With parts of Los Angeles County still smoldering from wildfires, the expected rain this weekend would seem like a welcome relief. But how the rain falls could make the difference between a disaster respite or a disaster repeat.

More rain fell Monday on parts of Southern California after causing mudflows over the weekend, helping firefighters but boosting the risk of toxic ash runoff in areas scorched by Los Angeles-area wildfires.

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.

Southern California’s first significant storm of the season brought weekend downpours that aided firefighters but caused ash, mud and debris to flow across streets in wildfire-burned areas.