When hills are healthy, vegetation anchors soil in place. But when that vegetation is burned off, hillsides become vulnerable to erosion, and slopes can come crashing down.

Forecasters released a breakdown Tuesday of where and when heavy rainfall this week is expected to hit areas of Southern California left burn-scarred by recent wildfires, raising the risk of potentially dangerous debris flows in those parts of the region.

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.