Emergency crews shovelled mud from a section of Interstate 5 north of Los Angeles after flash flood debris blocked the important highway, stranded hundreds of vehicles, and forced some motorists to take refuge on top of their cars.

Excavator trucks scooped and hauled away mud in the darkness Thursday night and the cleanup continued well into Friday, leaving thousands of drivers searching for alternative routes.

About 15 cars remained wedged in the debris and needed to be towed out.

A vehicle is stuck along a muddy road in the mountainous community of Green Valley, Calif., about 65 miles northwest of downtown Los Angeles on Thursday. Flash flooding in northern Los Angeles County filled several roads with mud, stranding vehicles and blocking traffic on one of the state's main highways. (KABC-TV/Associated Press)

There were no reports of deaths or injuries from the flooding Thursday at Fort Tejon, about 120 kilometres north of downtown Los Angeles, brought on by a weather system that produced powerful afternoon thunderstorms in the mountains and deserts.

A second highway was closed Thursday night following a separate mudslide. And a number of secondary roads were left impassable from mud and some residents were trapped in their homes.

Officials expect the freeway to be reopened around 2 p.m. Friday, California Highway Patrol Officer Tony Polizzi said.

The affected section of I-5, one of the state's major north-south arteries, carries traffic among steep mountains over a pass rising to an elevation of more than 1,250 metres between the Central Valley and metropolitan Los Angeles.

After removing the debris, a geologist will check the stability of nearby slopes before the freeway is reopened, California Department of Transportation spokeswoman Lauren Wonder said earlier.

A truck hauling a bulldozer navigates over a washed out section of Lake Hughes Road north of California's Castaic Lake in the Angeles National Forest after a storm caused flash floods in the area on Thursday. (Katharine Lotze/Santa Clarita Valley Signal/Associated Press)

"There could always be more slide that comes down onto the road," she said. "Our engineers are always very careful so they make sure in a flood situation, any hillside is secure."

The National Weather Service said a flash flood watch would be in effect again Friday afternoon and early evening for the mountains and deserts because of the threat of more severe and slow-moving thunderstorms, which raises the potential for flash floods and debris flows.

Photos of Interstate 5 posted on social media showed the freeway in disarray, with semi-trucks and cars sitting askew, stuck in mud that in some cases surpassed their wheels. Not an inch of asphalt was visible.

'It's getting pretty hairy out there.'- Robert Rocha

One of the worst-hit areas was rural Lake Hughes, a tiny mountainside community in northern Los Angeles County. Robert Rocha, a 37-year-old resident, said he was driving home from work when the storm arrived.

"It was getting pretty hairy out there," he said. "I've never seen it rain that hard in such a short period of time, the hail and wind — it was coming down hard," he said. "The debris was just intense — chunks of wood and rock flowing everywhere."

Los Angeles County Fire Department Capt. Keith Mora said the agency rescued four people and two dogs from atop one car. Many more were able to walk to safety after waiting out the flood on top of their own vehicles, he said.

"They were able to use their vehicles as a security blanket, to stand on top of and stay higher than the flood water," Mora said.

In all, the agency reported rescuing 14 people and eight animals. Los Angeles County firefighters were expecting to go house to house Friday morning to check on any stranded occupants.

'It's kind of like a tailgate party without the party.'- Jennifer Stewart

The storm was instigated by a low pressure system absorbing moisture from the south. As much as 3.7 centimetres of rain fell in a quick span of time near where the most intense flooding occurred.

The system was expected to shift slowly eastward through Friday, National Weather Service meteorologist Joe Sirard said.

State Route 58 in Kern County was closed near the town of Mojave, about 150 kilometres north of downtown Los Angeles. In a statement, the California Department of Transportation said it expected the State Route 58 shutdown would be "a long term closure" and advised motorists to seek alternative routes.

Back in Lake Hughes, Jennifer Stewart said she had just picked up her 17-year-old daughter from school when the storm hit.

"The hail was so bad I thought it was going to crack my windshield," she said.

Stewart said she was among about 30 motorists who were left stranded on a local road. Everyone was calm, she said.

"It's kind of like a tailgate party without the party," she said. "Everyone's just quietly sitting in their cars talking among themselves."