Los Angeles (AFP) - Dozens of evacuation orders were in place Friday as a powerful atmospheric river, known as a "Pineapple express," surged into an already-sodden California, sparking warnings of widespread flash flooding.

Up to nine inches (23 centimeters) of rain were expected in some parts of the state, with several feet of heavy, wet snow falling over mountains where a huge snowpack has built up over months of near-record storms.

Forecasters said the Pineapple Express -- so called because it is bringing warm, subtropical moisture from Hawaii -- could cause some of that monster snowpack to melt, overwhelming river systems.

"Rainfall totals of 4-9 inches, atop areas with saturated soil and deep snowpack will cause widespread and severe flooding impacts," the National Weather Service (NWS) warned.

"Higher elevations in northern California and the Sierras will receive a heavy, wet snow leading to difficult travel."

US President Joe Biden on Friday approved an emergency declaration for the state, clearing the way for federal assistance to help local agencies.

The move came after a request from California Governor Gavin Newsom, who said he was mobilizing resources to help anyone in need.

"California is deploying every tool we have to protect communities from the relentless and deadly storms battering our state," Newsom said. 

The bulk of evacuation orders affect northern California, with seven rivers expected to burst their banks.

Many of them flooded at the start of the year as a daisy chain of atmospheric rivers dumped massive rain on the state.

More than 20 people died as back-to-back storms washed out communities, bringing down trees and causing landslides -- and emergency services now are worried the fresh storms could cause more problems.

'Stay at home'

San Mateo County Sheriff said two people whose car was crushed by a falling tree in Redwood City were expected to be okay.

"Stay home tonight if you can, folks," the sheriff's office tweeted, alongside a photo of the mangled car.

"If you must drive in the storm, deputies are here for you, day or night."

The Sierra Nevada mountain range has been buried in unusually heavy snow for weeks.

As more moved in on Friday, a number of ski resorts shut their gates, citing weather worries.

In South Lake Tahoe, the weight of the accumulated snow brought down the roof of a gas station, sparking a fire, the San Francisco Chronicle reported.

Local fire marshal Kim George said crews had responded to similar calls over the last few days.

"No one has been hurt in any of these incidents, which is remarkable," the Chronicle quoted her as saying.

In the San Bernardino Mountains, in the south of the state, most communities have dug out after days or even weeks of being cut off.

But local sheriffs said one man appeared to have taken things a bit far when he stole a municipal snowplow.

The suspect, named by law enforcement in Big Bear as Jonathan Hernandez, allegedly drove off with the vehicle on March 4.

He didn't manage to cover his tracks all that well, though -- the plow had a GPS device and when officers followed the signal they found the stolen vehicle with Hernandez still sitting inside.

Organizers of the Oscars were contending with rather un-Hollywood-like weather as they installed the huge, tented red carpet ahead of this Sunday's awards show.

Buckets were placed along the walkway to catch rain dripping through cracks and threatening to dampen life-size statuettes.