Rain falling on Southern California is expected to aid firefighters mopping up multiple wildfires. But potentially heavy downpours on charred hillsides could bring new troubles such as toxic ash runoff.

Rain eased on Monday after 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 burn areas.

Rain was falling across Southern California on Sunday, bringing some relief to thousands of firefighters battling multiple major blazes for nearly a month in the Los Angeles area, but also prompting new threats of floods and mudslides across burn-scarred lands.