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


Placeholder while article actions are loading

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

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

“This risk is increasing and has already been undervalued,” 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 lead the way.”

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 away or become inaccessible.

Swain is working with emergency response officials and the National Weather Service, explaining that the question is not if a megaflood will happen, but when..

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

What’s driving the massive, destructive rainfall across the country?

His paper built on the work of other scientists, who examined sediment layers along the coastline to determine how often megafloods occurred. They found evidence of extreme freshwater runoff, which washed soil and stony material out to sea. Those layers of material were buried under years of sand. The depth of the layers and the size of the pebbles and other material contained within provide insight into the severity of past flooding.

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

On the west coast, there are usually atmospheric rivers, or streams of moisture-rich air in the middle of the atmosphere with connections to the deep tropics. For a California megaflood to happen, you would need a near-stationary low-pressure zone in the northeastern Pacific Ocean, which would meander 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” ravaged 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 waddling for a few weeks and allowing winter storm after winter storm across the northeast Pacific to California.”

The paper warns of “extraordinary effects” and reports that such an episode “could turn the inland valleys of Sacramento and San Joaquin into a temporary but vast inland sea nearly 500 miles long and [inundate] much of the now densely populated coastal plain in present-day Los Angeles and Orange County.”

The effects of a months-long barrage of soaking storms can be disastrous, but Swain notes it’s possible to get forewarned.

“This is something that we will see coming for three to five days, and I hope a week and maybe even two weeks, with a probabilistic kind of forecast,” Swain said. “We would have a fair amount of warning for it.”

Atmospheric rivers that drench the west coast are rated on a scale of 1 to 5, like hurricanes

Swain’s simulations showed that the probability of a megaflood is much higher in winters dominated by El Niño than in winters affected by La Niña. El Niño is a large-scale chain reaction atmosphere-ocean pattern that can dominate the atmosphere for several years in a row, and it usually starts with higher-than-normal sea surface temperatures in the eastern tropical Pacific.

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

The impact of human-induced climate change also plays a role: Swain says it raises the ceiling in a megaflood.

“We have multiple scenarios. The future one is much larger, in accordance with [climate change]”In the historical scenario, the smaller ones, certain parts of the Sierra Nevada will see 50 to 60 inches of liquid-equivalent precipitation … but in the future, some places will see 70 to 80 and a few 100 at a time.” of 30 days. Even places like San Francisco and Sacramento can see 20 to 30 inches of rain, and that’s just in a month. ”

An independent study published Friday in Scientific Reports concluded that human-induced 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 air can dry up the landscape more quickly — hence California’s extended drought — but when rain comes, the deck is stacked to promote an exceptional event.

“Moisture isn’t the limiting factor in California,” Swain said. “Even in the dry years there is enough moisture. The absence is a lack of mechanism. It is more a lack of storm than moisture.”

Alan Rhoades, an expert on atmospheric rivers and not involved in either study, said the study highlights the importance of not forgetting major floods, which are also central to California’s history.

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

He added that compared to previous megafloods in the late 1800s, “California has massively 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 hit, forecasters are confident it will. There is a 0.5 to 1.0 percent chance of it happening in any given year.

Swain said one of his goals is to get officials to prepare. He suggested working with the National Oceanic and Atmospheric Administration to “run through simulations like a real tabletop on ground-based disaster scenarios.”

“We’ll figure out where the points of failure would actually be because one of the things we want to do is lead the way,” he said.

Kasha Patel contributed to this report.

The Valley Voice
The Valley Voice
Christopher Brito is a social media producer and trending writer for The Valley Voice, with a focus on sports and stories related to race and culture.


Please enter your comment!
Please enter your name here

Share post:


More like this