Mudslides, flooding and strong winds are battering southern California as a second day of storms drenches one of the most populous parts of the US.
Heavy rain of up to 1in (2.54cm) an hour is expected to hit areas around Los Angeles for at least 24 hours from Monday.
The "atmospheric rivers" causing the storms have already seen swathes of the state hit by rain, wind and snow.
A state of emergency has been declared in 10 of the state's counties.
Forecasters have said torrential rain is the main risk facing California on Monday.
On Sunday, the Weather Prediction Service (WPC), part of the National Weather Service (NWS), issued a level 4 warning for rainfall in southern California.
The WPC said potentially "life-threatening" and "catastrophic" flash flooding threatened the Los Angeles Basin and surrounding area. It said mudslides and debris flows were also a risk.
The centre added that "very heavy" snows were expected in the Sierra Nevada mountains, rendering travel "dangerous to impossible".
The NWS said strong winds of up to 70mph (112kmph) could cause further power cuts and downed trees, though winds would decrease significantly by Monday night.
Large steep waves would make areas along the coastline "extremely dangerous," it added.
On Sunday, flights from Santa Barbara Airport were cancelled and drivers stranded by flooding in Los Angeles had to be released by rescuers.
Hundreds of thousands were without power as the storm knocked out electricity networks.
Officials in Orange County issued a voluntary evacuation warning for some areas, but said that the warning could become an order with "little or no notice".
The storm is due to an "atmospheric river" effect, caused by airborne currents of dense moisture.
Atmospheric rivers are a phenomenon in which water evaporates into the air and is carried along by the wind, forming long currents that flow in the sky like rivers flow on land.
A first atmospheric river hit California last week. The renewed bout of bad weather is caused by a second.
In a statement declaring the state of emergency in eight counties, including Los Angeles and Orange, Governor Gavin Newsom said: "This is a serious storm with dangerous and potentially life-threatening impacts.
"California is ready with a record number of emergency assets on the ground to respond to the impacts of this storm."
Two other states have declared their own states of emergency.
The NWS said Monday would be "one of the most dramatic weather days in recent memory".
NWS officials urged people in affected areas to heed evacuation orders, stay off the roads and be prepared for potential power cuts.
