California declares emergency as massive storm slams into coast

California's governor has declared a state of emergency as residents brace for a major storm - the second to drench the state in the past week.

The storm is expected to develop into a bomb cyclone, and spark rain and snowstorms that could cause flooding and landslides in already soaked areas.

Northern California, which saw a deadly levee breach over the weekend, is expected to be hardest-hit.

The storm is also bringing strong winds to areas up and down the US west coast.

The National Weather Service (NWS) has issued a flood watch and high wind warning for the entire Bay Area in northern California- saying that the gusts are expected to down trees and cause power outages.

"To put it simply, this will likely be one of the most impactful systems on a widespread scale that this meteorologist has seen in a long while," the NWS forecaster for the region said in a weather advisory. 

"The impacts will include widespread flooding, roads washing out, hillside collapsing, trees down (potentially full groves), widespread power outages, immediate disruption to commerce, and the worst of all, likely loss of human life," he added.

"This is truly a brutal system that we are looking at and needs to be taken seriously."

It is expected to form into a bomb cyclone, a type of rapidly intensifying storm that is more commonly seen along the east coast of the US and Canada.

In a statement declaring a state of emergency on Wednesday, Governor Gavin Newsom said his order "will allow the state to respond quickly as the storm develops and support local officials in their ongoing response."

An atmospheric river - a long narrow stream of warm air bringing moisture from the tropics - is expected to fall on ground that has already been saturated by rainfall.

It is expected to hit hardest overnight on Wednesday into Thursday morning before extending into southern California on Thursday night.

Just a year after California recorded one if its driest year's on record, San Francisco on Saturday saw its second-wettest day in over 170 years.

San Francisco Mayor London Breed said the city is "preparing for a war" and issuing sandbags across the city to prevent flooding.

Rivers along the coast are forecast to see widespread flooding due to tidal surges. The storm is expected to dump up to 6 inches (15 cm) of rain in coastal areas, and gust up to 80mph (128kmph) in the coastal hills and mountains.

An evacuation order has been issued in Santa Cruz County, south of San Francisco, due to the "high probability" that some neighbourhoods will become inaccessible due to flooding.

Over 105 million people across the US are currently at risk of severe weather, according to the NWS.

Further east, some 30 million people are facing large storms that have already produced tornadoes in several states.