Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 309 AM EST Mon Jan 03 2022 Valid 12Z Mon Jan 03 2022 - 12Z Thu Jan 06 2022 ...Pacific Northwest to the Central Rockies... Days 1-3... Widespread snowfall across the Pacific Northwest through the Central Rockies is likely the next three days as a closed low southeast of Alaska slowly opens and pivots onshore WA Tuesday, shedding sheared vorticity eastward into the Rockies later on Tuesday. This will be accompanied by modest Pacific jet energy and mid-level confluent flow to drive moisture into the region, with IVT approaching 750 kg/m/s. Snow levels through the period will remain low, generally 1500-2500 ft initially, before climbing, especially in OR and points east, D3 to 4000 ft as the more significant moisture plume pivots onshore. A prolonged fetch of moisture coincident with the favorable lift will drive impressive snow totals for the Olympics, OR/WA Cascades, the northern CA ranges as well as eastward into the Sawtooth of ID. Several feet of accumulations are likely for the important Cascade mountain passes with some isolated 4-5 feet totals possible for the OR Cascades. Late on D2 into D3, the better moisture spills eastward across the northern to central Rockies. Tightening baroclinic zone and upslope component will support snowfall across the High Plains of WY into portions of the Plains. High probabilities for 4 inches extend across southeast WY into western/central NE with even a moderate signal for at least 6 inches. ...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 3 day period. The latest WPC probabilities show slight to moderate probabilities for 4 inches across the northern Plains with the main low pressure system then high probabilities off Lake Superior. In fact, probabilities for 8 inches are high and a significant LES event is likely, especially across the U.P. and the Keweenaw Peninsula. ...Southern Appalachians through Coastal New England... Day 1... ...Significant Winter Storm Today for the Mid-Atlantic... Low pressure deepening this morning over the Southeast is forecast to track northeast off the Mid-Atlantic coast later today while aloft, an anomalously deep, compact closed low pivots through the Southern Appalachians. This is also accompanied by an impressive poleward arcing jet streak over southern Maine. Altogether, this forcing for ascent will continue to drive widespread banded precipitation today. A surface front has dropped southward through the Mid-Atlantic and this will set the stage for a changeover from rain to wet snow this morning. A narrow zone of intense frontogenetical forcing will drive intense snow rates this morning, most likely from central VA northeast toward the DC metro and southern MD, DE, and southern NJ where the HREF probabilities show near certain 1"/hr snow rates with even moderate to high probabilities of 2"/hr rates at times. It is these rates that will help overcome the low-level warmth and hostile antecedent conditions to produce heavy snowfall. Overall for this forecast iteration, there wasn't much change to the footprint of the heaviest snow axis as the 00Z guidance has locked in on the general track and strength of the system. This gives a bit more confidence in some of the higher QPF values seen and as a result, a focused area of higher totals was incorporated for portions of central VA and southern DelMarVa. The signal for localized 8-12" totals has increased there. The latest WPC probabilities are high for at least 6 inches for the 24-hour period ending 12Z Tuesday with now a moderate signal for 8 inches from southern DC metro toward central NJ. The system will brush portions of eastern Long Island and across the Cape and Islands as well with a few inches of snow. WPC probabilities for 4 inches have increased to near 70 percent during the day 1 period. Taylor ~~~ Key Messages for the Southern Appalachians 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.