Southern California Rainstorms Raise Risks of Mudslides

A slow-moving rainstorm system settled over Southern California on Sunday, bringing a reprieve from a lengthy dry spell but also the risk of mudslides in areas scarred by this month’s wildfires.

The showers were expected to continue into Monday afternoon, with light rain across the region and intermittent bursts of heavy rain, forecasters said. The rain could reduce fire risks and help vegetation parched by the driest start to a rainy season on record in Los Angeles.

But the National Weather Service also assessed there was a 10 to 20 percent chance of significant mudslides in several Los Angeles County burn scars, sensitive areas where fires devoured trees and brush.

In the burn scars, the charred soil could act like slick pavement when soaked by rain, creating the conditions for mudslides, said Marc Chenard, a meteorologist with the service.

“You just don’t get any absorption of the water,” Mr. Chenard said. “It just all immediately turns into runoff.”

The highest intensity rain was expected between 4 p.m. on Sunday and 4 p.m. on Monday, according to the service. Los Angeles and Ventura Counties were expected to get up to an inch of total rainfall, and up to three inches was forecast in the mountains around Los Angeles.

The burn scars include areas scorched by the Palisades fire in the Pacific Palisades section of Los Angeles; the Hurst fire near the Sylmar area of the city; the Sunset fire near West Hollywood; the Eaton fire near Pasadena; the Hughes fire near Castaic Lake; and the Franklin fire near Malibu, among others.

Burn scars outside Los Angeles County had a 5 to 10 percent chance of experiencing mudslides, the Weather Service said.

Residents were urged to stock up on supplies and protect property with sandbags. A flood watch was in effect in Los Angeles County until Monday afternoon.

Light rain arrived in Ventura County, north of Los Angeles, on Saturday evening, and picked up across the region on Sunday, the service said.

The system was drifting southeast through Los Angeles County, delivering lightning and hail in some areas, according to the service. Through Sunday afternoon, the highest rates of rainfall — about three-quarters of an inch per hour — were limited to isolated areas.

Rates over half an inch of rain per hour in the burn scars could pose “some significant issues,” Mr. Chenard warned.

The Los Angeles region had endured a brutal drought for months, feeding this month’s devastating wildfires, which burned across thousands of acres and displaced more than 100,000 people.

Before Saturday, there had been no measurable rain in downtown Los Angeles this year, said John Feerick, a senior meteorologist at AccuWeather. He described the rain as welcome news.

“In general, this is beneficial rain,” Mr. Feerick said. “It should help with the fire situation immensely.”

“Now, with that comes the risk, because there are burn scars,” he added.