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 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.

Southern California’s first full rainy day of the winter has arrived. The day, months in the making, has brought steady storms across the region while the Southland experiences drought conditions. The mostly welcome rain began Saturday night and is expected to sustain until Monday evening.

The rain that is expected to hit the scorched Los Angeles landscape this weekend may bring relief to the fire fights, but it could also bring flash floods and mudslides. Although forecasts show that the risk is relatively low, local officials are taking the warnings seriously.