Residents at the base of a Santa Barbara County hillside that burned in this week’s Cave fire have been prompted to evacuate over concerns about debris flows as a strong storm begins to unleash rain across Southern California.

County officials issued the evacuation warning for neighborhoods in Santa Barbara and Goleta shortly after 10 p.m. Tuesday, noting that “public safety officials have determined there is a risk for potential debris flows in and below the Cave fire burn area.”

The affected neighborhoods are south of Cathedral Oaks Road/ Foothill Road, east of South Patterson Avenue, west of Las Positas Road and north of the ocean. That section is below a swath of homes under mandatory evacuation orders from the Cave fire, which ignited Monday afternoon and has charred more than 4,000 acres in the Los Padres National Forest.

“If you feel uncomfortable or unsafe at any time, do not wait, leave the area and move outside the fire evacuation order and debris flow evacuation warning area,” the county wrote in a notice. “If you live or are near creeks and streams, be aware that waterways may experience high flows and can rise quickly.”

Rain began falling along the Central Coast about 2 a.m. Wednesday. About four hours later, the bulk of the precipitation for the day appeared to be winding down in Santa Barbara, said Andrew Rorke, a senior forecaster with the National Weather Service in Oxnard.

“We’re not greatly concerned about debris flows in burn areas at this point,” Rorke said, noting that conditions could change quickly if a strong cell or thunderstorm makes its way over a burn scar and lingers.

“That would be the biggest threat to the burn area, but the odds of one hitting are very low,” he said.

When hills are verdant and healthy, vegetation can anchor the soil, even during heavy storms. But when the protective blanket of vegetation is burned off, hillsides become vulnerable to erosion, and slopes can come crashing down in a torrent of mud, rocks and dead branches, destroying homes.

Some residents in other parts of Santa Barbara County — including Montecito, which does not fall in the evacuation zone — received emergency alerts late Tuesday warning them about debris flows. Residents in those areas say it quickly resurfaced fears about the devastating mudslide that swept through the affluent coastal community in January 2018. Twenty-three people died and at least 130 homes were destroyed when the river of mud and rock flowed through Montecito during a rainstorm after the Thomas fire.

After that mudslide, a Times investigation found that government officials did not heed decades-old warnings to build bigger basins that could have made the mudslides far less catastrophic — and that Santa Barbara County failed to thoroughly empty the existing basins before the disaster, drastically reducing their capacity to trap debris.

Late Tuesday, television footage from the scene showed construction crews clearing dirt from debris basins in the region in case a mudslide occurs.

In Los Angeles County early Wednesday, motorists had a damp drive under cloudy skies as the beginning of rain from the storm fell during rush hour. The rain is expected to continue through the morning before tapering off in the afternoon, Rorke said.

“It’s going to be a really messy rush hour,” he said. “But the real impacts will be tomorrow.”

Temperatures are expected to drop into the mid-50s, and precipitation is expected to pick up again on Thanksgiving Day. The storm is expected to diminish by the evening, but scattered showers could linger through Friday. In all, 1 to 2 inches of rain are expected in the coast and valleys, with up to 3 inches in the foothills and lower elevations in the mountains.

The storm is also expected to drop snow levels to 2,000 feet, bringing travel challenges for people commuting before and during Thanksgiving.

By 6 a.m. Wednesday, flurries had begun to fall along the Grapevine section of the 5 Freeway. Light snow is expected to continue in the area throughout the day, but forecasters say Thursday’s conditions will deteriorate as snow levels continue to drop. The storm is expected to dump up to 6 inches of snow along the Grapevine, with the bulk of it falling Thursday, Rorke said.

The storm will drop a foot or more of snow at the top of Mt. Palomar, nearly a foot on Mt. Laguna and 4 to 6 inches between the 3,000- and 5,000-foot level, affecting Julian, Pine Valley and the Alpine area of Interstate 8, according to the weather service.

Some ski resorts in the San Bernardino Mountains could receive as much as 2 feet of snow.

“You have to pity people flying in from places like Massachusetts who are trying to escape the snow coming to L.A. and getting 55-degree temperatures,” Rorke said.

Times staff writer Rong-Gong Lin II contributed to this report