California is due for a ‘megaflood’ that could drop 100 inches of rain
“This risk is increasing and was already underappreciated,” said Daniel Swain, one of the study’s two authors and a climate scientist at the University of California at Los Angeles. “We want to get ahead of it.”
In such an event, some in the Sierra Nevada could end up with 25 to 34 feet of snow, and most of California’s major highways would be washed out or become inaccessible.
Swain is working with emergency management officials and the National Weather Service, explaining that it’s not a question of whether a megaflood will happen but when.
“It already has happened in 1862, and it probably has happened about five times per millennium before that,” he said. “On human time scales, 100 or 200 years sounds like a long time. But these are fairly regular occurrences.”
His paper built on the work of other scientists, who examined layers of sediment along the coastline to determine how frequently megafloods occurred. They found evidence of extreme freshwater runoff, which washed soil and stony materials out to sea. Those layers of material became buried beneath years of sand. The depth of the layers and the sizes of the pebbles and other material contained in them offer insight into the severity of past floods.
“It hasn’t happened in recent memory, so it’s a little bit ‘out of sight, out of mind,’ ” Swain said. “But [California is] a region that is in the perfect area … in a climatological and geographic context.”
On the West Coast, there commonly are atmospheric rivers, or streams of moisture-rich air at the mid-levels of the atmosphere with connections to the deep tropics. For a California megaflood to occur, you’d need a nearly stationary zone of low pressure in the northeast Pacific, which would sling a succession of high-end atmospheric rivers into the California coastline.
“These would be atmospheric river families,” Swain said. “You get one of these semi-persistent [dips in the jet stream] over the northeast Pacific that wobbles around for a few weeks and allows winter storm after winter storm across the northeast Pacific into California.”
The paper warns of “extraordinary impacts” and reports that such an episode could transform “the interior Sacramento and San Joaquin valleys…
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