Homes destroyed in the Palisades Fire in Pacific Palisades, California, on Jan. 27. (Credit: Mario Tama/Getty Images via Bloomberg)

(Bloomberg) — Weeks after wildfires that killed at least 29 people and destroyed entire neighborhoods, Southern California is facing another extreme-weather threat: Too much rain.

See also: Insurance considerations for mudslides and landslides

After drenching the northern part of the state, a massive Pacific storm will start to move south into Los Angeles this week, where it can transform burned hillsides into rivers of mud and debris. Rain has already started in San Luis Obispo and is forecast to soak Santa Barbara and Ventura counties before reaching Los Angeles.

There is a 5% to 15% chance rain will trigger debris flows across the burn scars, or scorched land left by the wildfires, the National Weather Service said. A mudslide can fill an average house in seconds.

“It is really going to be about the burn scars,” said Brian Hurley, a senior branch forecaster at the U.S. Weather Prediction Center. “It doesn’t take as much to reach flash flooding.”

This is in part because the vegetation that holds earth in place has been burned away, so even a small amount of rain can send it careening down hillsides. A less-powerful storm about two weeks ago led to small mudslides that closed roads in the Los Angeles area.

The storm raising the mudslide risk this week is known as an atmospheric river, a long plume of moisture from the Pacific that can contain as much water as flows through the mouth of the Mississippi River. While these weather systems have the potential to be destructive, they can also bring much-needed rain and snow to parched areas. On Friday, a state report showed California is lagging in the snow it requires to meet its water needs for the coming year.

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