FOUS11 KWBC 230755 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 255 AM EST Sat Nov 23 2024 Valid 12Z Sat Nov 23 2024 - 12Z Tue Nov 26 2024 ...Pacific Coast through the Intermountain West and Rockies... Days 1-3... Strong upper low off the Pacific Northwest finally begins to unravel while ejecting shortwaves inland across the West, with persistent onshore flow and lowering snow levels leading to widespread heavy mountain snowfall. The first shortwave ejecting through the base of an anomalous trough off the Pacific Coast will swing eastward into the Northwest Saturday and then begin to amplify into a potent but fast-moving impulse across the Northern Rockies and into the Northern Plains Sunday. This deepening system will drive downstream ascent through height falls, PVA, and divergence. At the same time, a zonally- oriented jet streak will pivot south of the region, placing its favorable LFQ atop the far northern High Plains and southern Canada, interacting with increasing baroclinicity as a front wavers near the international border. The overlap of synoptic lift with the enhanced mesoscale ascent through fgen will result in weak cyclogenesis, leading to an expanding area of precipitation falling as snow. The column will be plenty cold for wintry precipitation thanks to high pressure from Canada extending southward. As the WAA along the warm front lifts north, an exceptionally deep DGZ will support fluffy aggregate dendrites which should accumulate efficiently, but the most favorable environment and most impactful snowfall will likely be just north of the U.S.-Canada border. The greatest impacts over the U.S. from this system appear to occur over the northern Rockies on D1, where upslope flow will enhance ascent. WPC probabilities for at least 8 inches of snow are high (<70%) across northwest MT and Glacier NP, as well as the Salmon River and Clearwater Mts of Idaho. The Pacific mountain ranges spanning from the Cascades to the Sierra Nevada will see a few separate shortwaves move inland before the parent upper low finally pushes eastward on D3. The ongoing AR over central CA will come to an end by the start of D1 and allow for snow levels to fall back to around 4000-5000 ft across the Sierra and lower (2500-3000 feet) across the Cascades. However, IVT values will also become very weak along the West Coast until a resurgence across the Sierra on D3 in response to the parent upper lows eastward movement. A few locally enhanced axes of moisture are also possible across the Pac NW D2-D3 due to the lingering low off the coast. This results in high probabilities (>70%) for at least a foot of total snowfall over the next three days across the Sierra Nevada, Shasta Siskiyous of northern California, Oregon Cascades, and Olympic Mts. Areas above 5000 feet can expect the potential for up to two feet of snow. Leftover moisture spilling across the Intermountain West and central Rockies could combine with favorable west-east upslope enhancement over the Wasatch Mts of Utah and Colorado ranges on D2. WPC probabilities for at least 8 inches of snow are low (10-30%). ...Northern New England... Days 1-2... Stacked/wrapped upper and sfc low over the Northeast will complete its counterclockwise loop this morning before a separate sfc low develops and lifts north through Atlantic Canada along with the larger upper low this weekend. Upslope snow into the central Appalachians will persist through this evening via NW flow as moisture has circulated from the Atlantic up and around southeastern Canada back across the lower Great Lakes. A few additional inches of snow are possible in elevations above 3000 ft in West Virginia, with marginal surface temperatures limiting the potential for heavier amounts. Into New England, trailing vorticity lobe will aid this new coastal sfc low east of Cape Cod that will track northward into Nova Scotia by tonight, favoring increased northerly/cooler flow over interior portions of Maine southwestward into VT. The column is fairly mild to start over New England via SE flow ahead of the initial front, but that will change on D1 with rain changing to snow as the low approaches and deepens. QPF will be focused closer to the coast, but light amounts of snow are likely for the interior and especially the higher terrain above 2500ft or so (White Mountains and northern ME) where WPC probabilities of at least 4 inches are >40%. Additionally, lake effect snow showers are possible downwind of Lake Ontario D1-D2 due to this persistent west-northwest flow around the large low pressure system to the east. Warm boundary layer temperatures will limit snowfall accumulations as upper ridging quickly builds from the west. In, fact most showers may simply fall as rain outside the higher elevations of Upstate New York or within some of the locally heavier bands where dynamic cooling can lower surface temperatures to near freezing. ...Upper Great Lakes... Day 3... The fast-moving shortwave exiting the northern Plains on Sunday is forecast to reach the Upper Great Lakes on Monday while also taking on a negative tilt in response to a piece of upper energy quickly rounding the southern end of the trough over the Midwest. Surface low is expected to then deepen over the Great Lakes by the end of D3 with upper diffluence and lake enhancement off Lake Superior supporting periods of light to moderate snow over the U.P. of Michigan. WPC probabilities for at least 4 inches of snow are low (10-30%) for most of this region on D3, with medium chances (40-60%) over the Porcupine Mts. Snell $$