"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.
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.
The storm was instigated by a low pressure system absorbing moisture from the south. As much as 1.45 inches 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 95 miles 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.
California Highway Patrol Lt. Sven Miller says said cars and 75 trucks are stuck on the highway between the towns of Mojave and Tehachapi.
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."