The US state has already suffered a week of torrential downpours and damaging winds that killed 12 people in 10 days, Governor Gavin Newsom said.
More than 120,000 people are still without power as of Monday morning.
The governor warned on Sunday that the most brutal weather is due in the next 48 hours.
“We expect to see the worst of it still ahead of us,” Governor Newsom said at a news conference. “Don’t test fate.”
This new round of severe weather will bring heavy rain on already flooded rivers, damaging winds that are expected to topple trees and power lines, and heavy snow in northeast California.
The US National Weather Service (NWS) said the heaviest and most widespread rain will likely occur around Tuesday morning and afternoon, and have issued a flood warning in areas around Los Angeles, including Orange County and the San Bernardino County Mountains.
The Sacramento Valley is also under a flood advisory. Schools in and around Sacramento have cancelled classes on Monday in anticipation of the storm and amidst widespread power outages.
US president Joe Biden declared a state of emergency for California on Monday, which allows the Federal Emergency Management Agency, also known as FEMA, to provide disaster relief.
What are atmospheric rivers and bomb cyclones?
In the last week, California has experienced two overlapping weather phenomena – an atmospheric river, where an airborne stream of dense moisture flows in from the ocean, and a bomb cyclone, a storm with a rapid drop in pressure that creates a cyclone effect.