Much-needed rain forecast for Southern California this weekend

Some much-needed rain is in the forecast this weekend for Southern California, which could bring welcome relief to firefighters who have spent nearly three weeks battling devastating wildfires in the Los Angeles area.

But the expected precipitation also brings new risks of mudslides, flash flooding and toxic runoff.

Up to 1.5 inches of rain could fall over the San Gabriel Mountains, while most of Los Angeles County could see around half an inch starting Saturday afternoon and lasting possibly through Monday, according to the National Weather Service.

“There is also a risk of thunderstorms (10-20% chance), capable of producing small hail and isolated brief heavy downpours with rainfall rates up to 0.50 in/hr,” the weather service said in its weekend forecast.

The rain will likely help efforts to put out the deadly wildfires that have burned tens of thousands of acres in Los Angeles, Ventura and Riverside counties. The largest blaze, the Palisades Fire, ignited on Jan. 7 and was 72% contained as of Thursday.

Southern California has been mired in prolonged drought, with abnormally dry conditions over the past eight months. The last time Los Angeles recorded more than one-tenth of an inch of rain was in early May.

But while the rain will likely aid firefighting efforts, the precipitation brings new risks. Because recently burned ground is typically less stable and has less vegetation to anchor soil, rainfall over these burn scars can more easily erode hilly landscapes. That can cause dangerous mudslides “capable of transporting boulders and large woody debris,” which can damage structures, block drainage ways and endanger human life, according to the U.S. Geological Survey.

One landslide was reported in Pacific Palisades earlier this month — it damaged a home that had survived the fires. Officials with the Los Angeles Fire Department said they expected landslides in the burn scars to be a major concern as the rainy season picks up steam.

While this weekend’s forecast includes only a 10% to 20% chance of thunderstorms, the National Weather Service said storms could produce “isolated brief heavy rain,” which increases the risk of flash flooding and carries a “low but non-zero risk of debris flows over recent burn areas.”

California Gov. Gavin Newsom said Wednesday that the state will be ready with sandbags, plastic sheeting and other resources to protect burn scar areas.

“Even before we saw rain on the horizon in Los Angeles, I directed my departments and agencies to act fast to protect neighborhoods from potentially hazardous debris flows,” Newsom said in a statement.

He added that silt fencing and other physical barriers will be used to filter runoff and shield waterways from toxic pollution.

When homes and neighborhoods burn, the charred remains often contain toxic ash laced with harmful substances like lead and asbestos. Fires can also unleash hazardous chemicals from paint, electronics, cars and other household items. Without adequate protections, post-wildfire rain in recently burned areas can wash these toxic materials into the ocean, streams and other waterways.

“The City is taking aggressive action ahead of rainy weather,” L.A. Mayor Karen Bass wrote Tuesday on X. “Crews are preparing to stem the flow of toxins from burned areas to our water, beaches and ocean.”

Bass said efforts are underway to install reinforced barriers, remove fire debris with heavy-duty equipment and capture and divert stormwater into sewer systems.