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 climate change can multiply how bad it is for someone.

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, says a new paper published Friday. Scientific advances It seeks to shed light on hidden danger.

“This risk is increasing and already underestimated,” Daniel Swain, a professor of atmospheric sciences at the University of California, Los Angeles and one of the study’s two authors, said in an interview. “We want to go further than that.”

In such an event, some in the Sierra Nevada could end up with 40 or 50 feet of snow, and most of California’s major highways would be washed out or inaccessible.

Swain already works with emergency management officials and the National Weather Service, explaining it’s not a question A mega flood will happen – it’s just a matter of when.

It already happened in 1862, and before that it probably happened 5 times a millennium,” he said. “On human timescales, 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 built on the work of other scientists who studied sedimentary layers along the coast to determine how often megaflood events occurred. They found evidence of intense freshwater flow that washed mud and rock material into the ocean. Those layers of dirt were buried under the sand. The depth of each layer and the amount of pebbles and materials it contains provide insight into the severity of past flood events.

“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 right climate and geographical environment.”

Along the west coast, atmospheric rivers or currents of moisture-rich air are generally connected to the deep tropics in the mid-atmosphere. For a California megaflood, you’d need a near-stationary low-pressure zone in the northeast Pacific that would connect a series of high-level atmospheric rivers along the California coast.

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

The paper warns of “extraordinary implications” and such an episode “[transform] Inland the Sacramento and San Joaquin valleys form a temporary but vast inland sea nearly 300 miles long and [inundate] Much of the coastal plain is now densely populated in present-day Los Angeles and Orange counties.”

Swain notes that the impacts of a month-long barrage of drenching storms can be devastating, but early warning is possible.

“We’re looking at it coming out three to five days, and hopefully a week and maybe even 2 weeks out with a probability-type forecast,” 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 showed that the odds of a megaflood event during an El Niño winter during a La Niña are much higher. El Niño is a large-scale chain-reaction atmosphere-ocean pattern that dominates the atmosphere for years at a time, and typically begins with warmer sea surface temperatures than normal in the eastern tropical Pacific.

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

The impact of human-caused climate change also plays a role: Swain says it increases the ceiling on a large megaflood event.

“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 scenario, some places will see 70 to 80 and some places up to 100 in thirty 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, scientific reports released Friday showed.

A warmer atmosphere has a greater capacity to store moisture. In the absence of storms, the wind quickly dries out the landscape — hence California’s prolonged drought — but, if rain falls, 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.”

While they can’t say when the next California flood will hit, forecasters believe it will happen again. 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 simulations as a real tabletop in land disaster scenarios.”

“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.

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