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.

California's deadly wildfires could come to an end in the coming days when a weekend storm douses historically dry terrain, but the storm will come with the potential for a different problem: flash floods.

Cooler, wet weather is forecast to bring moisture to the parched landscape and reduce the fire threat. But heavy rainfall could also trigger flash floods in burn scars.