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ALAMEDA, Calif. — Thousands of Californians were told Monday to evacuate their homes ahead of a storm that officials said is likely to pummel a region already devastated by wildfires and mudslides.

Robert Lewin, the Santa Barbara County emergency management director, said the area is expecting more rainfall in a single storm than the county typically sees all year.

"Ten inches of rain falling on a burn area is something that we don't understand exactly what's going to happen," he said during a news conference.

Related: Deadly rains in Southern California send rivers of mud into homes, trigger fire, flooding

An "atmospheric river" is expected to arrive Tuesday and last until Thursday, officials said, bringing heavy rain and possible flash flooding, along with tumbling rock and mudslides from San Luis Obispo County south to Los Angeles.

La Conchita is expected to get around 4.67 inches of rain, at 0.4 inches an hour; Ojai is expected to receive 6.70 inches; and the Fillmore area is expected to get 4.78 inches, the Ventura County government said Tuesday. The Matilija Dam north of Ojai is forecast to receive a little more than 8 inches during the storm, the county government said.

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Mark Jackson, a meteorologist with the National Weather Service, said the storm’s bull's-eye will be an area scorched last year by a wildfire that was the largest in California history, at 440 square miles.

Two deaths, of a civilian and a firefighter, were blamed in the so-called Thomas Fire. More than 1,000 buildings — many of them homes — were destroyed.

In January, with the blaze still simmering, a powerful storm swept across Santa Barbara and Ventura Counties, triggering mudslides that killed 21 people.

With mountain channels cleared out by the January storm, Jackson expects rocks and soil to cascade toward the coast with even more ease than they did two months ago.

Lewin concurred. “Those mountains are locked and loaded with debris,” he said.

The National Weather Service warned Tuesday that most concerning are the areas burned by the so-called Thomas Fire as well as burn areas from the past Whittier, La Tuna, Sherpa, Fish and Sand fires.

“If you are near these burns, DEBRIS FLOWS ARE LIKELY, stay alert to the weather and heed messages/evacuations from emergency officials,” the weather service tweeted on Tuesday

Where are the recent burn areas? Most concerning: Thomas, Whittier, La Tuna, Sherpa, Fish, and Sand burns. If you are near these burns, DEBRIS FLOWS ARE LIKELY, stay alert to the weather and heed messages/evacuations from emergency officials. #larain #cawx #thomasfire #Montecito pic.twitter.com/UiEIltKpGi — NWS Los Angeles (@NWSLosAngeles) March 20, 2018

Mandatory evacuation orders will go into effect on Tuesday in Santa Barbara County, clearing 7,400 residents out of their homes, or roughly 17,000 to 18,000 people, Sheriff Bill Brown said.

“This storm is not the storm to question,” Lewin said. “Please heed the mandatory evacuation orders.”

CORRECTION (March 20, 2018, 4:45 p.m. ET): An earlier version of this article misstated the number of people who died in the Thomas Fire. There were two deaths, not 18.