Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 446 AM EST Sat Dec 11 2021 Valid 12Z Sat Dec 11 2021 - 12Z Tue Dec 14 2021 ...Pacific Northwest to the Northern Rockies and the Sierra Nevada... Days 1-3... An active period of weather will impact a large portion of the western U.S. through the weekend and into early next week. 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 this morning across the Olympics and Washington Cascades will shift south and east as moisture spills over and the system itself shifts southward. This will produce heavy snow on D1 across most of the terrain above 4000 ft from the Olympics eastward to the Northern Rockies and southward as far as the Trinity/Siskiyous. WPC probabilities are high for more than 12 inches of snow across these ranges with 2 ft possible in the highest terrain. D2 /Sunday into Monday/ will feature a continuation of broad moist advection into the west as the longwave trough persists offshore and pieces of shortwave energy shed eastward in the confluent and moist mid-level flow. This will persist heavy snowfall from the Olympics southward to the northern Sierra, and eastward into the Sawtooth and Tetons. WPC probabilities D2 for 12 inches are highest in the Sierra where locally 2 ft is possible, but are moderate to high in the Sawtooth where upslope enhancement is likely and parts of the Cascades, Tetons, and Olympics. The offshore trough sharpens into a closed low Monday driving a surface low to near the OR/WA coast. At the same time, the associated Pacific jet streak will intensify and begin to surge poleward placing the diffluent RRQ atop a region of compressed mid-level flow producing a surge of moisture onshore with PW anomalies rising to +1 to +1.5 standard deviations. This will manifest as a weak but long duration AR angled directly into the Sierra Nevada. Snow levels within the AR will rise to 6000 ft, but will remain low elsewhere, and may drop to 1500-2000 ft north of the AR in coastal OR and northern CA. This could allow for some light accumulations into the lower terrain near the OR/CA coasts including Sexton Mountain Pass where WPC probabilities are moderate for 4 inches. However, the heaviest snowfall is likely in the Sierra where WPC probabilities for 18 inches are above 90%, and locally more than 5 feet of snow is possible in the higher terrain, including many of the central Sierra passes. ...Great Lakes... Days 1... The ongoing major winter storm will continue to lift northeast today, ejecting into Ontario by this evening. An intense deformation axis noted on current radar imagery will trail the surface low, producing a swath of heavy snowfall after 6am CST which should be confined to parts of central WI northeast through the U.P. of MI. This deformation axis is progged to become more intense as modest instability lifts northward, which will lead to intense forcing and impressive snowfall rates of 1-2"/hr or possibly greater according to the WPC prototype snowband probabilities. The heaviest snow is likely in a narrow corridor from near Green Bay, WI northeast along the Door Peninsula and into the eastern U.P. where WPC probabilities for 4 inches are more than 50%. A secondary maxima of more than 4 inches is possible further west across the U.P. where CAA behind the departing system will cause some LES enhancement. ...New England... Day 1... Warm advection precipitation associated with the system impacting the Upper Midwest will lift southwest to northeast along a warm front into New England through the aftn. Low level cold air will be entrenched early as high pressure slowly retreats, such that precipitation will likely fall as freezing rain across the terrain and elevated valleys of VT/NH and much of interior ME. The p-type should eventually transition to all rain, limiting significant freezing rain accretion. However, WPC probabilities are 20-40% for more than 0.1" in parts of the White Mountains of NH and points northeast across the higher elevations of ME. ~~~Key Messages for December 9-11 Winter Storm from the Central Rockies to the Great Lakes~~~ ...Significant winter storm will wind down... The strong low pressure system will eject into Canada this evening bringing an end to the heavy snow. ...Heavy Snow will persist across the Upper Great Lakes... Additional accumulations of 3-6 inches are likely across northeast WI and the U.P. of MI. This will bring event total snowfall to 12-18 inches, with locally higher amounts, in many areas. ...Travel impacts... Travel will remain dangerous today as heavy snow rates of 1-2"/hr combine with strong winds to produce slippery roads and near zero visibility. Blowing snow may persist hazardous travel even after the snow ends. ...Impacts to infrastructure... The heavy and wet snow may produce scattered power outages. ~~~Key Messages for Dec 11-14 Winter Storm across the Western U.S.~~~ ...A prolonged Atmospheric River will bring heavy precipitation to much of the Western U.S... Heavy precipitation across WA and OR this morning will spread into ID, MT, and CA on Saturday. A second, even more impressive, surge of precipitation will lift onshore CA Monday and spread into the Great Basin by Wednesday. ...Heavy mountain snow likely... Significant snow accumulating to 1-3 feet is likely in the Olympics, Cascades, and Northern Rockies. Heavy snow of 2-4 feet or more is likely beginning Sunday in the ranges of CA including the Sierra Nevada. ...Widespread hazardous travel... Travel will become dangerous, especially for the mountain passes, and is discouraged. Roads are likely to experience closures. Whiteout conditions, downed trees, and power outages are all possible. ...Heavy rain expected for the valley locations in California... Several inches of rainfall will create the potential for flash flooding and debris flows, particularly near recent burn scar areas. Weiss