Over a decade ago, the U.S. Geological Survey (USGS) and state agencies developed the idea of the ARkStorm, an extreme storm and flood scenario for California based on previous rain events. A USGS video—disconcertingly set to hard rock music—created in 2010 and recently uploaded to YouTube paints a Biblical picture of what the devastating storm could potentially look like: “a fury rivaling that of hurricanes, beginning a process of destruction that will last for weeks.” As heavy storms caused by atmospheric rivers hit California this week–many regions are under flood warning—many are worried about the possibility of such a catastrophic event occurring now. So what is it, and is it really something you need to worry about?
An ARkStorm is a hypothetical scenario for California caused by a series of atmospheric rivers that drown much of the state with recurring rainstorms: “The storm is estimated to produce precipitation that in many places exceeds levels only experienced on average once every 500 to 1,000 years,” the USGS explains. “ARkStorm” stands for “Atmospheric River 1,000 years storm,” or, the type of storm that would occur roughly every thousand years. Climatologists have warned that climate change is making extreme weather more common, and California has been increasingly plagued by years’ worth of drought followed by intense rains that cause landslides in wildfire-tarnished landscapes, flooding of parched soil, etc.
Atmospheric rivers are weather patterns that are common on the West Coast that cause huge deluges of rain. The difference in an ARkStormis that, in a worst-case scenario, Pacific jet streams essentially cause a series of back-to-back-to-back storms in which the rain rarely lets up: “they will be characterized by a week-long sequences of recurrent, strong to extreme ARs during the cool season and coinciding with a persistently strong Pacific jet stream,” researchers explain.
In its original paper, the USGS explains that weeks’ worth of heavy rain causes mass flooding, chaos, damage, and destruction throughout the state and putting large swaths of land underwater. The storm is modeled on one that happened in the winter of 1861-1862, which turned Sacramento into a giant river and flooded huge swaths of the West Coast:
“Extensive flooding results. In many cases flooding overwhelms the state’s flood-protection system, which is typically designed to resist 100- to 200-year runoffs. The Central Valley experiences hypothetical flooding 300 miles long and 20 or more miles wide. Serious flooding also occurs in Orange County, Los Angeles County, San Diego, the San Francisco Bay area, and other coastal communities. Windspeeds in some places reach 125 miles per hour, hurricane force winds. Across wider areas of the state, winds reach 60 miles per hour. Hundreds of landslides damage roads, highways, and homes. Property damage exceeds $300 billion, most from flooding. Demand surge (an increase in labor rates and other repair costs after major natural disasters) could increase property losses by 20 percent. Agricultural losses and other costs to repair lifelines, dewater (drain) flooded islands, and repair damage from landslides, brings the total direct property loss to nearly $400 billion, of which $20 to $30 billion would be recoverable through public and commercial insurance. Power, water, sewer, and other lifelines experience damage that takes weeks or months to restore. Flooding evacuation could involve 1.5 million residents in the inland region and delta counties. Business interruption costs reach $325 billion in addition to the $400 billion property repair costs, meaning that an ARkStorm could cost on the order of $725 billion.”
The ARkStorm hypothesis is similar to the Great Shakeout, explains UCLA climate scientist Daniel Swain.
“It’s conflated as being a specific weather forecast for some specific date or right now, and it’s not either of those things,” Swain explains. “It’s never been and has never intended to be a specific prediction. It’s a scenario that outlines the kind of events that are possible in this part of the world at the high end of what’s plausible, not what’s actually heading towards California in this moment.”
The ARkStorm scenario was updated to “ArKStorm 2.0” in August of last year in a paper published in Science Advances that researchers say is “a new severe storm and flood scenario reimagined for the climate change era.” In this scenario, the researchers explain that an ARkStorm could be caused by significantly heavier-than-usual rains in 16-20 days out of a 30-day period, and that the ArKStorm scenario in general is more likely to occur because of extremes caused by climate change.
“Climate change is robustly increasing both the frequency and magnitude of extremely severe storm sequences capable of causing megaflood events in California. Our analysis suggests that the present-day (circa 2022) likelihood of historically rare to unprecedented 30-day precipitation accumulations has already increased substantially and that even modest additional increments of global warming will bring about even larger increases in likelihood,” they write.
“While earthquake risk always exists, the risk of mega flood increases with each passing year,” Swain said. “The whole point is so we’re not blindsided when the big one really comes.”
At the moment, California is indeed experiencing recurrent, heavy, destructive storms that are flooding parts of Northern California and San Francisco, while the rest of the state is receiving rainfall that is far heavier than usual.
While California is poised to experience escalated floods and storms, it’s unlikely that the state will be encountering something as devastating as an ARkStorm, Swain said. That being said, there are parallels between an ARkStorm and what's happening now, namely that both are not just one big storm, but a series of storms. However, the ARkstorm scenario is of greater magnitude and duration than what the West coast is currently experiencing.
“It would have to get much more intense and be of much greater duration than what we’re currently seeing,” Swain says. I’ll never say never, but that is not the current expectation.”
As for what’s really happening?
“The storm sequence is expected to continue and there could be additional powerful storms, one this weekend and another early next week in quick succession,” Swain explains. “That is when the flood risk may increase further. There is more uncertainty that far out but there is potential for much more significant and widespread flooding late this week and early next week, especially in Northern California, if the current forecast holds. That is when the bigger rivers might start to have problems.”
Swain emphasized that while this storm is unlikely to replicate an ARkstorm scenario, it still is indicative of how precipitation extremes are becoming even more extreme due to climate change.
“Events like this that are potentially significant flood events, but aren’t on the level of the worst case scenario we explore in ARkStorm, hopefully are a wake up call,” Swain said. “It’s a reminder that California is a place that can see a lot of water really quickly and that potential is rising due to a warming climate.”