Cars head south on Lakewood Blvd. near Long Beach, Calif. airport on Wednesday. (Jeff Gritchen/Associated Press/Long Beach Press-Telegram)More than 1,000 homes remained under evacuation orders Thursday as an already-drenched Southern California prepared for a fourth powerful storm carrying a threat of mudslides below fire-scarred mountains and big waves pounding the coast.

The brunt of the storm was expected by midmorning. The U.S. National Weather Service predicted up to 76 millimetres of rain throughout the day, accompanied by gusts up to 81 km/h and up to 51 centimetres of snow in the mountains.

Waves up six metres tall pounded the coast, bringing the threat of flooding in beach communities.

'We're crossing our fingers.'—Nicole Nishida, L.A. County Sheriff's Office

Flash flood watches were up for foothill communities below mountains that were denuded by wildfires last year. A week's worth of rain has soaked hillsides in the San Gabriel Mountains northeast of downtown Los Angeles, where 647.5 square kilometres of forest burned in a summer wildfire.

The rain filled catch basins with muddy slop.

But the basins, sandbags and concrete barriers along foothill streets were holding and no serious problems were reported by early Thursday morning.

"We're crossing our fingers," Los Angeles County sheriff's spokeswoman Nicole Nishida said. "I think if we can get through today, we'll be OK."

California Highway Patrol officers keep vehicles from entering the Northbound Interstate 5 at Lake Hughes Road in Castaic, Calif., on Wednesday. (Hans Gutknecht/Associated Press/LA Daily News)

Evacuations ordered

Homes remained under mandatory evacuation orders in foothill areas of Los Angeles, Glendale and La Canada Flintridge. Sheriff's deputies manned street barricades and conducted roaming patrols to protect emptied homes.

Farther north, the California Highway Patrol was escorting cars through the icy, snowy Tejon Pass section of Interstate 5, the main highway between Los Angeles and Northern California. The pass, known as the Grapevine, was closed for hours Wednesday because of poor conditions.

On Wednesday, five to 76 millimetres of rain fell in many areas.

Since the beginning of the week, more than 300,000 Southern California Edison customers had lost power.