Updated Jan 9, 2023 - Science

"Most potent" in barrage of atmospheric rivers threatens California

A sunken section of a parking lot after a rain storm at Seacliff State Beach in Aptos, California, on Sunday. Photo: Nic Coury/Bloomberg via Getty Images

California Gov. Gavin Newsom is asking President Joe Biden to make an emergency declaration in response to a series of deadly storms, as the National Weather Service warned the "most potent system" would arrive Monday.

The big picture: The latest in a barrage of destructive atmospheric rivers that've caused widespread power outages since late last month began unleashing more powerful winds and heavy rains on California Sunday night — prompting Sacramento County to issue evacuation orders for Wilton-area residents, with flooding "imminent."

A screenshot of a tweet from a Sacramento utility provider warning of widespread outages from the storm's powerful winds.
The Sacramento Municipal Utility District said many customers would experience extended outages due to destruction from the high winds pummeling the area. Photo: SMUD/Twitter

State of play: More than 424,000 Californians were without power, over 2,900 others were evacuated from their homes and 12 people had been confirmed killed in the storms, said Nancy Ward, director of the governor’s Office of Emergency Services, at a briefing Sunday.

  • An estimated 100,000-plus customers were still without power on Sunday night, as the Office of Emergency Management issued evacuation warnings for several areas in Santa Barbara County "due to potential flooding and debris flows."

Threat level: The atmospheric river arriving Sunday night "will cause significant impact to travel" and infrastructure due to heavy rain, heavy mountain snow and damaging wind gusts through Tuesday," per the NWS. Road closures, tree damage, mudslides and "potential for major river flooding" were expected.

  • "In this weather pattern, additional rain on saturated soils will lead to considerable flood impacts, including rapid water rises, mudslides and burn scar debris flows," the NWS said in an update Sunday evening.
  • "Widespread mountain snow and high winds will also produce issues across the state," it said.
  • The California Department of Water Resources noted in a statement announcing that the agency was working with local county emergency services on preparations, "We're working under conditions of intense saturation such that even moderate levels of rainfall can produce significant flooding impacts."
A screenshot of an NWS tweet warning, " Total rainfall expected tonight through Tuesday. 2-5" coast/valleys and 5-9" foothills and mountains."
Photo: National Weather Service Los Angeles/Twitter

Zoom in: There's a moderate risk of excessive rainfall in most of central California on Monday, extending southward towards the Transverse Ranges of southern California, per the NWS.

  • "As moisture continues to sink southward on Monday night and another push of rainfall enters on Tuesday, flash flooding is increasing likely over the southern California coastal ranges through early this week," the weather service said.
  • "Susceptible terrain and areas near recent burn scars will be most at risk for debris flows and rapid runoff."

Meanwhile, for the higher terrain of the Sierra Nevada, extremely "heavy snow and intense snowfall rates" was expected to "make travel very dangerous to impossible at times," with total snowfall amounts expected greater than 6 feet possible above 7,000 feet, according to the NWS.

  • "This amount of additional accumulating snow on top of an already well built snowpack is likely to increase the threat of avalanches and strain infrastructure," the weather service said.
  • "The combination of saturated soil and gusty winds could exacerbate the tree damage threat."
"California is expecting a stronger and more widespread atmospheric river that will bring strong winds, heavy rain, and thunderstorms," the governor's office warns in this screen-shotted tweet.
Photo: California Governor's Office/Twitter

Flashback: Newsom declared a state of emergency last week in response to an atmospheric river storm associated with a bomb cyclone that brought heavy rains and hurricane-force winds to California.

Between the lines: Atmospheric rivers are potent because narrow currents in the air can carry vast amounts of water vapor thousands of miles, from the tropics to mid-and-northern latitudes.

Go deeper: Australian official says climate change making historic flooding "less natural"

Editor's note: This article has been updated with additional details throughout.

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