100 inches of rain in California due to ‘megaflooding’

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Mention of California usually conjures up images of wildfires and drought, but scientists say the Golden State is the site of once-a-century “megafloods” — and how bad climate change can multiply.

The idea seems unimaginable – a month-long storm that dumps 30 inches of rain on San Francisco and 100 inches of rain and/or melted snow on the mountains. But it’s happened before — most recently in 1862 — and if history is any indicator, we’re overdue for another one. Research Published Friday in Science Advances, it seeks to shed light on the lurking danger.

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

In such an event, some parts of 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 inaccessible.

Swain works with emergency management officials and the National Weather Service to say it’s not a question of if megafloods will happen, but when..

It already happened in 1862, and maybe five times a millennium before that,” he said. “In human time scales, 100 or 200 years seems like a long time. But these are very common occurrences.

What caused the massive, destructive rainfall across the country?

His thesis builds on the work of other scientists who study sedimentary layers along the coast to determine how often floods occurred. They found evidence of extreme freshwater flows that washed mud and rock material into the ocean. Layers of the material were buried beneath the sand. The depth of the strata and the sizes of pebbles and other materials within them provide insight into the intensity of past floods.

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“It hasn’t happened in recent memory, so it’s a little bit ‘out of sight, out of mind,'” Swain said. “But [California is] An area that is in the right climate and geographical environment.”

Along the west coast, atmospheric rivers or currents of moisture-rich air are generally associated with the deep troposphere in the mid-atmosphere. For the California megaflood to occur, you would need a near-stationary low pressure area in the northeast Pacific, which would cause a series of high-level atmospheric rivers along the California coast.

Videos posted on social media on October 24, 2021, showed storm damage and flooding caused by an “atmospheric river” in California and Oregon. (Video: The Washington Post)

“These are atmospheric river families,” Swain said. “You get one of these for half [dips in the jet stream] A few weeks wobble over the Northeast Pacific and after the winter storm allows the winter storm to cross into California across the Northeast Pacific.

Warning of “extraordinary impacts,” the paper suggests “the interior would transform the Sacramento and San Joaquin valleys into a temporary but vast inland sea nearly 300 miles long.” [inundate] Much of the coastal plain is now densely populated in present-day Los Angeles and Orange counties.”

Swain notes that the effects of month-long storms can be devastating, but early warning is possible.

“We’ll see it play out three to five days, and I believe it could play out a week or two with a probabilistic type of projection,” Swain said. “We need a good amount of caution for that.”

Atmospheric rivers that drench the West Coast are rated hurricane-like on a scale of 1 to 5

Swain’s simulations show that the odds of a megaflood occurring in El Niño-dominated winters are much higher than in La Niña-dominated winters. El Niño is a large-scale chain-reaction atmosphere-ocean pattern that dominates the atmosphere for years at a time, and typically begins with above-normal sea surface temperatures in the eastern tropical Pacific.

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

The impact of human-caused climate change also plays a role: It increases the ceiling on a mega-flood, says Swain.

“We have so many scenarios. The future is so big, so flexible [climate change],” he said. “Historically, parts of the Sierra Nevada see 50 to 60 inches of liquid-equivalent precipitation … but in a future event, some places could see 70 to 80, and some places 100 in 30 days. . Even places like San Francisco and Sacramento can see 20 to 30 inches of rain in just one month.

A Independent study Human-caused climate change will intensify atmospheric rivers and double or triple their economic damage in the western United States by the 2090s, a study published Friday in Scientific Reports said.

A warmer atmosphere has a greater ability to store moisture. In the absence of storms, the wind can quickly dry out the landscape — hence California’s prolonged drought — but if it rains, the deck is stacked in favor of an exceptional event.

“Humidity is not the limiting factor in California,” Swain said. “There is plenty of moisture even during droughts. What is lacking is a lack of mechanism. It is a lack of storms rather than moisture.”

Alan Rhodes, an expert on atmospheric rivers who was not involved in either study, said the research highlights “the importance of not forgetting the big flood events that are central to California’s history.”

“The main concern is how much climate change will change the frequency of these event events and how much they will trigger and amplify the impacts of the next record system. [atmospheric river] event,” Rhodes, a hydroclimate research scientist at Lawrence Berkeley National Laboratory, wrote in an email.

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Compared to the megafloods of the late 1800s, “California has vastly expanded its rural, urban and agricultural land, making it more prone to loss of life and property,” he added.

While researchers can’t say when the next California megaflood will hit, forecasters are confident it will happen. 0.5 to 1.0 percent is likely to happen in any given year.

Swain said one goal of his work is to prepare officers. He suggested working with the National Oceanic and Atmospheric Administration to “run a realistic tabletop in land disaster scenarios through simulations.”

“We’ll work on where the points of failure really are 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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