California is due for a ‘megaflood’ that could drop 100 inches of rain

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A mention of California might usually conjure images of wildfires and droughts, but scientists say that the Golden State is also the site of extreme, once-a-century “megafloods” — and that climate change could amplify just how bad one gets.

The idea seems inconceivable — a month-long storm that dumps 30 inches of rain in San Francisco and up to 100 inches of rain and/or melted snow in the mountains. But it has happened before — most recently in 1862 — and if history is any indicator, we’re overdue for another, according to research published Friday in Science Advances that seeks to shed light on the lurking hazard.

“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.”

What’s driving the massive, destructive rainfalls around the country

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.

Videos posted to social media on Oct. 24, 2021, showed storm damage and flooding in California and Oregon as an “atmospheric river” pummeled the region. (Video: The Washington Post)

“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 into a temporary but vast inland sea nearly 300 miles in length and [inundate] much of the now densely populated coastal plain in present-day Los Angeles and Orange Counties.”

The effects of a month-long barrage of soaking storms could be disastrous, but Swain notes that it’s possible to have advance warning.

“This is something we’d see coming three to five days out, and I’d hope a week and perhaps even two weeks out, with a probabilistic type of prediction,” Swain said. “We’d have a decent amount of warning for it.”

Atmospheric rivers that drench the West Coast are rated on 1 to 5 scale like hurricanes

Swain’s simulations showed the odds of a megaflood occurring are far greater in winters dominated by El Niño than in winters influenced by La Niña. El Niño is a large-scale chain-reaction atmosphere-ocean pattern that can dominate the atmosphere for several years at a time, and it usually begins with higher-than-normal sea surface temperatures in the eastern tropical Pacific.

“When you look at the top eight monthly precipitation totals in simulations, eight out of eight occurred in El Niño years,” Swain said.

The influence of human-caused climate change also plays a role: Swain says it boosts the ceiling in a megaflood.

“We have multiple scenarios. The future one is much larger, consistent with [climate change],” he said. “In the historical scenario, the lesser one, certain parts of the Sierra Nevada see 50 to 60 inches of liquid-equivalent precipitation … but in the future event, some places see 70 to 80 and a few see 100 in a 30-day period. Even places like San Francisco and Sacramento could see 20 to 30 inches of rain, and that’s just in one month.”

An independent study published in Scientific Reports on Friday concluded that human-caused climate change will intensify atmospheric rivers and could double or triple their economic damage in the western United States by the 2090s.

A warmer atmosphere has a greater capacity to store moisture. In the absence of storms, that means the air can more quickly dry up the landscape — hence California’s prolonged drought — but should rain occur, the deck is stacked to favor an exceptional event.

“Moisture isn’t the limiting factor in California,” Swain said. “There’s plenty of moisture around even in the drought years. The absence is a lack of mechanism. It’s a lack of storms rather than moisture.”

Alan Rhoades, who is an expert on atmospheric rivers and was not involved in either study, said the research highlights the “the importance of not forgetting about major flood events, which are also central to California’s history.”

“The major worry is how much climate change will alter the frequency of these event occurrences and how much it will fuel and amplify the impacts of the next record-setting [atmospheric river] event,” wrote Rhoades, a hydroclimate research scientist at Lawrence Berkeley National Laboratory, in an email.

He added that compared to previous megafloods in the late 1800s, “California has vastly expanded its rural, urban, and agricultural sprawl, which could lead to more potential for loss of life and property.”

While researchers can’t say when the next California megaflood will strike, forecasters are confident that it will happen. There’s a 0.5 to 1.0 percent chance of it happening in any given year.

Swain said one goal of his work is to push officials to prepare. He suggested working with the National Oceanic and Atmospheric Administration to “run through simulations as a real tabletop on the ground disaster scenarios.”

“We’ll work through where the points of failure would actually be, because one of the things we want to do is get ahead of the curve,” he said.

Kasha Patel contributed to this report.

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