Areas charred by last month's wildfires in Southern California now face a new threat from a powerful, but fast-moving Pacific storm. Here's what you need to know about this threat.

Most of the California coast is under a flood alert as torrential rain and mountain snow intensified over California Thursday morning, part of yet another atmospheric river storm, bringing a significant risk of flooding and landslides in areas recently affected by devastating wildfires.

The second peak of an ongoing atmospheric river storm arrived in Northern California Monday afternoon, prompting flood watches and a winter storm warning up in the Sierra.