Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 309 PM EST Thu Dec 16 2021 Valid 00Z Fri Dec 17 2021 - 00Z Mon Dec 20 2021 U.P. of Michigan... Day 1... Lake-enhanced snow lingers over the northern end of the U.P. into Friday with localized accumulations of 2-4 inches. ...Western U.S.... Day 1... Weakening upper trough will continue to de-amplify into westerly flow this afternoon as it moves west from Oregon before crossing the northern Rockies tonight. Residual moisture allows terrain heavy snow from ID to north-central CO including northern UT and western WY/MT where Day 1 snow probabilities are moderately high for 6 or more inches. ...Northwest U.S.... Days 2/3... The next northern Pacific upper low will cross Vancouver Island Saturday as reinforcing low pressure shifts well off the West Coast. 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. Day 2 WPC probabilities are moderately high 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 before expanding Day 3 over northern ID/northwest MT ranges and down the OR Cascades. ...Northern Plains... Days 1/2... Shortwave crossing the northern Rockies this evening will shift ESE across the northern Plains through Friday evening 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. Day 1.5 WPC probabilities of at least 4 inches of snow remain 10-40 percent over the eastern ND/SD border. ...Northeast... Days 2/3... Upper shortwave moving out of the northern Plains Friday night will shift east through the Great Lakes Saturday, supporting surface low development along a boundary over the Ohio Valley Friday night, developing as it pushes east across the northern Mid-Atlantic by Saturday evening. This will support warm advection snow and some freezing rain over northern PA/western NY Saturday before shifting east over New England Saturday night. 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, particularly in valley should 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. Day 2 WPC probabilities for at least 0.1" ice is 20 to 50 percent along the PA/NY border with Day 2.5 snow probabilities for 4 or more inches moderately high from the Adirondacks eastward over VT/NH through =southern Maine -- generally 60-90 percent. Jackson