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

The highest intensity rain was expected between 4 p.m. on Sunday and 4 p.m. on Monday, according to the service. Los Angeles and Ventura Counties were expected to get up to an inch of total rainfall, and up to three inches was forecast in the mountains around Los Angeles.

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.