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.

Rainfall is needed and generally welcomed across Southern California. But following two historic fires, it also poses risks and it heightens the concern over contaminants that could make their way into storm drains,

The first significant storm of the season has brought snow and downpours to Southern California that doused wildfires and caused some ash and mud to flow across streets in the Los Angeles area.