Updated Feb 5, 2024 - Energy & Environment

Extreme atmospheric river storm slams California with flooding rains, high winds

A person walks through flood waters as a powerful long-duration atmospheric river storm, the second in less than a week, impacts California on February 4, 2024 in Santa Barbara, California.

A flooded street in Santa Barbara, as a powerful long-duration atmospheric river storm, the second in less than a week, impacts California on Sunday. Photo by Mario Tama/Getty Images

Editor's note: See Axios' latest coverage on the California storm.

California's governor proclaimed a state of emergency for eight southern counties as a rapidly intensifying storm off the central coast brought a "high risk" of potentially deadly flooding to Los Angeles Sunday and Monday.

Threat level: It is rare for any major city, let alone a city as large as LA, to be under a high risk for excessive precipitation, but LA is in that category for Sunday and Monday, indicating the prolonged nature of the flood threat.

  • High risk days are only issued on 4% of days, but they account for 39% of flood-related fatalities and 83% of flood-related damages, per the Weather Prediction Center.
  • In this case, the high risk is related to the atmospheric river associated with the powerful storm system.

Of note: On Sunday morning local time, the National Weather Service's LA forecast office raised the rainfall forecast for its forecast region to 4 to 8 inches in coastal and valley locations, and 8 to 14 inches in foothill and mountain areas.

  • This increased the likelihood of potentially deadly flooding once the heavy rain reached SoCal over Sunday night.
  • A key reason the forecast totals were raised is the likelihood that the heaviest rain bands will slow down or stall as they move through heavily populated portions of Southern California.
  • The WPC stated Sunday that "Locally catastrophic flash and urban flooding impacts are possible across Los Angeles."

Zoom in: California Gov. Gavin Newsom's emergency proclamation covers Los Angeles, Orange, Riverside, San Bernardino, San Diego, San Luis Obispo, Santa Barbara and Ventura counties.

  • Because it was intensifying quickly so close to the coastline, the storm is directing high winds into the Golden State, with hurricane force wind warnings in effect for offshore areas of Central California, and warnings issued for gusts up to 95 mph in the mountains just north and west of Los Angeles.
  • Even lower elevations, including San Francisco south to Big Sur, Monterey and San Luis Obispo, and potentially onto LA's doorstep, are seeing damaging wind gusts up to 70 mph or higher.
  • On Sunday, winds were gusting as high as 94 mph in Grapevine, Calif., and tree damage has been widespread across central and southern parts of the state.
  • As of 10pm Sunday local time, more than 800,000 customers were without power, per poweroutage.us.

Between the lines: "All systems are go for one of the most dramatic weather days in recent memory," the NWS LA stated in a forecast discussion Sunday morning, noting the extremely high difference in air pressure, known as a gradient, between LA and areas to the north, as the low continues to strengthen.

  • Tight pressure gradients result in high winds, as air rushes from high pressure to low pressure.
  • The heavy rainfall is forecast to cause mudslides, rockslides, flash flooding, urban flooding and additional hazards, especially because the storm is hitting an already water-logged state.

The intrigue: The atmospheric river, which is a narrow highway of moisture at mid-levels of the atmosphere, will be aimed at the California coast for a prolonged period, sliding from north to south over time.

  • This air will contain a tremendous amount of water vapor, which will be enhanced by unusually high sea surface temperatures between California and Hawaii.
  • "The warmer the surface ocean is, the more potential evaporation there is off of it," said UCLA climate scientist Daniel Swain.
  • He said human-caused climate change plus El Niño are likely causing this year's trend.
Schematic diagram of what an atmospheric river looks like hitting a coastline.
What an atmospheric river looks like hitting the California coast. Image: Sara Grillo/Axios

Zoom out: Human-caused climate change is making extreme precipitation events more common and intense across the U.S. and worldwide and is predicted to make atmospheric rivers even wetter in coming years.

  • One 2022 study found that atmospheric rivers that hit California in 2017 were made up to 15% wetter due to human-caused climate change, suggesting such an influence may already be detectable.

What they're saying: "This is a DANGEROUS SYSTEM with major risks to life and property," the NWS LA said in a social media post Sunday afternoon.

Go deeper: In photos: Atmospheric river triggers evacuations and outages in California

Editor's note: This a breaking news story. Please check back for updates.

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