Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 350 AM EST Thu Dec 01 2022 Valid 12Z Thu Dec 01 2022 - 12Z Sun Dec 04 2022 ...Pacific Northwest, California, Central and Northern Rockies... Days 1-3... Active period will continue for much of the West into the weekend. Upper trough along the WA/OR coast will push into the northern Great Basin today into early Friday and weaken as it looses favorable upper dynamics. Cold front will continue eastward and southeastward carrying a weak atmospheric river through northern California. Snow levels down to near sea level will support additional light snow for the metro areas along I-5 from Seattle southward to Portland but the heaviest amounts are expected over the Oregon Cascades on Day 1. Modest snow amounts are also expected farther east across central Idaho as well as around Yellowstone and points south into the northern Wasatch and Uintas in UT. Height falls and ample moisture will complement lower-level frontogenesis and orographic upslope over these and other higher elevations. WPC probabilities for at least 8 inches of snow are high over many mountainous areas. Back into California, the Sierra will feel the full brunt of the moisture flux orthogonal into the terrain, and heavy accumulations are likely with snow levels around 4000-6000ft to start before crashing as the cold front works through the area. WPC probabilities for at least 18 inches of snow are high above around 5000-6000ft to the north (7000ft+ to the south). By Day 2, the upper trough and cold front will move across the central Rockies where light to modest amounts are possible, especially over Colorado. Concurrently, another upper low will drop southward out of the eastern Gulf of Alaska along 130W just offshore WA/OR late Friday into early Saturday. Trend has been for a bit less QPF over the Pac NW, and mostly confined to the Olympics where significant accumulations are still possible. By Day 3, that upper low will wobble toward the coast but the tail-end of the lead system off California may support another wave of precipitation into the Golden State, focused primarily over the Sierra but also into the Shasta Siskiyous and Klamath Mountains. WPC probabilities for at least 6 inches of snow are moderate (>40%) to high (>70%) over these areas on Day 3. ...Great Lakes... Day 1... West to WNW flow behind a cold front and around its parent low over northern Ontario will continue lake effect snow off of Lake Ontario through today before winding down this evening as ridging moves in. WPC probabilities for 4 inches or more are moderate (>40%) over parts of the Tug Hill with localized 6+ inch totals likely during the first part of the Day 1 forecast. ...Northern Minnesota... Day 2... A quickly-moving positively-titled trough will race through the Northern Plains/Upper Midwest/western Great Lakes late Friday into Saturday. Trend has been for a quicker progression and less QPF overall since yesterday. However, modest upper divergence in association with the retreating northern jet coupled with cyclogenesis over the Plains lifting the surface low through the Corn Belt will support an area of accumulating snow to the northwest of the low. Ratios will rise as colder air moves in but at the same time as QPF exits/diminishes. WPC probabilities for 4 inches or more of snow are low across much of northern MN as well as over the western U.P. of Michigan due to lake effect. For Days 1-3, the probability of significant icing exceeding 0.10 inch is less than 10%. Fracasso ***Key Messages for Nov 29 to Dec 2 Winter Storm Affecting Western U.S. into Friday*** - Mountain travel disruptions 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 today which will lead to continued impacts from lower elevations to mountain passes. - Northern California mountains, including the Sierra Nevada, will have major impacts through tonight. At the peak, very heavy snow rates in excess of 3 inches per hour are forecast, leading to dangerous or impossible travel. Backcountry avalanches are also possible. - Heavy snow and moderate impacts to travel will spread southeastward across the Intermountain West and Northern Rockies through Friday.