Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 312 AM EST Fri Jan 04 2019 Valid 12Z Fri Jan 04 2019 - 12Z Mon Jan 07 2019 ...New England... Day 2... Deep upper low and associated surface reflection will lift northeast from the Mid-Atlantic states to near Nova Scotia Saturday into Sunday. At the same time, a northern stream shortwave will dig through southeast Canada, with the interaction between these two features likely drawing moisture northward into New England. Despite the surface and mid-level low tracking well south of Maine, a robust and persistent TROWAL north of the 700mb low combined with at least subtle low-level frictional convergence along the coast will drive ascent into the moistening column. This will produce a good chance for snowfall across southern and eastern Maine, but accumulations should just be an inch or two, with WPC probabilities above 10 percent for 4 inches only in downeast Maine. Some light freezing rain is also possible across central New England as surface temperatures drop behind the departing low and moisture lingers, but WPC probabilities remain just 20-30 percent for 0.1 inches of accretion. ...Western U.S.... Days 1-3... Today, a low moving into British Columbia will drive a weakening cold front into Washington State. Moisture rotating onshore south of this low will produce snowfall into the Washington Olympics and Cascades and spilling across to the Northern Rockies. Total snowfall will be modest however as forcing wanes this evening in response to shortwave ridging. WPC probabilities are high for 4 inches of snow above 4000 feet. A stronger low will reach the CA/OR border Saturday preceding a full latitude trough moving onshore Saturday night. Snow levels rise ahead of this feature, but will fall behind the associated cold front which will move into California on Saturday. Significant moisture within a Pacific Jet maximum will advect onshore, and intense lift within the left exit region of the jet max combined with increasing 700mb flow will produce heavy snow from the Sierra Nevada to the OR Cascades. The heaviest snow is likely in the Sierras where low-level winds will be orthogonal to the ranges to produce efficient upslope enhancement, and WPC probabilities are moderate to high for 12 inches on each of Days 2 and 3. Elsewhere across the northern California ranges and into the Oregon Cascades, probabilities are high for 8 inches or more. As the full latitude trough lifts eastward during Sunday, moisture and associated heavy snow will spread into the rest of the Mountain West, with heavy snow likely as far south as the Mogollon Rim of AZ, and as far north as the Canadian Rockies. Snow levels will fall to as low as 4000 feet in AZ/NM, with 1000-2000 feet widespread from Utah and points north. WPC probabilities show a moderate risk for 8 inches in the highest terrain, with widespread high probabilities for 4 inches elsewhere. ...Upper Midwest... Day 3... A trough moving through the Mountain West will push eastward creating height falls across the Northern Plains and into the Western Great Lakes. A warm/occluded front will lift northward associated with WAA Sunday night, causing precipitation to overspread the region. The antecedent column is not very cold, so the low-level thermal advection will cause much of the precipitation to be rain. However, areas from northern ND southeast towards northern WI will be cold enough initially for snow. Ascent through this WAA combined with height falls will cause moderate snow, but the system is progressive so the temporal duration of snow will be minimal. A few inches of snow is possible, but WPC probabilities for 4 inches are only above 50 percent along the north shore of Lake Superior where low-level upslope enhancement into the Iron Ranges will increase snowfall accumulations. Weiss