California braces for powerful atmospheric river storm

Another atmospheric river storm is threatening California with flooding, landslides, and power outages on Wednesday as millions of residents recover from several destructive storms, one of which caused a levee breach last weekend.

Flood watches are in effect across central and Northern California, where the ground has become more saturated and vulnerable to flooding and rapid runoff. The new storm is expected to compound the damage in the already-battered region and is forecast to bring up to four inches of rain and winds up to 40 miles an hour to the valleys, according to the National Weather Service Weather Prediction Center.

Officials in the city of Watsonville and in Santa Cruz County have issued mandatory evacuation orders for communities at high risk of flooding. San Mateo County, located south of San Francisco, declared a local state of emergency and activated its emergency operations center.

“Prepare now for flooding, downed trees and power outages,” National Weather Service forecasters in the San Francisco Bay Area said Tuesday evening, warning residents that the weather system would bring major rain and wind.

In south Sacramento County, responders are attempting to repair part of a 34-mile levee system along the Cosumnes River, which protects land made up mostly of vineyards and cattle ranches, before the storm is set to arrive on Wednesday.

The new storm is part of a series of atmospheric river storms that climate scientists forecast will continue this month. Atmospheric rivers are relatively long, narrow streams in the atmosphere that transport most of the water vapor outside of the tropics, according to the National Oceanic and Atmospheric Administration.

“These storms typically bring heavy or extreme rainfall and snowfall rates over relatively short periods of time,” said Daniel McEvoy, a climatologist with the Western Regional Climate Center.

Atmospheric rivers contribute up to 50% of California’s annual precipitation and drive about 84% of flood damage in the Western states. While recent heavy rains have brought a degree of relief to drought-imperiled California, scientists warn the storms won’t immediately alleviate severe deficits in the state’s groundwater and soil moisture.

Daniel Swain, a climate scientist at UCLA, said there are at least two additional atmospheric river storms forecast in the next five days that could lead to more severe flooding.

“This is not just a one-and-done. This is an ongoing sequence,” Swain said. “The big flood risk comes with this succession of progressively stronger storms, which looks like the pattern we’re in.”

Gov. Gavin Newsom’s office said on Twitter that the state’s operations center is at its highest emergency level and its flood operations center is coordinating efforts like bringing sandbags to local residents. Officials said the state is setting up shelters and is prepared to deploy staff to hospitals and ambulance strike teams.