Los Angeles, California - Torrential rains unleashed flash floods and warnings of debris flow across southern California, particularly in fire-scarred areas, with further downpours forecast for Thursday as authorities declared a state of emergency in several counties.

Driven by an atmospheric river known as "the Pineapple Express," which moves heavy moisture from the tropical climes of Hawaii to the West Coast, the storm was expected to deliver months' worth of rain over a few days.

Early Thursday, the National Weather Service (NWS) warned about the risk of excessive rainfall over parts of southern California, including in Los Angeles, the second-most populous city in the US.

The NWS warned of a "broad plume of moisture" producing heavy rain in California on Christmas Day, adding there was a "moderate risk" of excessive rainfall over the southern parts of the state.

Environment and Climate Major California storm brings deluge of rain and evacuations before Christmas

"Numerous flash flooding events are possible. In addition, many streams may flood, potentially affecting larger rivers. The flooding may include debris flows in or near recently burned areas," the bulletin added, referring to areas affected by wildfires.

The rain was forecast to continue on Friday, the service said.

State authorities declared a state of emergency in several counties, including Los Angeles.

Heavy rain lashed southern California on Wednesday, spawning floods and debris flows. Some communities had already received 10 inches of rain in the first storm, forecasters said.

The Los Angeles Times reported that at least three people had died in storm-related incidents, including a San Diego man who was killed by a falling tree.