Updated Jan 24, 2024 - Energy & Environment

Atmospheric rivers to threaten California, raising flood concerns

Map showing jet stream winds blowing across the Pacific Ocean into North America and across it.

Map showing jet stream winds blowing across the Pacific Ocean into North America and across it, featuring extremely strong winds over the North Pacific associated with potential storms. Image: Tropicaltidbits.com

San Diego's dangerous flash flood on Monday may not be the state's last such event of the next couple of weeks, as a weather pattern favorable to atmospheric rivers takes shape across the Pacific Ocean.

Why it matters: Atmospheric rivers are responsible for the majority of the Golden State's precipitation, and are associated with some of its worst floods on record.

Zoom in: On Monday, a relatively weak atmospheric river, along with favorable winds in the mid-to-upper atmosphere, combined to deliver a deluge to San Diego, with the city seeing 2.73 inches of rainfall.

  • This ranked as its fourth-wettest day on record, as well as the wettest January day.
  • Most of the rain there fell in just a three-hour period and focused on downtown San Diego, a region with poor stormwater runoff issues. This led to severe flooding and more than two-dozen water rescues, as Axios' Andrew Keatts reports.

The big picture: During the next two weeks, a powerhouse jet stream is projected to set up across the Pacific Ocean. In a characteristic pattern for El Niño-influenced winters, computer model simulations show it blowing largely west-to-east across the world's largest ocean basin, at speeds up to 200 mph.

  • This weather pattern will be ideal for carrying copious amounts of water vapor and associated storminess into California, potentially in a series of storms.
  • Such a scenario poses flooding risks, but also could prove to be a boon for ski areas if snow levels drop low enough, and for water managers concerned about the 2024 dry season.
  • The first time period for an atmospheric river event looks to be Jan. 31 to Feb. 5, which the National Weather Service has highlighted for the risk of heavy rain and snowfall for much of the West Coast, extending into Utah, Arizona, Nevada and New Mexico.
  • "An impactful atmospheric river event will likely move southward along the West Coast," the NWS stated in a social media post on Tuesday. The storm, the NWS said, may bring "Heavy rain and high winds to California and interior regions of the Southwest, along with heavy snow at higher elevations."

Of note: An atmospheric river is a long, narrow highway of moisture in the sky, typically at about 10,000 to 15,000 feet above the surface.

  • These can stretch for thousands of miles and strong ones can even transport an amount of water vapor "roughly equivalent to 7.5 to 15 times the average flow of liquid water at the mouth of the Mississippi River," according to NOAA.

Between the lines: Precipitation outlooks through the first week of February show the potential for the evolution of a deep dip, or trough, in the jet stream across the West, with a ridge of high pressure further east.

  • This would bring wetter than average conditions to the West, along with above average temperatures and drier conditions in the East.
  • While no atmospheric river storm is guaranteed to bring an extreme flood situation like San Diego faced Monday, it is noteworthy that this was the second California city to pick up so much rain in less than a day, with Ventura, California, having seen a similar situation in December.
  • As climate scientist Daniel Swain stated via X, the unusually mild ocean temperatures in the tropical Pacific Ocean, which are associated with El Niño, as well as above average water temperatures closer to California may be helping to fuel such deluges.
  • In addition, climate change is boosting the odds and severity of precipitation extremes both nationally and globally. Warmer waters and air temperatures provide more moisture to fuel storms.

The bottom line: January is likely to end stormy, and February is likely to start that way in the U.S. West. It may stay that way into mid-month at least.

Editor's note: This story was updated with a statement from the NWS.

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