Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 427 PM EST Tue Dec 29 2020 Valid 00Z Wed Dec 30 2020 - 00Z Sat Jan 02 2021 Day 1... ...Pacific Northwest to the Northern Rockies... As a compact, deepening low approaches northern Vancouver Island, organized heavier precipitation is expected to move into western Washington by early Wednesday, with lighter precipitation spilling across the northern Intermountain Region and into the northern Rockies by late in the day. 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 8-inches or more across the Cascade passes. ...Southern Plains... An upper trough will continue to advance east across the Plains -- driving a cold front southeast across the southern Plains. By late Wednesday, an upper low closing off within the southern stream over northern Mexico will begin to direct moisture north of the surface boundary and into the low-to-mid level front centered over West Texas. Thermal profiles support snow accumulating snow, with WPC PWPF showing high probabilities for accumulations of 2-4 inches over and around the Davis Mountains. ...Upper Midwest/Great Lakes... Phasing energy over the Upper Midwest will support a deepening area of low pressure moving east across northern Minnesota into the Great Lakes. A warm front lifting ahead of the low will support a period of light to moderate snow changing to mixed precipitation moving northeast from eastern Iowa and northern Illinois to southern Wisconsin and Lower Michigan Tuesday evening into the overnight. WPC PWPF is indicating snow accumulations of 4-inches or more are likely before the changeover and/or precipitation ends across southern Wisconsin to central Lower Michigan. Accumulating ice is more likely farther to the south, with WPC PWPF indicating some higher probabilities for 0.10-inch of ice or more centered over central Illinois. Day 2... ...Pacific Northwest to the Northern Rockies... Even as the previously noted system quickly weakens, onshore flow along with energy aloft will continue to support precipitation spreading across the region. Additional heavy snow accumulations 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. ...Southern 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. WPC PWPF shows high probabilities for accumulations of 6-inches or more across portions of this region. An icy mix is more likely farther to the east, with accumulating ice expected from the Texas Hill Country northward to central Oklahoma. ...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 supports northerly surface winds, 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-inch or more during this period. Day 3... ...Southern and Central Plains to the Mid Mississippi Valley and Southern Great Lakes... A deep low centered over North Texas Thursday evening is forecast to lift north-northeast across the Ozark region and into the mid Mississippi valley on Friday -- drawing plenty of moisture and warm air aloft into the eastern Plains, mid Mississippi valley and Great Lakes. Strong upper forcing along with low-to-mid level frontogenesis is expected to produce widespread precipitation moving north across Oklahoma, eastern Kansas and the lower Missouri and mid Mississippi valleys Thursday night into Friday morning, with shallow cold air supporting a wintry mix and accumulating ice. By Friday morning, significant ice accumulations can be expected across portions of eastern Kansas and across northern and central Missouri, including the KC metro. Deepening cold air on the backside of the system is expected to support a changeover to snow before precipitation ends. While widespread heavy snow accumulations are not expected, at least an inch or two can be expected from the eastern Texas Panhandle and Northwest Texas to northwest Missouri and southern Iowa. This precipitation shield is likely to continue shifting north during the day on Friday -- impacting the southern Great Lakes region, including Chicago, with a wintry mix of accumulating ice and snow. ...Central Appalachians... As the area of low pressure approaching the Great Lakes continues to direct moisture and warm air aloft from the central into the eastern U.S., models show high pressure over the central U.S., shifting into the Northeast, with a cold air damming signature setting up along and east of the central Appalachians Friday morning. This is likely to produce wintry precipitation, with areas of accumulating ice. Guidance shows the biggest threat for significant ice accumulations centered from western Maryland to the Southern Tier of New York and the Catskills. Pereira