ARkStorm

An ARkStorm is a hypothetical, but scientifically plausible weather event that could potentially devastate the state of California with catastrophic flooding. While the name invokes images of the biblical Noah’s ark, the term is actually shorthand for “atmospheric river 1,000,” a reference to atmospheric rivers (AR) of saturated tropical air creating a storm that occurs on average once every one thousand (k) years. In an ARkStorm scenario, waves of tropical moisture from the Pacific would pummel California with wind and rain for weeks, potentially leaving some areas under ten to twenty feet of water. Estimates of damage from such a storm range from $725 billion to $1 trillion. ARkStorms have occurred several times in California’s past, the most recent being in 1861 and 1862. During that event, more than ten feet of water flooded the central part of the state, creating an inland “sea” hundreds of miles long and dozens of miles across. The flood was so devastating, it bankrupted the state and changed California’s economy forever.

Background

Much of the Earth’s weather is caused by the movement of air masses across its surface. An air mass is a large body of air that takes on the temperature and characteristics of the geographical terrain below it. For example, an air mass that originates over the Arctic is cold and dry; one that forms near the equator is warm and moist. The movement and interaction of different air masses is responsible for causing everything from blizzards to thunderstorms.

Atmospheric rivers are long, relatively narrow bands of moist air that contain large amounts of water vapor. These rivers carry the water vapor—the gaseous state of water—from the warm tropical regions to other parts of the globe. Atmospheric rivers can hold an amount of water vapor equivalent to the average water flow at the mouth of the Mississippi River. When an atmospheric river makes landfall, it can interact with a continental air mass and produce large amounts of rain or snow. Most are relatively weaker systems and can bring needed precipitation to replenish water supplies. However, if a strong atmospheric river stalls over a low-lying area, it can cause significant flooding.

Overview

Atmospheric rivers have had a profound impact on the western United States, often bringing seasonal rainfall to coastal areas and huge snowfalls to the Sierra Nevada mountain range. Meteorologists have nicknamed this phenomenon the “Pineapple Express” because the warm, moist air originates from the tropical regions near Hawai'i. Water vapor condenses into liquid water as it rises and cools, so the moisture-filled atmospheric rivers can produce massive amounts of rain or snow when they encounter the slopes of California’s central mountains.

An ARkStorm scenario can occur when an exceptionally potent atmospheric river batters the California coast with waves of storms that could last for a month or longer. Some worst-case calculations predict such a storm could cause hurricane-force winds and dump up to ten feet of rain on the region. While scientific predictions are based on hypothetical models, devastating superstorms have impacted the region at least ten times in the past. Geological evidence has shown ARkStorm events occurring as far back as 212 BCE, with the last recorded event happening in 1861–1862. Scientists speculate the possibility of the next ARkStorm event occurring by the year 2060 as a fifty-fifty proposition, with the risk increasing due to climate change.

According to estimates by the United States Geological Survey (USGS), an ARkStorm could potentially flood 25 percent of all the buildings in the state. The storm would cause substantial inland flooding that would overwhelm California’s rivers and inundate coastal areas as the water rushed out to the sea. In addition to flooding, the storm would cause deadly landslides and destroy thousands of square miles of farmland. The USGS estimated that an ARkStorm could destroy more than fifty levees and result in the evacuation of more than 1.5 million people. Damage from the storm could top $725 billion, with some estimates approaching $1 trillion. As of 2019, about 40 million people lived in the areas that could potentially be affected by the storm, with the USGS predicting a “substantial” loss of life in such an event. While California would bear the brunt of such a storm, the impact of the flooding could also be felt in Arizona, Nevada, Oregon, and Utah. The USGS based its predictive data on past storm events that occurred in 1969 and 1986. The agency’s research will allow meteorologists to better forecast any potential ARkStorm event in the future.

The last ARkStorm event occurred over forty-three days in late 1861 into 1862. That storm inundated parts of California with more than ten feet of rain, and caused floodwaters to rise as high as twenty feet in some spots. The state’s central region was turned into an inland “sea” that stretched for 300 miles (483 kilometers) and was 60 miles (97 kilometers) wide at some places. The floodwaters extended from north of Sacramento down to near Bakersfield. The state’s government officials were forced to flee the capital of Sacramento and temporarily relocate to San Francisco. About one-third of all buildings in the state were believed to have been destroyed.

At the time, California was not a densely populated state, so the final death toll was officially listed at twenty-one, although far more people likely died in the disaster. However, the state’s livestock industry was decimated, with an estimated toll of more than 700,000 sheep and cattle killed. The floods also devastated the region’s fishing and mining industries. The economic damage was so bad that California was forced to declare bankruptcy. In the wake of the disaster, the state’s central region transitioned from a predominantly ranching economy to an agriculture-based economy.

By the mid-2020s, atmospheric river storms were common in California. These impacted the state with increasing frequency and turbulence. 12 atmospheric river storms deluged the state in the first three months of 2023. A year later in February 2024, California received five consecutive days of rain in which at least nine people were killed. Los Angeles received over 9 inches of rain, representing more than half its average annual rainfall. 562 mudslides were reported. A beneficial side-effect has been the replenishment of many water sources like rivers and lakes that had reached historic lows due to drought.

Bibliography

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“California Storm: After 5 Days, the Rain Has Stopped. In Its Wake, 9 Dead, a Trail of Destruction.” The Los Angeles Times, 5 Feb. 2024, www.latimes.com/california/story/2024-02-05/california-storm-weather-los-angeles-2024. Accessed 21 May 2024.

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Mckenna, Corey. “USGS’ ARkStorm Scenario Turns California Valley Into Inland Sea.” Government Technology, 16 Feb. 2016, www.govtech.com/em/disaster/USGS-ARkStorm-Scenario-California-021611.html. Accessed 10 Mar. 2021.

Mendez, Emely. “A Climate Expert Explains Why Atmospheric Rivers Are Causing Historic Rainfall in California.” Columbia Climate School, news.climate.columbia.edu/2024/02/05/a-climate-expert-explains-why-atmospheric-rivers-are-causing-historic-rainfall-in-california. 5 Feb. 2024, Accessed 21 May 2024.

Perry, Douglas. “California Now Worries About Massive ‘Arkstorm’ Flood, Which Could Devastate Oregon Too.” Oregonian, 19 Feb. 2019, www.oregonlive.com/weather/2019/02/california-now-worries-about-massive-arkstorm-flood-which-could-devastate-oregon-too.html. Accessed 10 Mar. 2021.

Porter, Keith, et al. “Overview of the ARkStorm Scenario.” United States Geological Survey, 7 Dec. 2016, pubs.usgs.gov/of/2010/1312/. Accessed 10 Mar. 2021.

Ralph, F. Martin. Atmospheric Rivers. Springer, 2020.