Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 433 AM EST Wed Dec 30 2020 Valid 12Z Wed Dec 30 2020 - 12Z Sat Jan 02 2021 Day 1... ...Pacific Northwest to the Northern Rockies... A compact area of low pressure moves slowly east towards Vancouver Island, with moist onshore flow ahead of the low advecting precipitable water values of 05-0.75 inches into WA State. The high moisture is also accompanied by warm temperatures, so snow is expected to be confined to higher elevations of the WA-OR Cascades. The 300 mb jet axis crosses OR and southern ID driving upper divergence maxima across the ranges of northeast WA and northern ID, where up a foot of snow may fall. As jet energy continues into MT, several inches may fall in the mountains of northwest MT. The trailing cold front, along with a well-defined shortwave moving south of the low, will start to bring heavier precipitation farther south into western Oregon and northwestern California late Wednesday. Heaviest snow accumulations during the Day 1 period are expected to fall across the Olympics and Washington Cascades, where WPC PWPF shows high probabilities for 12 inches across the higher elevations. ...Southern Plains... An upper trough will continue to advance east across the Plains -- driving a cold front southeast across the southern Plains. By late today, an upper low closing off within the southern stream over northern Mexico will begin to advect moisture north of the surface boundary and into the low-to-mid level front centered over Southwest Texas. Thermal profiles support snow accumulating snow, with WPC PWPF showing high probabilities for accumulations of 6-10 inches in the Davis Mountains. Precip type uncertainty and a likely mixture leads to lower snow amounts in surrounding valley areas. ...Great Lakes/Northeast... Low pressure existing the Great Lakes and moving north of New York and New England produces warm advection that results in a period of light to moderate snow changing to mixed precipitation moving across Ohio and PA north across New York and New England. Primary snow accumulations are reserved for higher elevations like the Tug Hill Plateau and Adirondacks. Light icing is expected in western New York to portions of the Adirondacks/Greens/Berkshires, where low probabilities exist for up to a tenth of an inch of icing. Day 2... ...Southern to Central Plains... An upper low will continue to deepen as it moves across northern Mexico Wednesday night into Thursday morning, assuming a negative tilt as it lifts into South Texas on Thursday. The depth of the cold air is expected to remain sufficient for snow on the northwest side of the developing comma head, with several inches of accumulation likely across the Stockton and Edwards plateaus into the Concho valley. As the low moves north towards OK, the precip changes from mixed types over to snow in western OK as cooling aloft occurs in tandem with the low. WPC PWPF shows probabilities for accumulations of 4 inches or more across portions of western TX to western OK and adjacent south central KS. Higher probabilities are closer to the low but less in OK and KS due to the mixed precip types. An icy mix is more likely farther to the east, with accumulating ice expected from the Texas Hill Country northward to central Oklahoma, southeast Kansas and northern Missouri. There is potential for a quarter inch of icing in portions of northern Missouri to eastern Kansas. ...Ozarks to the Lower Ohio Valley... A trailing baroclinic zone left behind by the low moving east of the Great Lakes is expected to linger across the region Wednesday night into Thursday. As the developing low to the southeast begins to send moisture and warmer air aloft into the region, while high pressure to the north reinforces cold air in place. This will set the stage for a period of wintry precipitation and stripe of accumulating ice from the Ozarks across southern Missouri into the lower Ohio Valley. From the Ozarks to southern Illinois, WPC PWPF shows high probabilities for ice accumulations of 0.10 inches during this period. ...Pacific Northwest to the Northern Rockies... Even as the previously noted system quickly weakens, difluent flow aloft in the vicinity of a 300 mb jet streak crossing WA and OR will continue to support precipitation spreading across the region the first half of the period before the jet departs and drying aloft commences. Several additional inches of snow are possible along the Washington Cascades, with locally heavy accumulations likely to develop farther south along portions of the Oregon Cascades and farther east over the northeastern Oregon and northern Idaho ranges. Day 3... ...Central Plains to the Mid Mississippi Valley and Great Lakes... A deep low centered over mid Mississippi valley on Friday morning is forecast to move northeast towards the Great Lakes and gradually weaken. Along the 700 mb low track, deformation and cooling leads to a period of snow with several inches possible just west of the 700 mb low track from the mid MS Valley to the Great Lakes. East of the low, warm advection over the cold surface is expected to produce icing across northern MO and southeast Iowa up across northern IL, northern Indiana and lower Michigan. The Chicago area is likely to receive a wintry mix of accumulating ice and snow. The axis/orientation of both snow and sleet/freezing rain depends on the low track, with east-west spread remaining that results in different models having differences locations with heavier snow and freezing rain potential. Risks aren't high yet due to these typical forecast differences. ...Central Appalachians/Mid Atlantic/Northeast ... As the area of low pressure approaching the Great Lakes continues to advect moisture and warm air aloft into the eastern U.S., models show high pressure into the Northeast, with a cold air damming signature setting up along and east of the central to northern Appalachians Friday. This is likely to produce wintry precipitation, with areas of accumulating ice. Guidance shows the biggest probability for a quarter inch of icing across the Laurel Highlands of central PA. Areas of a tenth to a quarter inch of icing are possible in much of the rest of central PA to the southern tier of NY, the Catskills, Adirondacks, and Berkshires. Snow accumulations are expected north of the ice area in northern NY, VT, NH, and southern ME. Low probabilities of 4 inches of snow are shown in these mountains. ...Pacific Northwest... The respite provided by the upper ridge on Day 2 comes to a close as the next upper trough is forecast to move across the northeast Pacific towards the WA-OR coast Sat morning. As the upper trough approaches, moisture advection once again increases, with the 00z ECMWF showing a strip of 0.75 inches of precipitable water streaming onshore in western WA-OR. The low level warm/moisture advection into the Cascades raises snow levels, so the threat for extended snow is reduced in the OR Cascades and primarily in the WA Cascades/higher elevations of the WA Olympics. Moderate to high probabilities of 8 inches are shown in the norther WA Cascades due to better than average model/ensemble agreement on the forecast heavier snows there. Petersen