the mountain areas is expected throughout western WA and is anticipated to shift south down the coast through the Cascades Wednesday night, continuing in northern California on Thursday and the Sierra Nevada through early Friday. Moisture increasingly reaches inland areas such as eastern Washington and Idaho as the southwesterly jet downstream of the trough axis strengthens Wednesday and Thursday. Moisture flux anomalies per the ECENS reach around +1-1.5 standard deviations above the climatological mean tonight across western OR an WA. The better moisture transport then occurs across the Sierra Nevada on Thursday as the sharpening trough taps into better subtropical Pacific moisture. ECENS moisture flux anomalies increase to around +2-3 standard deviations at this time frame. 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 Olympics and Washington Cascades and moderate for the ranges of north-central and northeast Washington. The latest WPC snow probabilities are also high for 8 or more inches over the California Cascades and the Sierra Nevada, as well as moderately high for the Sawtooth and Bitterroot Ranges of ID/MT. Storm total snowfall for the event through Day 3 (00Z Saturday) 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. ...Great Lakes... Days 1-2... A strong low pressure system is set to rapidly strengthen north of Lake Superior tonight as a shortwave pushing into the central Plains turns the longwave trough into a negative tilt while a closed upper-level low churns just north of MN. Lingering moderate to heavy snowfall is likely across the western U.P. of MI and northern WI associated with this system early this evening, before low and mid-level moisture rapidly dries behind a quickly advancing cold front. Once this deepening system treks further into Ontario, Canada, strong west-northwesterly winds will turn on the lake effect snow machines. An impressive 1030+ mb high settling over the mid Mississippi Valley will result in a strong pressure gradient and westerly flow will set up a favorable period of lake effect snow lasting through at least Thursday night. These LES bands will first begin downwind of Lake Superior and Lake Michigan Wednesday morning, before a potentially higher magnitude event occurs downwind of Lake Erie and Lake Ontario between Wednesday night and most of the day on 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. Meanwhile, moderate WPC probabilities for at least 8 inches exists throughout the Tug Hill Plateau off Lake Ontario where better westerly fetch exists. While not nearly as extreme as the most recent LES event, parts of the Tug Hill could receive just over a foot of snow through the end of the day on Thursday. 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*** -Widespread travel disruptions, especially at higher elevations and in mountain passes, spread across much of the Western U.S. into 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 be affected mainly through Wednesday. 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 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 across the Intermountain West and Northern Rockies Wednesday into Friday.