The "largest storm of the season" has made its way to Southern California bringing heavy rainfall and prompting evacuation warnings in burn scar areas across the region, weather officials say.

A swift debris flow swept a Los Angeles Fire Department vehicle off the Pacific Coast Highway and into the ocean as heavy rain fell across Southern California on Thursday, creating deluges of water and mud on roads at the bottom of hillsides left barren by recent wildfires.

Snow and ice in whiteout conditions contributed to as many as 30 cars involved in a pileup on a major highway connecting Oregon and Idaho, reportedly injuring several people, as a winter storm descended on the Pacific Northwest Thursday.