LA's Overdue "Mega-Storm" Could Be More Than Just a Storm, Says Expert

Scientists raise the alarm about a future "ARkStorm."

A recent report from the US Army Corps of Engineers warned that Los Angeles is overdue for an epic storm that could displace 1.5 million people with extreme flooding, and it could last for weeks. This hypothetical catastrophe is sometimes referred to as a “mega-storm,” but that informal term doesn’t capture the scale of such an event. Its actual classification as an “ARkStorm” details just how much damage it could cause if the city doesn’t prepare.

As the LA Times reported Monday, a “rare LA mega-storm” could overwhelm the Whittier Narrows Dam, a major factor in the city’s flood control system. The Corps, which deemed the dam unsafe in 2017, has been figuring out how to prepare LA for such a storm for several years. It released an impact statement describing the dam’s potential for damage in December 2018, and two public hearings held in January reiterated that the dam no longer meets the agency’s “tolerable-risk” guidelines and would not protect the city if a “mega-storm” hits.

A Mega-Storm Is an “ARkStorm”

Whittier Narrows Dam
The Whittier Narrows Dam, opened in 1956, wouldn't withstand a "mega-storm" or "ARkStorm," warn experts.

Atmospheric scientist David Kristovich, Ph.D., an adjunct associate professor at the University of Illinois’ Department of Atmospheric Sciences, tells Inverse that he’s “not aware of any formal definition of a mega-storm,” since the term isn’t listed in the glossaries of either the American Meteorological Society or NOAA’s National Weather Service. This hypothetical future storm, however, is characterized by the ARkStorm project, which the United States Geological Survey runs to help cities prepare for the damage caused by future storms before it’s too late.

ARkStorm specifically addresses the possibility of a massive West Coast storm like the legendary 43-day storm that devastated California in December 1861. According to a 2013 Scientific American analysis by UC Berkeley climate change expert Lynn Ingram, Ph.D., that storm put central and southern California underwater for six months, and geologic evidence suggests that those floods have occurred in the state “every 100 to 200 years.” That’s the kind of historical data that the ARkStorm project uses to predict what a future storm — one exacerbated by climate change — could look like.

According to the USGS, such a storm could “realistically flood thousands of square miles of urban and agricultural land, result in thousands of landslides, disrupt lifelines throughout the state for days or weeks, and cost on the order of $725 billion.”

Introducing “Atmospheric Rivers”

atmospheric river
Atmospheric rivers are streams of water vapor with the potential to dump massive amounts of precipitation.

The historical data suggests that such a storm would be fueled by an “atmospheric river” — that’s what the “AR” in ARkStorm stands for — a huge stream of water vapor that floats a mile above the Earth’s surface. After these rivers of water vapor are generated over the warm, moist Pacific Ocean, they travel across the globe, causing severe flooding — and especially so on America’s West Coast. According to the NOAA, they can carry “an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River.”

Importantly, Kristovich adds, an ARkStorm “is not necessarily a single storm but could be a series of severe storms cumulatively having severe impacts.”

Atmospheric rivers, for their part, don’t always cause catastrophe when they make landfall. In fact, the spillover from atmospheric rivers supply a large amount of California’s yearly precipitation and are currently helping pull California from its ongoing drought. But not all atmospheric rivers are created equal. ARkStorm refers specifically to “Atmospheric River 1,000” — an event that will produce levels of precipitation experienced on average only once every 500 to 1,000 years.

Preparing for the Other “Big One”

California’s “other Big One,” as the USGS puts it (the Big One is the massive earthquake), will be a statewide disaster and an economic catastrophe if the state doesn’t update its infrastructure now. There’s no predicting when exactly it will hit, but experts say it’s “plausible, perhaps inevitable,” noting that the geologic record shows that at least six storms as severe as the 1861-62 storm have occurred in California in the last 1,800 years.

The LA Times reports that the US Army Corps of Engineers is calling for $600 million from the federal government to upgrade the Whittier Narrows Dam, though this funding will require congressional approval.

Additionally, city officials are working with the federal government to create an emergency plan in case the dam isn’t repaired in time. Whether you call it a mega-storm or ARkStorm, the question is no longer if it will happen, but when.