Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 343 PM EST Fri Dec 10 2021 Valid 00Z Sat Dec 11 2021 - 00Z Tue Dec 14 2021 ...Pacific Northwest to the Northern Rockies and the Sierra Nevada... Days 1-3... A pair of strong storms are expected to impact a large portion of the western U.S. beginning later tonight and continuing through the weekend. The first system -- a strong frontal boundary associated with an amplifying upper trough over the eastern Pacific -- will bring heavy precipitation, including mountain snows to portions of the Pacific Northwest and the northern Rockies beginning later today. Heavy mountain snow is expected to develop first across the Olympics and Washington Cascades tonight, before spreading south into the Oregon and California Cascades, as well as the Klamath Mountains on Saturday. Heavy snows are also expected farther east across portions of the Blue Mountains, as well as the northern to central Idaho and the northwestern Montana ranges. As unsettled weather continues across the Pacific Northwest into the northern Rockies, heavy snows will continue to develop and edge farther south across Northern California, impacting the northern Sierra Nevada by early Sunday. The upper trough is expected to continue to amplify off of the Northwest Coast through the weekend, as a series of well-defined shortwaves dive south from the Gulf of Alaska into the base of the broader scale trough. A wave moving through the base of the trough early Monday will be accompanied by a reinforcing surge of deeper moisture and colder air, supporting additional heavy snows for portions of southern Oregon and Northern California beginning Sunday night and continuing into Monday, before extending farther south from the northern into the central Sierra Nevada. With snow levels already low, the reinforcing influx of colder air is expected to bring snow levels close to the Oregon and northwestern California coasts, with some potential for heavy amounts in the higher elevations of the coastal ranges. By late Monday, three-day snowfall totals will likely exceed several feet for many mountain locations, including the Olympics, Cascades, Klamath, northern and central Sierra Nevada, Blues, and the northern Rockies from northern Idaho and northwestern Montana to southern Idaho and northwestern Wyoming. Travel will likely disrupted by these storms, with WPC probabilities indicating heavy accumulations are likely for most of the Cascade and the northern to central Sierra passes. ...Upper Midwest... Days 1... The ongoing winter storm over the central Great Plains and Upper Midwest is forecast to shift northeast, bringing heavy snows to portions of the Upper Midwest/northern Great Lakes overnight into Saturday before tracking into eastern Canada later in the day. The forecast remains similar, with low pressure over the central Great Plains forecast to intensify as it turns northeast overnight. Phasing mid level streams over the central Plains and mid Mississippi Valley will support an amplifying, negatively-tilted upper trough moving across the Midwest -- driving a deepening surface low northeast from the mid Mississippi Valley to the Great Lakes. Models have been consistent in showing the potential for moderate to heavy bands setting up on the northwest side of the low. HREF guidance continues to show several members indicating snowfall rates 1-2 inch/hour translating northeast from southern Minnesota to northern Wisconsin and the U.P. of Michigan overnight. Evening and overnight accumulations of 6-inches or more appear likely from southeastern Minnesota to the U.P. Some of the heaviest snow totals are likely to center over the central U.P., where a prolonged period of northeasterly flow is likely to support some lake enhanced totals. ...New England... Day 1... Warm advection precipitation associated with the system impacting the Upper Midwest is forecast to spread across the Northeast on Saturday. Low level cold air is expected to remain in place long enough for some mixed precipitation, including freezing rain, at the onset across portions of northern New York and New England. While widespread significant icing is not expected, WPC probabilities do show some greater than 50 percent probabilities for ice accumulations of 0.10 inch or more along and east of the White Mountains, where a weak surface ridge and northeasterly flow may help the keep the low level cold air in place longer. ~~~Key Messages for December 9-11 Winter Storm from the Central Rockies to the Great Lakes~~~ ...Significant winter storm will continue... A strong low pressure system will track across the Central Plains to the Upper Midwest and Great Lakes tonight in Saturday. ...Heavy snow will expand into the Upper Midwest and Great Lakes through Saturday morning... Total accumulations of 8-14 inches of snow with locally higher amounts are expected from southeastern South Dakota through the Upper Midwest and into the U.P. of Michigan. ...Widespread impacts to travel and infrastructure... All forms of travel will become dangerous as heavy snow rates reaching 2"/hr and strong winds create near whiteout conditions at times. The heavy and wet snow may produce scattered power outages. ~~~Key Messages for Dec 11-14 Winter Storm across the Western U.S.~~~ ...Pair of strong storms will direct an Atmospheric River along the West Coast beginning later tonight through the middle of next week... The first surge will bring the heaviest precipitation to Washington, Oregon, and Northern California tonight through Sunday. The second wave arrives Monday to Wednesday, bringing much needed but heavy rainfall from Oregon to California. ...Heavy mountain snow likely to cause travel disruptions... Multiple feet of snow to the Cascades this weekend and then more heavy snow spreads southward through the Sierra early to mid next week. ...Difficult to dangerous mountain travel possible... Travel will become dangerous for the mountain passes. Whiteout conditions, downed trees, and power outages are all possible in these areas. ...Heavy rain expected for the valley locations in California... Other hazards with this storm include heavy rain and the potential for flash flooding and possible debris flows, particularly near recent burn scar areas. Pereira