The flood risk could be highest in Southern California, where wildfires over the past two years have left burn scars and loose soil that could add to the risk of runoff, Kleebauer said. The Central Valley, from Sacramento to Fresno, could also be at risk, as precipitation that falls on the Sierra Nevada rushes down.

A series of powerful winter storms threatened to slam California with relentless rains, heavy winds and mountain snow as holiday travelers hit the road for Christmas.