The deluge started last night and forecasters say the region could get 10 inches of rain before it tapers off Tuesday. That's almost as much as the total rainfall since last July.
Barricades are being set up and motorists are being warned to turn around if they see high water ahead.
"The water is swift and dangerous following heavy rainfall, and people need to stay away from streambanks until the water level subsides," said Jody Noiron, supervisor for Angeles National Forest.
The warnings cover the mountain areas of Orange, San Diego, Riverside and San Bernardino counties. There's particular concern in areas stripped by recent wildfires.
In Southern California, reports CBS News Meteorologist George Cullen, the end of the rain – once it does end – won't mean an immediate end to the danger of flooding, as runoff will spill into already swollen rivers and streams.
Hurricane-force winds have knocked out power to more than 100,000 homes and businesses in the San Francisco Bay Area. A 30-ton construction crane perched on the rooftop of a shopping mall in Cupertino, the Santa Clara County sheriff's office said. No one was injured in the accident but it did cause a hole in the mall roof, watch commander Tim Hayes said.
More than 100,000 Pacific Gas and Electric Co. customers were without power as of 11 p.m. Monday night, PG&E spokeswoman Jana Schuering said.
Most of the power outages were reported in the Bay Area, where wind gusts peaked at 98 mph at Angel Island, according to the National Weather Service. Winds at San Francisco International Airport reached 71 mph, leading to delays of up to two hours.
PG&E anticipated restoring power to most customers by Tuesday morning, Schuering said.
Until Monday, 10.7 inches of rain had fallen on the Los Angeles area since last July, 5.3 inches less than normal and 23 inches less than had fallen by this time last year.
Last year was the second-wettest year on record for Los Angeles, as 33.9 inches of rain fell on the city.