Rain in Southern California will aid firefighters but create a risk of toxic ash runoff

Rain began falling in parched Southern California on Saturday in a boon for firefighters who were mopping up multiple wildfires. But heavy downpours on charred hillsides could bring the threat of new troubles like toxic ash runoff.

Los Angeles County crews spent much of the week removing vegetation, shoring up slopes and reinforcing roads in devastated areas of the Palisades and Eaton fires, which reduced entire neighborhoods to rubble and ash after breaking out during powerful winds Jan. 7.

Most of the region was expected to get get around an inch (about 2.5 centimeters) of precipitation over several days, but “the threat is high enough to prepare for the worst-case scenario” of localized cloudbursts causing mud and debris to flow down hills, the National Weather Service said on social media.

“So the problem would be if one of those showers happens to park itself over a burn area,” weather service meteorologist Carol Smith said. “That could be enough to create debris flows.”

The rain started along the Central Coast before moving into Ventura and LA counties in the evening. It was forecast to increase throughout the weekend and possibly last into early Tuesday, Smith said. Flood watches were issued for some burn areas, and snow was likely in the mountains.

Los Angeles Mayor Karen Bass issued an executive order this week to expedite cleanup efforts and mitigate the environmental impacts of fire-related pollutants. LA County supervisors also approved an emergency motion to install flood-control infrastructure and expedite and remove sediment in fire-impacted areas.

Fire crews filled sandbags for communities while county workers installed barriers and cleared drainage pipes and basins.

Officials cautioned that ash in recent burn zones was a toxic mix of incinerated cars, electronics, batteries, building materials, paints, furniture and other household items. It contains pesticides, asbestos, plastics and lead. Residents were urged to wear protective gear while cleaning up.

Concerns about post-fire debris flows have been especially high since 2018, when the town of Montecito up the coast from LA was ravaged by mudslides after a downpour hit mountain slopes burned bare by a huge blaze. Twenty-three people died, and hundreds of homes were damaged.

While the impending wet weather ended weeks of dangerous gusts and reduced humidity, several wildfires were still burning across Southern California. Those included the Palisades and Eaton fires, which killed at least 28 people and destroyed more than 14,000 structures. Containment of the Palisades Fire reached 81%, and the Eaton Fire was at 95% containment.

In northern Los Angeles County, firefighters made significant progress against the Hughes Fire, which prompted evacuations for tens of thousands of people when it erupted Wednesday in mountains near Lake Castaic.

And in San Diego County, there was still little containment of the Border 2 Fire churning through a remote area of the Otay Mountain Wilderness near the U.S.-Mexico border.

The rain was expected to snap a near-record streak of dry weather for Southern California. Much of the region has received less than 5% of the average rainfall for this point in the water year, which began Oct. 1, the Los Angeles Times reported.

Most of Southern California is now either in “extreme drought” or “severe drought,” according to the U.S. Drought Monitor.