After weeks of windy and dry weather, rain has fallen in parched Southern California and is expected to aid firefighters who are mopping up multiple wildfires. But potentially heavy downpours on charred hillsides could bring new troubles such as toxic ash runoff.

Southern California is about to get its first significant rainfall in months, bringing desperately needed relief after dry conditions and hazardous winds fueled a series of wildfires in January. The rain could start as early as Saturday afternoon and last through Monday night with the heaviest rain coming on Sunday,

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.