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

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

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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’s happened before — most recently in 1862 — and, if history is any indicator, we’re overdue for another, according to a new paper published Friday in Science Advances that seeks to shed light on the lurking danger.

“This risk is increasing and was already underappreciated,” said Daniel Swain, one of the study’s two authors and a Professor of atmospheric sciences at the University of California in Los Angeles, in an interview. “We want to get ahead of it.”

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 is already working with emergency management officials and the National Weather Service, explaining it’s not a question of if a megaflood will happen — it’s a matter of when.

It already happened in 1862, and it probably happened about 5 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 rainfall around the country

His paper built off the work of other scientists, who examined layers of sediment along the coastline to determine how frequently megaflood events occurred. They found evidence of extreme freshwater runoff, which washed soil and stony materials out to sea. Those layers of dirt became buried beneath years of sand. The depth of each layer, as well as the size of the pebbles and material contained, offers 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 perfect area … in a climatological and geographic context.”

On the West Coast, there are commonly 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, you’d need a nearly stationary zone of low pressure in the Northeast Pacific, which would sling a series of high-end atmospheric rivers into the California coastline.

“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 impacts of a month-long barrage of soaking storms could be disastrous, but Swain notes 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 2 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 a 1 to 5 scale like Hurricanes

Swain’s simulations showed the odds of a megaflood event occurring are far greater in an El Niño winter versus during a 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 usually begins with warmer-than-normal warm 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,” he said.

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

“We have multiple scenarios. The future one is much larger, consistent with [climate change]they 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 that see 100 in a thirty-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 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 is not 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.”

While they can’t say when the next California megaflood will strike, Forecasters are confident it will happen again. 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 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.

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