A major cyclone is set to hit the US state of California with up to seven inches (18 centimeters) of rain on Wednesday, the National Weather Service said, after tens of thousands of people were placed under evacuation orders due to a barrage of storms that killed at least 17 people.

On Tuesday, torrential downpours caused flash flooding, closed key highways, toppled trees and swept away drivers and passengers -- including a five-year-old boy who remains missing in central California.

Around 66,000 homes and businesses in the most populous US state were without power early on Wednesday, according to tracking site Poweroutage.us.

The new storm will hit northern California and is forecast to bring several more feet of snow to the Sierra Nevada mountains, the NWS said.

The NWS described an "unrelenting series of atmospheric river events" that is the most powerful storm system since 2005.

"An enormous cyclone rotating well off the West Coast will bring the next round of heavy precipitation and gusty winds (on Wednesday), this time targeting northern California," the service's latest advisory said.

California Governor Gavin Newsom said at least 34,000 people had been told to flee the storms, with more danger expected.

"The fact is that we're not out of the woods; we expect these storms to continue at least through the 18th of this month," he told reporters on Tuesday.

"We now have 17 confirmed -- and I underscore 'confirmed' tragically -- just confirmed deaths."

The town of Montecito, home to Britain's Prince Harry and his wife Meghan Markle, was pounded by rain, threatening dangerous mudslides on hills already sodden by weeks of downpours and sparking an evacuation order.

"Because the mountains are right there, when it really rains, it comes down at a really high rate... it's pretty dangerous pretty quickly," resident Daniel DeMuyer told AFP.

"That's the price of living in such a beautiful place, when it rains like this, it causes a lot of destruction."

Montecito, where multi-million dollar properties are surrounded by breathtaking California countryside, is particularly vulnerable to mudslides because it sits at the foot of a mountain range that was ravaged by fire five years ago.

Hundreds of square miles were scorched, stripping the hillsides of the vegetation that normally keeps soil in place.

But an evacuation order for the town -- home to stars including Ellen DeGeneres, Gwyneth Paltrow, Katy Perry, and Rob Lowe -- was lifted on Tuesday.

Boy swept away 

There were tragedies across the state.

Authorities in San Luis Obispo County called off a search for a five-year-old boy because rushing waters were too dangerous for divers, Fox News reported, quoting a county official.

The child, who fled with his mother from their car as it was inundated by flood waters, has not been declared dead. His mother was rescued.

Two motorists died in a crash north of Bakersfield after a tree crashed onto a road.

Destruction was widespread, with whole communities flooded in some areas.

Dominick King said his restaurant in Capitola had been wrecked.

"It's a lot worse than expected," he told AFP.

"All my windows in the back are completely blown out. All my tables are kind of strewn around my floors... and our floors had warped, so I guess the waves came up from under the building.

"It's not just me, the whole block has been decimated."

Swaths of the Golden State were under flood warnings and forecasters said the misery would continue.

The extreme weather will not be limited to California, the NWS said, with the system that caused Tuesday's rainfall working its way through the country and likely to cause thunderstorms over central and southern areas.

Downpours in drought 

Heavy rain is not unusual for California during winter but these downpours are testing the state.

They come as much of the western United States is more than two decades into a punishing drought that has seen major increases in the frequency and intensity of wildfires.

Scientists say human-caused climate change, brought about by the unchecked burning of fossil fuels, has supercharged such wild swings in weather.

But even the recent rains are not enough to comprehensively reverse the drought.

Scientists say several years of above-average rainfall are needed to get reservoirs back to healthy levels.