LOS ANGELES (Reuters) - Thousands of residents in a Southern California county where 21 people died from mudslides in January were advised on Wednesday to leave their homes, ahead of a rainstorm that officials said could again trigger a cascade of mud and rocks.

The warning, which is one step below a mandatory order, affects about 30,000 people in Santa Barbara County, said Kelly Hoover, a spokeswoman for the county sheriff’s office.

“If at any time people feel threatened, take immediate action. Do not wait for a notification,” the Santa Barbara County Sheriff’s Office said in a written statement.

The latest warning covers areas along the Pacific Coast and the foothills, including the affluent communities of Montecito and Goleta. It follows criticism of Santa Barbara County officials that they did not adequately warn residents of the danger ahead of the January mudslides.

If the approaching storm is judged to be severe, Santa Barbara County officials, based on an emergency plan they are following, could bump up the recommended evacuation warning they have issued to a mandatory order about 24 hours before rains arrive.

A rainstorm is expected to bring about a third to two-thirds of an inch of rain an hour, beginning on Thursday afternoon or evening, Santa Barbara County officials said in a statement.

Precipitation early on Friday may be strong enough to trigger debris flows.

Areas below hillsides burned in recent wildfires are considered at risk, and those are the neighborhoods where officials are advising people to evacuate.

Santa Barbara County, which is about 60 miles (97 km) northwest of Los Angeles along the California coast and is home to celebrities Oprah Winfrey, Ellen DeGeneres and others, is still rebuilding from the January mudslides.

Some of the areas where people have been advised to leave their homes in the latest warning overlap with places hit by debris flows in January, Hoover said.