Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 409 PM EST Wed Dec 08 2021 Valid 00Z Thu Dec 09 2021 - 00Z Sun Dec 12 2021 ...Sierra Nevada to the Central Rockies... Days 1-2... A low amplitude shortwave embedded within progressive northwesterly flow over the eastern Pacific is expected to amplify as it moves onshore across California on Thursday. Favorable upper forcing along with the associated frontal band is expected to support mountain snows along the Sierra Nevada. With snow levels dropping to 5000 ft, or lower, in most locations, probabilities are high that many locations along the central Sierra, including the passes, will see snow accumulations exceeding 6 inches. Organized precipitation, including mountain snow, is expected to spread east through the Great Basin into the central Rockies. Southwesterly flow ahead of the advancing trough is forecast to direct deepening moisture into a low-to-mid level baroclinic zone extending across the region -- supporting areas of heavy mountain snow. The areas impacted are expected to include the central and eastern Nevada, Utah, western Colorado, and the south-central Wyoming ranges. For some areas, including the Wasatch into the Uintas in Utah, and the Elk Mountains northward to the Sierra Madre and Medicine Bow mountains in western Colorado and southern Wyoming, WPC probabilities indicate that local accumulations of 8 inches or more are likely by late Thursday. Snows are expected to quickly end across the Sierra Nevada and across much of the Great Basin, as the trough continues to move progressive east late Thursday. However, heavy snows are likely to continue and expand across the central into the southern Rockies through the overnight into early Friday. From the Sierra Madre south to the San Juan Mountains, WPC probabilities indicate that additional accumulations of 8 inches or more are likely late Thursday to late Friday. Two day totals are expected to exceed two feet across portions of the higher terrain. Meanwhile as the upper trough amplifies, accumulating snows, with locally heavy amounts are expected to spread south into the higher terrain of northern Arizona and along the Mogollon Rim late Thursday into early Friday. ...Central High Plains to the Upper Midwest... Days 1-3... Lee-side cyclogenesis over eastern Colorado in response to the advancing trough is expected to support snows developing farther to the north over southeastern Wyoming into the Nebraska Panhandle and southwestern South Dakota beginning Thursday afternoon and evening. Strong low-to-mid level frontogenesis will help support a stripe of moderate to locally heavy snows shifting east from the High Plains across southern South Dakota and northern Nebraska overnight into early Friday. WPC probabilities indicate that accumulations of 6 inches or more are likely, with heavier amounts possible, from the Laramie Mountains eastward into far southwestern South Dakota and the northern Nebraska Panhandle into the Sandhills. While guidance shows a stripe of significant snows continuing east across southeastern South Dakota and northeastern Nebraska late Friday, it does suggest a relative minimum across the region. Then as the system begins to take a negative-tilt, stronger upper forcing is expected to support an increase in snowfall rates across portions of southern Minnesota into central Wisconsin and the U.P. of Michigan Friday night into Saturday. WPC probabilities indicate a significant threat for accumulations of 6 inches or more across this area. ...Northeast... Day 1... Low pressure developing off of the Mid Atlantic coast this afternoon is forecast to deepen as it tracks north, remaining well offshore and tracking southeast of the Canadian Maritimes. As the system tracks southeast of Nova Scotia, models continue to show an area of localized heavier precipitation organizing along a surface trough extending back toward Downeast Maine. WPC probabilities continue to show that localized amounts of 4 inches or more are likely along the Downeast coast. ...Pacific Northwest... Days 1-3... Models show an upper trough centered over the Northwest being further amplified by a well-defined shortwave diving south from the Gulf of Alaska into the region on Thursday. This is expected to bring heavy snows to portions of the Olympics and the Washington and northern Oregon Cascades. WPC probabilities show that accumulations of 8 inches or more likely for portions of the region by late Thursday, including at both Snoqualmie and Stevens passes. Shortwave ridging is expected to support decreasing snowfall on Friday before a more amplified system drops into the region on Saturday -- bringing heavy snows back into the Olympics and northern Cascades, with accumulations of a foot or more likely. For Days 1-3, the probability of significant icing is less than 10 percent. ...Key Messages for Central Rockies to the Upper Midwest Winter Storm... 1. Significant winter weather storm expected. A winter storm will track from the Utah/Colorado ranges Thursday toward the north-central Plains Friday and the Upper Midwest Friday night into Saturday. 2. Heavy snow will span from the central Rockies toward the Upper Midwest. A foot of snow or more from the central Rockies, with isolated 2 feet across the highest terrain. A swath of 6-8 inches (locally high amounts possible) from the Wyoming Front Range toward the Upper Midwest. 3. Significant disruptions to travel expected. Heavy snow will lead to hazardous to difficult driving conditions and blowing snow may significantly reduce visibility in some areas. Pereira