Much-need rain has begun to fall over Southern California, bringing relief to the drought-stricken region but also the threat of toxic runoff.

Rain in Southern California is expected to help firefighters responding to multiple wildfires, but potentially heavy rain on burned hillsides could cause toxic ash runoff.

The National Weather Service said there is a 15-25% chance of thunderstorms across the region, which could bring periods of heavy rainfall that could overwhelm the burn-scar areas.