Disaster 'larger than any in world history' could hit California - USA Today
The rivers in the sky hold the key.
The strength, location and duration of atmospheric rivers – ribbons of water vapor that extend thousands of miles from the tropics to the western United States – will be the deciding factor in how much rain and snow falls in California this winter, and whether a disastrous "ARkStorm" flood could conceivably occur, experts told USA TODAY.
"Atmospheric rivers are the wild card for this winter," Scripps Institution of Oceanography research meteorologist Marty Ralph told USA TODAY. "If we get atmospheric rivers back to back, that sets the stage for potential flooding."
The El Niño powering up in the Pacific is prompting fears of a calamitous California winter, with potential flooding of historic proportions.
Could it cause an "ARkStorm" scenario? "I am concerned about this winter," Daniel Swain, a climate scientist with the University of California Agriculture and Natural Resources, told USA TODAY. "Due to the very strong El Niño, the risk of a severe ARkStorm-like flood resulting from weeks of heavy precipitation is more likely in a year like this one."
Atmospheric rivers are at the center of the so-called "ARkStorm" scenario – a theoretical weather set-up that "would result in widespread, catastrophic flooding throughout California – likely causing much greater damage, disruption, and economic losses than a large-magnitude earthquake near one of California’s major urban areas," Swain wrote in his blog "WeatherWest" in 2022.
The original ARkStorm scenario was developed in 2010 by scientists mostly from the U.S. Geological Survey, and reflected the potential for a flood event of truly biblical proportions. "ARkStorm," short for Atmospheric River (k) Storm, is a scenario developed to explore the impacts of an exceptionally severe series of atmospheric-river storms in California.
(The k represents 1,000, describing an event with a 0.1% chance of occurring in a specific location in any given year.)
Scientists used historic floods of the past, including a megaflood in 1862, to look at the potential impacts that could occur if or when a series of atmospheric rivers slam the California coast. The storms can pull in heat and moisture from the tropical Pacific and approach hurricane-like ferocity.
A second ARkStorm scenario (ARkStorm 2.0), in 2022, looked at whether climate change could make such an event more likely or more severe.
An atmospheric river is a long, narrow plume of concentrated moisture in the atmosphere that can produce heavy rain or snow when it reaches land. At 250 to 375 miles wide, they provide the fuel for the massive rain and snowstorms that can cause flooding along the West Coast.
Though beneficial for water supplies, such events can wreak havoc on travel, bring deadly mudslides and cause catastrophic damage to life and property, the National Oceanic and Atmospheric Administration said.
And atmospheric rivers don't dance to the tune of El Niño and La Niña, "but we're not sure why," Ralph said.
Indeed, the connection between the El Niño climate pattern and the propensity toward sustained strings of strong atmospheric-river landfalls on the West Coast remains "hazy overall," said Michael Dettinger, a research hydroclimatologist at the Scripps Institution of Oceanography, in an email to USA TODAY.
El Niño can increase the odds of atmospheric rivers hitting the West Coast, especially California and the Southwest, according to a 2017 study, but doesn't guarantee a busy atmospheric-river season. Individual atmospheric rivers remain difficult to predict months in advance.
Overall, however, the study concluded that when an El Niño brings warmer than normal water to the eastern Pacific, atmospheric rivers are more likely to impact the western United States.
The winter of 1861-62 featured the worst flood in California's history, as weeks of heavy rain left huge swaths of the state underwater. If such an event happened today, it is estimated that it would be a $1 trillion disaster, "larger than any in world history," according to a statement from the University of California, Los Angeles.
Specifically, intense rainstorms pummeled central California "virtually unabated" from Christmas Eve 1861 until January 1862, Scientific American chronicled in a 2013 story on "The Coming Megastorms."
In 2012, scientists determined that the extreme rainfall of 1861-62 was likely due to a series of atmospheric rivers that hit the entire West Coast, from Oregon to Southern California.
How much rain fell? According to a Weatherwise magazine article by meteorologist Jan Null of Golden Gate Weather Services, during the period from December 24, 1861, to January 21, 1862, rain occurred in the state on 28 out of the 30 days. San Francisco recorded nearly 34 inches of rain between December and January. Sacramento tallied over 37 inches for the 2 months.
"The 1861-1862 storms caused record or near-record flooding events across the state, from Eureka and Humboldt counties in the northwest, all the way to Orange and San Diego counties in the south," Null wrote in Weatherwise.
The flow of water also created "a huge inland sea … a region at least 300 miles long," leaving Central and Southern California underwater for up to six months, Scientific American said. Floodwaters stretched as wide as 60 miles across, the UCLA researchers wrote.
"Thousands of farms are entirely under water – cattle starving and drowning," scientist William Brewer, author of "Up and Down California in 1860-1864," wrote in a letter to his brother that Scientific American cited. "All the roads in the middle of the state are impassable; so all mails are cut off. The telegraph also does not work clear through. In the Sacramento Valley for some distance the tops of the poles are under water."
An estimated 4,000 people died and one-third of all property in the state was destroyed, including one-fourth of its 800,000 cattle, which either drowned or starved, wrote the SFGate news site in a retrospective.
That event is widely considered the benchmark for a “plausible worst case scenario” flood in contemporary California, Swain wrote in his blog WeatherWest.
El Niño isn't always the bogeyman for weather in California, as climate records indicate that 1861-62 was not an El Niño winter, Null said. In fact, only six of the 13 costliest California flood years since 1950 were during El Niño, Null told USA TODAY.
Null said the 1862 flood was a combination of the heavy rain, especially the roughly 30-day period from Dec. 22 to Jan. 22, with about 30 inches in 30 days. And just as important, there was very little in the way of flood controls, with few levees and dams.
More good news: "For flood control reasons alone, I do not think a similar meteorological event this year would rival the scope of 1861-62," Null told USA TODAY. "That is not to say there would not be huge widespread problems, but the important context would be the distribution of the rain across the winter."
The state has experienced major floods over the years, but nothing on the scale of the Great Flood of 1862, according to a statement from UCLA. The state’s population then was about 500,000, compared to nearly 40 million today. Were a similar event to happen again, parts of cities such as Sacramento, Stockton, Fresno and Los Angeles would be under water, even with today’s extensive collection of reservoirs, levees and bypasses.
Though no flood so large has happened since, climate modeling and the paleoclimate record – including river sediment deposits dating back thousands of years – shows that it typically happened every 100 to 200 years in the pre-climate change era.
In the end, what will the 2026-27 winter look like? Could it be a record-breaker for rain and snow?
The sky rivers will provide the answer: According to Ralph, in addition to being California's winter "wild card," atmospheric rivers can "make or break whether we're in drought or flood."
Doyle Rice is a national correspondent for USA TODAY, with a focus on weather and climate.
