California is bracing for a powerful atmospheric river storm that is expected to drench large swaths of the state with rain and bring several feet of snow to the mountains.
Much of northern California was under a winter storm warning because of the gusty winds and heavy snow in the forecast that the National Weather Service (NWS) said would lead to “difficult to impossible travel conditions”.
The NWS issued a flood watch in much of southern California through Thursday afternoon. The region was also anticipating dangerous driving conditions as well as road flooding and debris flows in areas affected by recent wildfires. Los Angeles placed several areas under evacuation warnings and orders due to the risk of debris flows from heavy rain, and announced the closure of several roads, including a portion of the Pacific Coast Highway.
Meanwhile, the state’s office of emergency services announced it had sent more than 400 workers, including swift-water rescue teams and urban search and rescue personnel, across the state in preparation for the storm. Authorities advised residents to have a go-bag ready and prepare for power outages.
Atmospheric rivers, long and relatively narrow bands of water vapor that take moisture from the ocean and dump massive amounts of rain, play a crucial role in replenishing the state’s reservoirs and snowpack. But they have also been behind some of the most devastating storms in California, including the extreme weather that killed about 20 people in 2023, and are increasing in frequency and severity.
A comprehensive study of atmospheric rivers in the current issue of the Journal of Climate found that the heavy rain and wind events most known for dousing California and other parts of the west have been getting bigger, wetter and more frequent in the past 45 years as the world warms.
Atmospheric rivers have increased in the area they soak by 6% to 9% since 1980, increased in frequency by 2% to 6% and are slightly wetter than before, the study said.
Scientists have long predicted that as the climate crisis caused by the burning of coal, oil and gas makes the air warmer, it holds more moisture, which means bigger, nastier atmospheric rivers in the future. The new research indicates that a wetter future is already here.
“This doesn’t mean that it’s necessarily all because of climate change. We didn’t study that, but it does line up, broadly speaking, with some expectations of how [atmospheric rivers] will change in a warming atmosphere,” according to the study’s lead author, Lexi Henny, an atmospheric scientist at the University of North Carolina who did her research while at Nasa.
What has happened already “is still small relative to the changes that we think are going to happen” in a future warmer world, Henny said.
While atmospheric rivers can bring much needed rain to drought-stricken places, they are often dangerous. Last year a series of atmospheric rivers caused hundreds of mudslides and killed several people in California. In the 1860s, California had to move its capital out of Sacramento because of atmospheric river-caused flooding.
These events happen all over the US and the world, though sometimes don’t get recognized as atmospheric rivers, Henny said. An atmospheric river in New England in 2023 brought a foot of rain and 50mph winds. A 2020 atmospheric river dumped 99in of snow on Alaska.
