Updated at 7:25 p.m. ET

Thirteen people have reportedly died as heavy rain drenched fire-ravaged Santa Barbara County in Southern California on Tuesday. Thousands of people are evacuating from their homes because the rain is raising the risk of mudslides on hills stripped by recent wildfires.

That area has seen heavy mudflows that have destroyed homes and prompted dramatic rescue operations. "Heavy rains have triggered massive runoff," Mike Eliason, a public information officer for the Santa Barbara County Fire Department, said on Twitter. "Access is difficult/delayed to due to — at some locations — waist deep mudflow, trees, and wires down."

Rescue workers in bright yellow jackets waded through deep mud and debris, searching for people in need of aid. Homes sat crumpled and destroyed in pools of dark muck.

According to the AP, some 21,000 people have evacuated in large swaths of Southern California, including vulnerable areas of Santa Barbara and Ventura counties. That is where the massive Thomas Fire raged last month. It was the largest in California's history, burning more than 280,000 acres and destroying more than 1,000 structures.

The AP quotes officials as saying only 10 percent to 15 percent of Santa Barbara County residents who were told to evacuate did so.

In Los Angeles County, several canyon areas were also put under mandatory evacuation orders, starting Monday, because of the heavy rains.

Just an inch of rain fell in Southern California last month, drying out the ground, freelance reporter Danielle Karson tells our Newscast unit. In areas where the fire burned, there is almost no ground vegetation. Meteorologist Joe Sirard tells Danielle that this can cause problems.

"When we get short-term heavy rain like we're getting right now, it will build up debris in the mud in these areas and it comes right down the mountain," Sirard says. "So it could be a dangerous situation."

On Tuesday morning, the National Weather Service warned of the potential for flash flooding "for much of Santa Barbara, Ventura and Los Angeles Counties," with rainfall rates as high as an inch and a half per hour.

Several houses have been destroyed in the deluge, and a Santa Barbara County spokesman says some residents are unaccounted for in neighborhoods that are hard to reach because of downed trees and power lines.

Residents of vulnerable areas are also piling up sandbags to try to protect their homes. The AP reports that the Ventura County Sheriff's Office says, "Jail inmates have been filling sandbags at the rate of 2,000 a day."

And the California Highway Patrol has closed the coastal 101 freeway in both directions because of "flooding and debris flows." Topanga Canyon "was also closed in both directions due to a mudslide north of Pacific Coast Highway," member station KPCC reports.

Authorities have also closed the Sepulveda Basin recreation area in the San Fernando Valley, which will cause closures on the nearby 405 freeway, according to the Los Angeles Police Department.

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