Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 346 AM EST Thu Dec 16 2021 Valid 12Z Thu Dec 16 2021 - 12Z Sun Dec 19 2021 Minnesota and the U.P. of Michigan... Day 1... Robust cyclone will race through Ontario early day 1 with wrap-around snow moving steadily eastward. Some lake-enhanced snow will linger over the U.P. through this evening with localized accumulations of several inches. WPC probabilities of at least 4 inches is about 20-50% over the Keweenaw Peninsula. ...Western U.S.... Days 1 and 3... Weakening ex-upper low will continue to de-amplify into westerly flow early today over Oregon as it heads toward the central/northern Rockies. Exiting jet streak will help provide broad-scale lift over the Sierra Nevada early day 1 and over the higher terrain of the Great Basin and Utah ranges before diminishing on Friday. WPC probabilities for at least 6 inches of snow are moderate to high over the Sierra and parts of the Wasatch and Uintas northward to the Tetons/Wind Rivers of WY, and Bitterroots along the ID/MT border. On day 3 (12Z Sat - 12Z Sun), next northern Pacific upper low will dive southeastward out of the Gulf of Alaska along with trailing energy exiting south of the Aleutians. Snow levels will briefly rise to around 4000-5000ft as a warm front lifts through western Washington, only to fall again by late Saturday as a cold front moves into the area (to around 2000ft). With a modest plume of moisture nosing into the area (PW values around 0.75" which are around +1 to +1.5 sigma), moderate snowfall is expected for mainly at and above pass level. WPC probabilities are moderate for 6 or more inches of snow over the higher Olympics and northern WA Cascades, but a bit lower over the northern tip of ID. ...Northern Plains... Day 2... Shortwave over British Columbia this morning will dive southeastward today and through Montana overnight into early Friday. By day 2, this will move through the Dakotas as a NW-SE surface trough forms to the northwest of a wave along a front over the mid-Mississippi Valley. Though progressive, models indicate a small area of a tenth to a quarter or so of QPF which, with temperatures in the teens to low 20s, will support a fluffy few inches of snow. WPC probabilities of at least 4 inches of snow are somewhat low -- around 10-40 percent. ...Northeast... Day 3... Upper shortwave moving out of the Plains on day 2 will move through the Great Lakes on day 3, supporting surface low development along a boundary at the tail-end of the departing day 1 system. This low will lift northeastward and then slip eastward across the Northeast or northern Mid-Atlantic (perhaps in two pieces), supporting warm advection snow and some freezing rain from northern PA northward and northeastward Saturday afternoon and overnight. Models/ensembles showed a modest amount of spread in the critical temperatures between the surface and 850mb, with maximum spread focused around 0C. Marginal surface temperatures near or just below freezing could support a period of freezing rain as warm air noses northward aloft as precipitation moves in from the southwest, favoring areas along and north of I-80 in PA through the Southern Tier. Farther north, with a deeper cold layer and sufficient QPF via a decent mid-level frontogenesis band, snow will be the dominant precipitation type north of I-90 across the North Country eastward through VT and NH into ME. WPC probabilities for at least 4 inches of snow are moderate to high from the Adirondacks eastward (especially over higher terrain) -- generally 60-80 percent. Fracasso