Another ‘Significant’ Atmospheric River Is Drenching California

Jul 21, 2023

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More heavy rain is soaking the state on Tuesday, with the ground still saturated from the storms and flooding last week.

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By Judson Jones

Judson Jones is a meteorologist and reporter for The Times.

Another powerful atmospheric river arrived overnight in California, threatening to deliver strong winds, heavy rain and a new round of floods, avalanches and mudslides on Tuesday, while some residents have yet to recover from the storms that hammered the state last week.

Light rainfall began falling before dawn in parts of the state, and was expected to intensify and become more widespread through the morning as the rapidly strengthening storm moves onshore. The high winds will threaten to topple trees and snap utility lines, while the spreading rain will increase the danger that creeks and rivers will overtop their banks in the afternoon, especially in the areas that were hardest hit last week.

Forecasters with the National Weather Service in Los Angeles are calling the latest storm, which is expected to last through Wednesday, a "significant atmospheric river event" that will drench large parts of the state and blanket mountain areas with yet more snow.

"In fact, it will be a big surprise if it does not do so," the Weather Service said.

Using 20 years of satellite observations, researchers identified periods of extreme wetness and dryness, and found they were becoming larger, more frequent and more severe.

Scattered highway and school closures and transportation delays caused by the weather were reported early on Tuesday, with more expected as conditions worsen. Officials in Santa Barbara County issued evacuation orders for people living near the burn scars of recent wildfires, which can be even more prone to flooding and mudslides than other areas.

After enduring a historically long stretch of drought, California is now experiencing one of its most ferocious winters on record. Total snowpack and rainfall in many parts of the state have reached levels not seen in decades. Months of precipitation have soaked hillsides, filled lakes and rivers, and covered the Sierra Nevada and other mountain ranges in the state with roof-level snow.

That means that each new storm is likely to compound the effects of the one before. The rain on Tuesday is falling on ground that is still fully saturated from last week's system and the earlier winter storms.

The impact of the new storm is expected "to eclipse and exceed the previous one, with potentially large-scale and long-lasting flooding impacts," forecasters with the Bay Area office of the Weather Service wrote on Monday.

Use these maps to follow the storm's forecast and impact.

Another compounding factor, especially along the Central Coast and in the Bay Area, is that thunderstorms and gale-force winds are expected. Wind gusts could reach 55 to 70 miles an hour or more along the immediate coastline and at elevations above 1,000 feet in the hills. The winds will spread into the Sierra Nevada, where there could be gusts over 100 miles an hour at mountaintops.

In some areas, residents are still coming to grips with the damage caused by last week's storms. The small farm community of Pajaro remains flooded after a levee broke early Saturday along the Pajaro River, between Santa Cruz and Monterey counties.

The latest atmospheric river — a narrow conveyor belt of water vapor in the sky — arrived first in Northern California late Monday, and then slowly spread into the central part of the state early Tuesday. It is expected to deliver rain in Los Angeles and other regions of Southern California later in the day.

All told, the storm is predicted to bring another two to five inches of rain along the coast and in valleys, and up to eight inches in the mountains. At elevations above 8,000 feet, the Sierra Nevada could receive another foot of snow or more, increasing the weight bearing down on roofs and the risk of roof collapses and roof avalanches, when large frozen sheets of snow and ice suddenly slide off buildings.

The Los Angeles region has already received at least twice the average amount of cumulative rainfall for this point in the year, according to Mike Wofford, a senior meteorologist for the Weather Service office in Los Angeles.

Anything that falls from the sky now is less likely than usual to soak into the ground, and will run off quickly instead, increasing the risk of flash floods. From there, water and snowmelt will flow into creeks and rivers that are also rising, increasing the risk that they will overflow. The combination means that water-related impact from the storm is a sure bet, forecasters from the Bay Area said on Monday.

Patrick Ayd, a meteorologist with the Weather Service in the Bay Area, compared the ground to a kitchen sponge that is completely full of water. It cannot absorb more moisture, he said, so any more water that arrives will just run off.

Waterways like the Salinas River, in the Central Coast region, have had a little bit of time to recede after the last storm, but were expected to begin rising again as the rain fell.

Mr. Ayd said that residents who saw flooding during last week's atmospheric river, like people living in the Santa Cruz Mountains or in Monterey County, should expect to be flooded again.

"They’re the ones, especially, that need to have that go-bag and be able to evacuate quickly," he said.

In mountainous regions of the state, hundreds of creeks are primed to rise quickly, Mr. Wofford said, and Santa Barbara County looks to be one of the areas that could see significant disruptions. U.S. Highway 101 is the main north-south route through the area, and it will be susceptible to closures this week because of creeks that may spill over their banks and onto the road.

Mr. Ayd noted that when the ground is saturated, trees are more susceptible to toppling over, especially after having been weakened by a previous storm system.

Judson Jones is a meteorologist and reporter for The Times, covering the most extreme storms across the globe. @thejudsonjones

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