Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 339 PM EST Mon Jan 03 2022 Valid 00Z Tue Jan 04 2022 - 00Z Fri Jan 07 2022 ...Pacific Northwest to the Central Rockies... Days 1-3... Widespread snowfall across the Pacific Northwest into the northern and central Rockies is likely through midweek as a closed low southeast of Alaska opens, with sheared energy spreading across the region. This will be accompanied by modest Pacific jet energy and mid-level confluent flow to drive moisture into the region, with PWs increasing to 1.5 to 2 standard deviations above normal across Oregon and Northern California by late tomorrow, followed by moisture deepening farther north and east as the period progresses. Snow levels are expected to remain low across the Pacific Northwest through Tuesday, before building across Oregon as an upper ridge axis begins to move onshore -- driving a frontal boundary northeast by late Wednesday. The front will continue to push farther north, with additional snow level increases across Oregon into Washington ahead of an approaching shortwave. This shortwave will accompanied by another shot of deeper moisture -- raising the threat for heavy precipitation. As the cold air aloft retreats, lingering cold air at the surface may set the stage for mixed precipitation, including accumulating ice across the valleys of Washington and northern Oregon. Heaviest snow accumulations through late Tuesday are expected to center along the Oregon Cascades, where WPC probabilities indicate that accumulations of 1-2 feet are likely. Locally heavy accumulations are likely farther north in the Olympics and northern Cascades, as well as south into the Klamath Mountains and northern Sierra. The eastern Oregon mountains as well as portions of the northern Rockies, including the Sawtooth, are expected to see locally heavy accumulations as well. Unsettled weather, including locally heavy snow is expected to continue across the Cascades into the northern Rockies on Wednesday. As energy and moisture continue to spread southeast, the threat for heavy snow is expected to increase across the western Wyoming, northern Utah, and north-central Colorado ranges -- with WPC probabilities indicating significant potential for accumulations of a foot or more by late Wednesday. Beginning late Wednesday and continuing into Thursday, an approaching shortwave will focus the threat for heavy snow farther north, with heavy snows likely for the Washington Cascades. Locally heavy snows are also likely for the Blue Mountains and portions of the northern Rockies, especially the northern to central Idaho ranges. ...Northern Plains and Great Lakes... Days 2-3... Shortwave trough coming from Alberta and sheared vorticity from the Pacific Northwest will work together to close off a 500 mb low over the western Great Lakes by Wednesday. Surface cyclogenesis over the Plains will deepen as it tracks through the Upper Mississippi River Valley. This favorable forcing for ascent, modest moisture in place within the very cold thermal profile will support a period of fluffy snow late Tuesday through Wednesday followed by increasing lake effect snows as the system departs. Plummeting 850 mb temperatures and a prolonged fetch of favorable wind flow off Lake Superior and Lake Michigan should lead to some hefty totals through the period. The latest WPC probabilities show high probabilities for 6 inches of snow or more by late Wednesday across the western U.P. of Michigan, including the Keweenaw Peninsula. The upper low is forecast to quickly open and lift northeast along the St. Lawrence Valley Wednesday night into Thursday. As northerly to northwesterly winds continue to support lake effect activity off of lakes Superior and Michigan, westerly flow and low level cold air will support the development of lake effect snows east of lakes Erie and Ontario into western New York and the Tug Hill region. ...Central High Plains to the Ohio Valley... Days 2-3... Increasing divergence along the left-exit region of an upper jet extending southeast from the northern Rockies, overlapping strong low-to-mid level frontogenesis, is expected to support the increasing potential for heavy snow development from southeastern Wyoming into the Nebraska Panhandle on Wednesday. WPC probabilities indicate at least a moderate threat for snow accumulations of 4-inches or more across this region. The threat for heavy snow is less certain farther east, but a stripe of at least light accumulations is expected across Kansas and Nebraska into Missouri late Wednesday into Thursday. By late Thursday, the probabilities for heavier amounts begins to increase from southern Missouri into the Ohio Valley as amplifying flow over the Plains into the Mississippi valley begins to tap some deeper moisture from the Gulf. Pereira/Taylor ~~~ Key Messages for the Southeast through Mid-Atlantic Winter Storm ~~~ -Winter storm will lead to significant travel disruptions and dangerous driving conditions today across the Southern Appalachians through the Mid-Atlantic. -6-8" totals are expected across the Southern Appalachians and portions of the Mid-Atlantic. Localized higher amounts up to 8-12" likely. -Very intense snowfall rates near 2"/hr will occur this morning and thunder-snow is likely. Rapidly deteriorating road conditions and severely reduced visibility should be expected. -The heavy wet snow will accumulate on trees and power lines and this could lead to power outages. -Very cold temperatures tonight may allow untreated roads to re-freeze, producing additional dangerous travel even after the snow has ended.