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

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A mention of California might typically conjure up images of wildfires and droughts, but scientists say the Golden State is also the site of extreme, once-a-century ‘megafloods’ – and that climate change could amplify how bad we become.

The idea seems inconceivable – a month-long storm dumping 30 inches of rain in San Francisco and up to 100 inches of rain and/or sleet in the mountains. But it’s happened before – most recently in 1862 – and if history is any indicator, we’re waiting for another, according to a study published Friday in Science Advances that seeks to shed light on the danger lurking.

“This risk is increasing and was already underestimated,” said Daniel Swain, one of the study’s two authors and a climatologist at the University of California, Los Angeles. “We want to get ahead.”

In such a case, 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 about whether a mega-flood will happen, but when..

It happened before in 1862, and it probably happened about five times every millennium before that,” he said. “On a human scale, 100 or 200 years sounds like a long time. But these are fairly regular occurrences.

What is driving the massive and destructive rains across the country

His paper built on the work of other scientists, who examined sediment layers along the coastline to determine the frequency of mega-floods. They found evidence of extreme freshwater runoff, which washed soil and stony material out to sea. These layers of material were buried under years of sand. The depth of the layers and the size of the pebbles and other materials they contain give insight into the severity of past floods.

“It hasn’t happened in recent memory, so it’s kind of ‘out of sight, out of mind,'” Swain said. [California is] a region that is in the perfect zone… in a climatological and geographical context.

On the west coast there are usually atmospheric rivers or currents of moisture-rich air at mid-atmospheric levels with connections to the deep tropics. For a California Megaflood to occur, an area of ​​nearly stationary low pressure would be needed in the Pacific Northeast, which would project a succession of high-end atmospheric rivers onto the California coast.

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

“These would be families of atmospheric rivers,” Swain said. “You get one of those semi-evergreens [dips in the jet stream] over the Pacific Northeast that falters for a few weeks and allows winter storm after winter storm across the Pacific Northeast into California.

The newspaper warns of “extraordinary impacts” and reports that such an episode could transform “the interior valleys of Sacramento and San Joaquin into a temporary but vast inland sea nearly 300 miles long 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 soggy storms could be disastrous, but Swain notes that it’s possible to have advance warning.

“It’s something that we would see coming three to five days, and hopefully a week and maybe even two weeks, with a probabilistic type of prediction,” Swain said. “We would have a decent amount of warning for that.”

Atmospheric rivers that flood the west coast are rated on a scale of 1 to 5 as hurricanes

Swain’s simulations showed that the likelihood of a mega-flood occurring is much greater during El Niño-dominated winters than during La Niña-influenced winters. El Niño is a large-scale chain reaction pattern between the atmosphere and the ocean that can dominate the atmosphere for several years at a time, and it usually begins with above-normal sea surface temperatures in the eastern tropical Pacific.

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

The influence of human-caused climate change also plays a role: Swain says he’s raising the ceiling for a mega-flood.

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

An independent study published Friday in Scientific Reports 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 ability to store moisture. When there are no thunderstorms, that means the air can dry out the landscape more quickly—hence California’s prolonged drought—but when there’s rain, the bridge is stacked to foster an exceptional event.

“Humidity isn’t the limiting factor in California,” Swain said. “There is a lot of humidity, even in drought years. Absence is a lack of mechanism. It’s a lack of thunderstorms rather than humidity.

Alan Rhoades, who is an expert on atmospheric rivers and was not involved in either study, said the research highlights “the importance of remembering major floods, which are also central to the story of California”.

“The main concern is how much climate change will alter the frequency of these events and how much it will fuel and amplify the impacts of the next record [atmospheric river] event,” Rhoades, a hydroclimatology researcher at Lawrence Berkeley National Laboratory, wrote in an email.

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

While researchers can’t say when the next California mega-flood will hit, forecasters are confident it will. There is a 0.5-1.0% chance of this happening in any given year.

Swain said one of the purposes of his job was to push officials to prepare. He suggested working with the National Oceanic and Atmospheric Administration to “run simulations like a real ground disaster scenario table.”

“We will work where the points of failure would really be because one of the things we want to do is get a head start,” he said.

Kasha Patel contributed to this report.

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