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 on its way to the Los Angeles area will be a welcome relief to firefighters still battling multiple catastrophic wildfires, but officials warned Sunday that flash floods and potential debris flows could be the trade-off.