California on Sunday braced for more severe weather after a week of torrential downpours and damaging winds killed at least 12 people in the past 10 days and knocked out power for hundreds of thousands of homes and businesses.

Forecasters with the National Weather Service warned that northern and central California were still in the path of a "relentless parade of cyclones," promising little relief for the region until the middle of the week.

Two overlapping phenomena - an immense airborne stream of dense moisture from the ocean called an atmospheric river and a sprawling, hurricane-force low-pressure system known as a bomb cyclone - have caused devastating flooding and record snowfall over the past week.

The latest storms vividly illustrated the consequences of warmer sea and air temperatures caused by climate change.

Despite the temporary deluge, the western United States remains in a two-decade drought. While climate change has resulted in extreme heat, drought and floods, experts say the west would need several exceptionally rainy years in a row to replenish aquifers and reservoirs.

At least 12 people have died from weather-related incidents in California in the past 10 days, Governor Gavin Newsom told a news conference. Among the victims was a toddler who was killed by a redwood tree that fell and crushed a mobile home in northern California.

A woman living in a homeless encampment along the Sacramento River died Saturday night during a raging storm when a tree branch fell on her tent.

Joe Costa, the woman's neighbor in the encampment, told Reuters on Sunday that he had found her barely breathing.

"I started yelling for 911 ... I opened her side of her tent and pulled her out, and she was unresponsive,' Costa recalled.

First responders attempted to resuscitate the woman before taking her to a hospital, where she was pronounced dead, according to local news reports.

Some 424,000 California homes and business remained without power as Sunday afternoon, state officials said at a news conference.

Another severe storm was supposed to hit on Monday, and another atmospheric river, the sixth of the season, was expected later in the week, state officials said.

"We expect to see the worst of it still ahead of us," Newsom said.

In the last week, severe weather spawned violent wind gusts that toppled trucks, flooded the streets of small towns along northern California's coast and churned up storm surge that destroyed a pier in Santa Cruz.

The heavy rain and snow have caused significant flooding and ground saturation, meaning the next storm to move through this week would bring an additional flood threat, the National Weather Service said.

Five feet (1.5 meters) of snow could fall on the Sierra Nevada mountains by Tuesday.

Newsom declared a state of emergency on Wednesday and said he had asked the White House to issue a federal emergency declaration ahead of the coming storms.