The El Nino weather phenomenon is likely to drench California for some time to come and trigger floods, but it still won't end the state's severe drought, experts say.
Some 200 vehicles were stuck on highways in central California late last week amid a deluge of water, mud and debris.
The deluge left trailers partly covered in sludge, while mobile homes were knocked over and houses damaged.
"That was really something -- six inches (15 centimeters) of precipitation per hour," said Tim Krantz, a professor of environmental studies at the University of Redlands.
"I'm afraid that could be just the first of serious storms" which could last all winter, he told AFP.
It can cause unusually heavy rains in some parts of the world and drought elsewhere.
According to Krantz, some spots in the Pacific have seen temperatures more than four degrees Celsius above average.
"We're experiencing El Nino in California, which increases the amount of precipitation, said Michael Schaffner of the National Weather Service.
The effects of El Nino are expected to last until next spring and forecasters have warned it could be one of the strongest on record. The last major El Nino episode took place in 1997 to 1998.