Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 337 AM EST Tue Jan 03 2023 Valid 12Z Tue Jan 03 2023 - 12Z Fri Jan 06 2023 ...Western U.S.... Days 1-3... An active pattern will continue for the West, with a very significant system approaching the coast day 2-3. A very deep and potent storm characterized by an anomalously deep upper level and surface low approaches the coast Wednesday night into Thursday. An atmospheric river will accompany this feature, bringing a narrow but intense plume of moisture onshore, particularly focused on California. IVT values approach 500-700+ kg/m/s, or near 4 standard deviations above normal with PWs approaching 3 standard deviations above normal. Generally higher snow levels will keep the greatest snow accumulations confined to the highest peaks of the Sierra and the Shasta/Trinity/Siskiyou region where totals exceeding 3 feet appear likely. Elsewhere across the region, moisture spilling eastward into the Great Basin and Intermountain West will support some heavy snow with probabilities of 4 inches are 20-40 percent for the higher elevations. The OR/WA Cascades could also see upwards of 8-12" over the day 2-3 period. ...New England... Day 3... A deep closed upper low over the Upper Midwest begins to shed vorticity lobes eastward around it to produce modest ascent across New England Wednesday night into Thursday. A secondary jet streak blossoming downstream of the primary trough axis will strengthen and advect northward from the Southeast, providing additional synoptic ascent as diffluence improves atop the shedding vorticity energy. This should result in a modest low pressure moving through the eastern Great Lakes, with WAA ahead of it driving widespread precipitation across New England and Upstate New York. Initially this will all be rain, but cold air draining southward from a surface high positioned south of Hudson Bay will keep at least central and northern New England cold enough for precip to change to wintry p-types, with snow, sleet, and freezing rain all possible. Guidance has trended slightly warmer this cycle, limiting the ice potential across far northern upstate NY and northern VT where up to 0.1" of ice accretion appears likely on Day 2 (an isolated signal is showing up for near 0.25" in localized places). By Day 3, an offshore low pressure is expected form as the longwave trough approaches the eastern U.S. and this low lifts toward the northeast. This may wrap around colder air and produce a wintry mix across portions of coastal New England including light ice accumulations and snow. Probabilities of 0.1" of ice approach 30-40 percent late on the day 3 period across southern New Hampshire and coastal Maine. ...Central Plains and Upper Midwest... Days 1-2... An anomalously deep closed low has ejected into the Plains early this morning per recent water vapor imagery and 500 mb heights. The system will continue to deepen and amplify as it lifts toward the Upper Midwest, reaching -2 to -3 standard deviations below climo today. At the surface, an area of low pressure organizing over the central Plains will also deepen as it races toward the northeast. Current radar imagery shows an elongated deformation band of mixed winter precipitation from the CO/WY Front Range toward the Upper Midwest. Highly anomalous moisture wrapping into the system combined with the impressive lift/forcing will help produce a large swath of heavy snow and localized significant ice accumulations today into tonight. The potential still exists for impressive snow rates up to 3"/hr where the strong lift overlaps within the DGZ per the latest WPC snow band tool. The heaviest snow is expected to fall today from northern Nebraska through eastern South Dakota into the central Minnesota including the MSP metro where a large footprint of 8-12" is expected. Isolated 12" totals appear most likely across southeast South Dakota per the latest WPC snow probabilities. As the surface low pulls away into Wednesday, the upper level feature lags and should produce additional light to moderate snowfall across much of the Upper Midwest. An additional several inches of fluffier snow is expected bringing storm total snowfall to near 10-12" for central Minnesota through northern Wisconsin. South/southeast of the heaviest snow swath, additional ice accumulations are likely. Based on the latest model trends including 00Z hi-res guidance, the best setup for additional ice accumulations looks to be from northwest Iowa through southern Minnesota into western/central Wisconsin where up to 0.10" ice accretions are possible, with the latest WPC ice probabilities between 20-30 percent. Weiss/Taylor ...Key Messages for January 1-4 Winter Storm... -A large swath of moderate to very heavy snow, freezing rain, and sleet will continue across the Plains today, extending across the Upper Midwest through Wednesday. -Intense snow rates of 1-3"/hr (accompanied by thunder) are likely from parts of Nebraska and South Dakota into Minnesota. Snowfall will rapidly accumulate to more than 12 inches over this region. -Gusty winds around 30 mph will produce areas of blowing and drifting snow. This will result in snow-covered roads, reduced visibility, and difficult to impossible travel. -Localized ice accumulations between 0.1-0.25†are likely for portions of eastern Nebraska through southern Minnesota. This may lead to dangerous travel, scattered power outages, and tree damage.