August 11, 2026
California’s Central Valley faces the greatest peril, though flooding, landslides, power outages, and transportation failures could strike every county. "It was always a when, not if,” said Dr. Daniel Swain, a UCLA climate scientist who co-authored the ARkStorm 2.0 study. "Before global warming, that ‘when’ might have been centuries away. Now it’s quite likely to be within my own lifetime.”
Atmospheric rivers normally deliver up to half of California’s annual water supply. When they stall or arrive in rapid succession, however, they become engines of catastrophe.
The strongest can transport roughly 15 times as much water as the Mississippi River at its mouth. The original ARkStorm scenario, developed by the U.S. Geological Survey and other experts in 2010, drew partly on the Great Flood of 1861–62, when storms pounded the state for about 45 days and turned the Central Valley into a lake nearly 300 miles long. Geological records show six even more severe events struck California over the past 1,800 years.
Researchers updated the scenario in 2022 to account for a warmer climate. Swain described one key change as the "expanding atmospheric sponge effect.” For every 1.8 degrees Fahrenheit of warming, the atmosphere can hold about seven percent more water vapor.
"You can essentially just imagine an ordinary kitchen sponge that grows progressively larger with warming,” he said. "It has the capacity to absorb much more water.” When that moisture is released over California, rainfall intensifies dramatically.
The strongest can transport roughly 15 times as much water as the Mississippi River at its mouth. The original ARkStorm scenario, developed by the U.S. Geological Survey and other experts in 2010, drew partly on the Great Flood of 1861–62, when storms pounded the state for about 45 days and turned the Central Valley into a lake nearly 300 miles long. Geological records show six even more severe events struck California over the past 1,800 years.
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