Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 345 PM EST Sat Dec 02 2023 Valid 00Z Sun Dec 03 2023 - 00Z Wed Dec 06 2023 ...Cascades to the Northern and Central Rockies... Days 1-2... A well-defined shortwave that is currently centered over the northern Rockies is forecast to dive southeast and reach the central Plains on Sunday. Strong ascent afforded in part by left-exit region upper jet forcing and low-to-mid level baroclinicity will support moderate to heavy snow developing over portions of the central Rockies tonight into Sunday. The areas most likely impacted by heavy snow from this evening into tomorrow include the southwest Wyoming, southeast Idaho, northern Utah, south-central Wyoming, and the northwest Colorado ranges. The WPC guidance shows widespread high probabilities (greater than 70 percent) for accumulations of 8 inches or more across these areas, with high probabilities for accumulations of a foot or more across the higher peaks. There are also locally high probabilities for 8 inches or more farther east across some of the northeast Nevada mountains. Some additional locally heavy amounts may carry over into late Sunday and Monday as additional energy and moisture moves southeast across the region. However, the overall threat for heavy snowfall will begin to diminish as a strong upstream ridge begins to move across the region and snow levels increase. In the Northwest, snow levels will begin to climb significantly as a shortwave ridge builds in the wake of the previously noted shortwave and ahead of a deep, compact upper low centered over the Gulf of Alaska on Sunday. Models continue to show strong moisture transport, fueling heavy precipitation in the Northwest, especially along and west of the Oregon Cascades. However, snow levels will be climbing to above 7000ft through much of the Cascades on Sunday. Following a brief dip late Sunday, snow levels are forecast to surge upward as a sharp ridge builds ahead of an amplifying trough over the eastern Pacific on Monday. ...Mid Mississippi Valley to the Upper Great Lakes... Day 1... A shortwave trough that is currently centered over the central and southern Plains is expected to lift out ahead of the trough now moving across the Rockies, assuming a negative-tilt as it moves from the mid Mississippi Valley to the Great Lakes on Sunday. Dynamic cooling is expected to support a changeover to snow on the northwest side of the precipitation shield beginning across eastern Iowa this evening, before an expanding area of snow lifts into portions of Wisconsin and lower Michigan on Sunday. Amounts for most areas are expected to be light (less than 4 inches), however WPC guidance indicates that at least an inch or two is likely across a good portion of south-central Wisconsin to the Green Bay area and across northern lower Michigan. ...Northern New York and New England... Days 1-2... The previously noted shortwave lifting out of the Plains will send a developing surface low into the lower Great Lakes on Sunday, before reaching the St Lawrence Valley Sunday night. Meanwhile, a coastal low is forecast to track northeast away from the Mid-Atlantic and east of Atlantic Canada. Models continue to show notable CAD signature with Canadian high pressure over Quebec helping to hold cold air in place across portions of northern New England. By late Sunday and continuing into Monday, strong ascent afforded in part by a coupled upper jet and low-to-mid level frontogenesis will support the development of heavy precipitation ahead of the parent low. The heaviest accumulations are expected to center from the White Mountains eastward into interior western and central Maine. WPC guidance shows high probabilities (greater than 70 percent) for accumulations of 6 inches or more across this area. For the Adirondacks and the northern Greens, lower totals are more likely, with WPC guidance under 50 percent for accumulations of 6 inches or more. For areas farther south, including much of Upstate New York and southern New England, thermal profiles indicate a mostly rain event with limited to no accumulating snow expected. Pereira ***Key Messages for Western U.S. Winter Storm*** --Heavy snow and blowing snow in the Rockies Periods of heavy and blowing snow will produce mountain travel impacts over portions of the northern and central Rockies through Sunday night. An additional 1-2 feet of snow is forecast for ranges in northern Utah to northern Colorado mainly through Sunday. --Atmospheric Rivers For Northwest Into Midweek Periodic heavy rain with rising snow levels is expected over the Pacific Northwest as back-to-back atmospheric rivers arrive through Wednesday. --Risk for Major River Flooding Additional multiple inches of rain in the lowlands and heavy rainfall melting recent snows at pass level will produce moderate to possibly major river flooding through midweek in western Washington.