Meanwhile, thousands in the Pacific Northwest remained without power or clean drinking water after a deadly wave of storms there.
In California, Orange County sheriff's patrol cars broadcast warnings through loudspeakers urging about 2,000 people to get out of three canyons because some of the heaviest rainfall in more than a year was expected to hit the area overnight.
In the Northwest, rescue and evacuation work ended as floodwaters continued to recede. Six people died in Washington, and another two died in Oregon . An elderly man was missing after he was believed to have fallen into a raging creek behind his house in rural Winlock, Washington .
About 640 people were still in shelters, 33,000 customers lacked power and about 18,900 had no safe drinking water, Gov. Chris Gregoire said. Fourteen water systems were shut down and people served by nine others were under orders to boil water. Some areas requested vaccines, especially for tetanus.
Helicopters took flood-stranded residents to safety at the height of the storm. By the time helicopter operations ended Wednesday evening, at least 300 people had been taken to safety in what Gregoire described as Washington state's largest aerial search-and-rescue operation in a decade.
Interstate 5, the West Coast's main north-south route, reopened Thursday with one lane of traffic in each direction but was limited to freight haulers.
The freeway is not expected to reopen to passenger vehicles until Friday.
Amtrak passenger train service between Seattle and Vancouver, British Columbia , resumed, and service to Portland, Oregon, was expected Friday or Saturday, Gregoire said.
Damage is likely to reach into the billions of dollars but remains to be tallied.
In California, the National Weather Service issued a flash-flood watch for coastal and mountain areas through Friday afternoon. As much as an 1 inch (2.5 centimeters) of rain in urban areas and as much as 3 inches (7.6 centimeters) in the mountains was predicted, said Stuart Seto, a Weather Service specialist.
Southern California has had a very dry rainy season, and it will be the first time since April 2006 that so much rain has fallen from a single storm, Seto predicted.
Areas denuded by fires were a special concern. Wildfires this year have stripped vegetation from thousands of acres of land. That land is now susceptible to excessive runoff and erosion.
"It doesn't take much to set off those mud and debris flows," Seto said.
In Los Angeles County , crews sandbagged hillsides in Griffith Park , where about 1,200 acres were scorched in May. Work was also under way in Malibu , where a wildfire destroyed 53 homes in November.