State officials said another severe storm was on the way on Monday, and another atmospheric river, the sixth of the season, was expected.
“We still expect to see the worst of it,” Newsom said.
Over the past week, inclement weather spawned violent wind gusts that overturned trucks, flooded streets in small towns along the northern California coast and churned up a storm surge that destroyed a pier in Santa Cruz.
The National Weather Service said the heavy rain and snow had caused significant flooding and ground saturation, meaning the next storm that moves through this week will bring the risk of additional flooding.
Up to five feet (1.5 m) of snow could fall in 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 approaching storms.