The first images have emerged showing California pummeled by potentially deadly rain and wind as about 37 million residents remain under flood alerts.

San Francisco Public Works crews tended to a downed acacia tree in the neighborhood of West Portal, equipped with a chainsaw, as rain pounded the city.

Videos from San Jose showed trees swaying and buckling under the gusts. And in Santa Cruz, water ponded on residential streets and poured out of drainpipes.

Atmospheric rivers began hitting the state last week, toppling trees and dousing roads. With the ground already soaked from the first storm, officials expressed concern about mudslides and flooding.

Weather researcher Ryan Maue said the 'bomb cyclone' - a term denoting a storm that rapidly intensifies over 24 hours - could dump more than 8 trillion gallons of precipitation on the state.

He likened the amount to the volume of Lake Mead - 8.5 trillion gallons. 

The Weather Prediction Center issued a Level 4 risk for excessive rainfall in Santa Barbara and Oxnard - an exceedingly rare advisory issued on fewer than 4 percent of days on average. 

A Level 3 risk was instated for much of the California coast, including San Francisco down through Los Angeles.

Speaking at a news conference Saturday, Nancy Ward, director of the Governor's Office of Emergency Services, said more than 8,500 members of public safety organizations had been mobilized in anticipation of the storm.

The threat of extreme weather was great enough that Governor Gavin Newsom  activated the state's emergency operation center, which would run 24 hours a day, Ward added.

Officials in Santa Barbara County heightened evacuation advisories to orders on Saturday.

The same day, the Ventura County Sheriff's Office issued evacuation orders for some communities, in place from 5pm local time Saturday to 5pm Sunday.