SAN FRANCISCO—A powerful storm barreled toward Southern California after flooding highways, toppling trees, and causing mudflows 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 a half foot (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 …