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

More than an inch of rain fell in parts of Los Angeles Monday afternoon, triggering flash flood watches and warnings in areas scorched by this month's wildfires.

With parts of Los Angeles County still smoldering from wildfires, the expected rain this weekend would seem like a welcome relief. But how the rain falls could make the difference between a disaster respite or a disaster repeat.