A powerful storm barreled toward Southern California after flooding highways, toppling trees and causing mud flows in areas burned bare by recent fires across the northern part of the state.

Drenching showers and strong winds accompanied the weekend's arrival of an atmospheric river – a long and wide plume of moisture pulled in from the Pacific Ocean. The National Weather Service's Sacramento office warned of "potentially historic rain."

Flooding was reported across the San Francisco Bay Area, closing streets in Berkeley, inundating Oakland's Bay Bridge toll plaza and overflowing rivers in Napa and Sonoma counties. Power poles were downed and tens of thousands of people in the North Bay were without electricity.

By Sunday morning, Mount Tamalpais just north of San Francisco had recorded 15 centimeters of rainfall during the previous 12 hours, the weather service said.

"Some of our higher elevation locations could see 6, 7, 8 inches of rain before we're all said and done," weather service meteorologist Sean Miller said.

About 241 kilometers to the north, the California Highway Patrol closed a stretch of State Route 70 in Butte and Plumas counties because of multiple landslides within the massive Dixie Fire burn scar.

The same storm system also slammed Oregon and Washington state, causing power outages affecting tens of thousands of people. 

In California's Colusa and Yolo counties, state highways 16 and 20 were shut for several miles due to mudslides, the state Department of Transportation said.

Burn areas remain a concern, as land devoid of vegetation can't soak up heavy rainfall as quickly, increasing the likelihood of flash flooding.

"If you are in the vicinity of a recent burn scar and haven't already, prepare now for likely debris flows," the Sacramento weather service tweeted. "If you are told to evacuate by local officials, or you feel threatened, do not hesitate to do so. If it is too late to evacuate, get to higher ground."

South of San Francisco, evacuation orders were in effect in the Santa Cruz Mountains over concerns that several inches of rain could trigger debris flows in the CZU Lightning Complex Fire burn scar when the storm moves through early Monday. Further south, parts of western Santa Barbara County saw evacuation warnings upgraded to orders in the area burned by this month's Alisal Fire.

Strong winds were also expected, with gusts of up to 97 kilometers per hour at the windiest spots in Northern California. Elevations above 2,745 meters in the Sierra Nevada could get 45.72 centimeters of snow or more from Sunday until Monday morning.

Recent storms have helped contain some of the nation's largest wildfires this year. But it remains to be seen if the wet weather will make a dent in the drought that's plaguing California and the western United States. California's climate is hotter and drier now and that means the rain and snow that does fall is likely to evaporate or absorb into the soil.

California's 2021 water year, which ended September 30, was the second driest on record and last year's was the fifth driest on record. Some of the state's most important reservoirs are at record low levels.

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