An atmospheric river was set to begin pummeling Southern California with heavy rain on Thursday, prompting evacuations and school closures as officials warned about life-threatening flooding and mudslides in areas charred by the wildfires that broke out across the region last month.

For the second time this year, Southern Californians are evacuating their homes as heavy rain threatens new destruction in areas still recovering from last month’s devastating wildfires.

A winter storm, anticipated to be the strongest of the season to date, began dropping water over Southern California overnight into early Thursday morning, bringing a half-inch of rain to some areas in Orange County, the Inland Empire and the Los Angeles County mountains by sunrise.