Extreme-weather advisories for floods, high wind and winter storm conditions were posted on Monday across parts of California and southwestern Arizona where some 35 million people live.
The National Weather Service documented staggering rainfall amounts from the storm, which lashed northern California on Sunday with hurricane-force gusts of wind, along with heavy precipitation that intensified as the system moved south on Sunday night and Monday.
The National Weather Service (NWS) said more than 25.4cm of rain had fallen since Sunday across the Los Angeles area with much more expected before showers were due to taper off on Tuesday.
"Significant flooding is ongoing and expected to expand," the NWS said in a notice posted online.
The intense rainfall, with heavy snow in high-elevation mountain areas, was carried to California by a storm system meteorologists call an atmospheric river, a vast airborne current of dense moisture funnelled inland from the Pacific.
Winds gusting to 121km/h on Sunday downed trees and utility lines across the San Francisco Bay Area and California's Central Coast, knocking out power to roughly 875,000 homes at the storm's peak in that region.
At least two people were killed by wind-toppled trees on Sunday - an 82-year-old man in the former gold rush town of Yuba City and a 45-year-old man at Boulder Creek in the coastal Santa Cruz Mountains.
The greatest flash-flooding threat on Monday centred on southern California, the NWS said, as the system slowly pivoted and pushed farther into the interior of California, but forecasters said "catastrophic" impacts were unlikely.
"There is widespread moderate flooding, but right now things seem to be somewhat manageable," said Daniel Swain, a meteorologist and climate scientist at the University of California, Los Angeles, during a briefing on YouTube on Monday.
Still, rescue teams have pulled dozens of people to safety statewide, mostly motorists stranded in cars by rising waters when they tried to drive through flooded roadways, according to Brian Ferguson, spokesperson for the Governor's Office of Emergency Services.
He said evacuation orders also were in effect for several southern California neighbourhoods considered to be at particularly high risk of flash floods and mudslides.
"We're not out of the woods yet," Ferguson told Reuters by telephone. "There could continue be very dangerous impacts all through southern California today and tomorrow."
Overnight, at least five homes sustained damage from a landslide in the Beverly Crest, a neighbourhood in the Santa Monica Mountains west of downtown Los Angeles. Three more homes were damaged by a landslide in Encino, a neighbourhood in the San Fernando Valley region, the city fire department said.
Earlier, authorities urged residents to be prepared for extremely dangerous conditions, especially in the Hollywood Hills of Los Angeles and in the Santa Monica Mountains.
California Governor Gavin Newsom on Sunday declared a state of emergency in eight counties with a combined population of more than 20 million people.
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