US President Joe Biden approved an emergency declaration for California after a week of storms that have killed at least 12 people since December 31 and knocked out power for hundreds of thousands of homes and businesses in the state.

The emergency declaration authorises the Federal Emergency Management Agency (FEMA) to coordinate disaster relief efforts and mobilise emergency resources, the White House said in a statement.

More than 120,000 homes and businesses were still without power in California early on Monday, according to data from PowerOutage.us, after a massive storm last week that disrupted road travel with flash floods, rock slides and toppled trees.

The National Weather Service warned of a “relentless parade of atmospheric rivers” — storms that are long plumes of moisture stretching out into the Pacific Ocean and capable of dropping staggering amounts of rain and snow.

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