Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 415 PM EDT Tue Mar 17 2020 Valid 00Z Wed Mar 18 2020 - 00Z Sat Mar 21 2020 ...Northern California, Northeastern Nevada, into Southern Oregon... Day 1... A piece of energy wrapping around the stalled upper low off the CA coast will shift northeast across the Northern Great Basin and close off before circling back to the south across CA Thursday. Moderate ascent through jet diffluence, height falls, and PVA will move slowly overhead in conjunction with a nearly stationary region of enhanced 700-500mb RH. This forcing and moisture will combine to produce heavy snow in the mountains of Southern OR, NE California, and into ID/MT, with snow levels falling to around 3000 ft. WPC probabilities are high for 8 inches in the Modoc Plateau and into the Fremont Mountains. Total snowfall may exceed 18 inches in the highest terrain. ...Four Corners States through the Northern Plains and Western Great Lakes... Days 1-3... A shortwave ejecting from the closed low near Baja will swing northeast from the Southwest into the Central Plains while amplifying. This feature is expected to deepen into a closed low near the High Plains of Wyoming, before rapidly opening and ejecting northeast towards the Great Lakes on Friday. The falling heights and increasing south to southwest flow leads to Pacific moisture fluxes across far northwestern Mexico into AZ/UT Wednesday before shifting across CO Wednesday night. Precip breaks out in the Desert SW early Wednesday with snow levels 5000 to 6000ft and AZ/UT and 6000 to 7000ft in CO. Day 1 to 1.5 snow probabilities for 8 or more inches are moderate to high for the Mogollon Rim and White Mtns of AZ and Kaibab Plateau (north of Grand Canyon North Rim) up the UT Wasatch to the Uintas before expanding to the San Juan/Sangre De Christos and west-central CO Ranges to the Front Range. As this system slides northeast, the combination of height falls, PVA, and increasing jet level diffluence through an increasingly coupled jet structure will lead to lee cyclogenesis across eastern CO. This surface low will then lift northeast to interact with a cold front, and produce an intense band of frontogenesis between 750-600mb. While the low-level thermals from eastern WY through the Central and Northern Plains may initially be too warm for snow, this enhanced fgen collocated with the synoptic ascent will drive the column to cool and change p-type over to snow. Once this occurs, a swath of heavy snow is likely from far NE CO/SE WY through Nebraska and northeast towards Lake Superior. The heaviest snow from this system is likely in the High Plains of WY and into much of Nebraska where WPC probabilities are high for 6 inches on D2. Further northeast, there is somewhat lowered confidence due to the strength of a developing TROWAL and its placement, but WPC probabilities are moderate to high for 4 inches from the Buffalo Ridge into the U.P. of MI, with snowfall in the U.P. likely enhanced by Lake Effect behind the low on Friday. Further north, a stripe of modest freezing rain is likely in ND and into western MN where low-level moisture increases through modest deformation and frontogenesis. Temperatures will be cold enough for snow, but on Wednesday into Thursday, the DGZ is likely to be too dry for snowfall creation. This should lead to a period of light freezing rain which may accrete to near 0.1 inches. ...Northeast... Day 2... A quick moving wave of low pressure beneath a modest shortwave racing eastward Thursday morning will spread precipitation across Upstate New York, Southern and Central New England. The precipitation will be driven primarily by WAA and intensifying frontogenesis as the surface wave moves east from Pennsylvania to east of Cape Cod. The thermal profiles are marginal, and strong WAA will eventually transition nearly the entire area from snow to rain late Thursday morning. However, briefly intense forcing despite modest thermals should allow for a burst of moderate to heavy snow, especially in the terrain of the Catskills, Berkshires, Worcester Hills, and southern Adirondacks, Greens, and Whites. Snowfall will generally only be around 2 inches, but WPC probabilities indicate a low risk for 4 inches in the higher terrain of New Hampshire and into central Maine. Weiss