No one knows when it’ll happen—it could be five months or five decades. No one can say exactly how bad it will be, although most predictions put it somewhere between a Roland Emmerich disaster movie and a diluvial apocalypse. At this point, only one thing is for sure.
It’s going to get wet. Very, very wet.
It’s called the ARkStorm scenario, a catastrophic weather event that, according to a terrifying, just-released report compiled by scientists at UCLA, could dwarf California’s droughts, fires, and even earthquakes in overall destruction. Triggered in part by global warming, the storm will theoretically begin brewing off the coastline, where it will form a series of “atmospheric rivers,” massive squalls stretching hundreds of miles, that go on for weeks. It’ll dump 100 inches of rain, an unprecedented deluge, triggering mudslides and washing debris from mountains across the region. Electrical grids will be darkened. Cell towers will be toppled. Roads and highways will be impassable. Millions will be displaced. Entire areas—including large swaths of Los Angeles—will be under water.
“This would be worse than anything Southern California has experienced since it became a densely populated urban area,” notes the lead author of the UCLA report, Daniel Swain, who predicts that the ARkStorm could be four times as catastrophic as “The Big One,” the giant, long-anticipated earthquake that will someday likely erupt along the San Andreas Fault. “There’s really no historical analogy for what we’re talking about here.”
Scared yet? We talked to a bunch of geomorphologists, seismologists, historians, and flooding experts for a glimpse of what to expect. You may want to crawl under your blankets for the rest of this.
“This would be worse than anything California has experienced.”
Let’s start with the good news: unlike the case with earthquakes, residents of L.A. will probably get ample notice—perhaps a week or even more—from when the ARkStorm forms off the coast until it ultimately makes landfall. The bad news: Just leaving town won’t be enough. The sheer size and duration of the storm means it’ll stretch across pretty much the whole coastline and farther inland.
Upon reaching the L.A. Basin, the storm system will slam into the San Gabriel and San Bernardino mountains. The steepness and elevation of those ranges will cause the air in the system to rise quickly, cool, and then reduce pressure, triggering a downpour unlike anything the region has experienced in modern history. Because of Southern California’s semiarid climate and shallow soil, it won’t take long for the ground to become saturated, resulting in unparalleled amounts of runoff.
“L.A. is sitting at the base of a mountain range, which is the perfect flood-generating machine,” explains Jeffrey Mount, senior fellow at the Public Policy Institute of California’s Water Policy Center, who lays out a frightening sequence of events.
Neighborhoods ranging from La Cañada Flintridge to Rancho Cucamonga, nestled in the alluvial fans around the base of the San Gabriel range, will be in the most precarious positions. They’ll be the first point of contact for the runoff.
“Look at [the 2018 mudflow in] Montecito,” says Mount. “The ability [of a storm] to move debris out of steep mountains, especially if they recently burned, is extraordinary.”
All eyes will then turn to the L.A. River. Most experts agree that it will be only a matter of time before the channel is overwhelmed. “It’s not just a backdrop for movies,” says Swain. “The whole idea of [the L.A. River] is flood control, and it has worked really well up to a point. But would it be able to handle this? That’s an open question.” With perhaps too obvious an answer. The L.A. River wasn’t engineered to deal with debris. A single shopping cart, let alone the rubble and detritus of a biblical-level storm, could create a jam almost anywhere along its nearly 50-mile course running from the San Fernando Valley to Long Beach.
If the waters do breach the river’s channel walls, it could lay waste to the surrounding areas, flooding thousands of homes and wreaking havoc on everything from petrochemical plants to movie studios.
Mount predicts that the first wave of evacuations will occur about a week into the ARkStorm, starting in low-lying areas like Long Beach, Compton, Venice, and Marina del Rey, which will rapidly turn into lakes. After week two, the city’s defenses, Mount believes, will be completely overwhelmed. “Bad assumptions that were made in the 1930s are going to come back to bite us 100 years later,” he says.
But wait, it gets even worse. Major highways that connect Los Angeles to the north and east could be shut down for several weeks due to landslides, flooding, and debris flows. Erosion leading to cliff failure could close Pacific Coast Highway for a month or more, isolating residents of Malibu, Oxnard, and Ventura. To avoid being stranded—or worse, seeing their homes slide away—residents of the Hollywood Hills better hope their engineers did the right math. Emergency vehicles would be rendered obsolete, so fleets of boats and helicopters would be mobilized to aid those stranded in the most severely impacted communities, assuming boats and helicopters could operate in such weather conditions.
As the storm wears on, clean and drinkable water would become scarce as wastewater treatment plants, which, deprived of the chemicals they need to operate, would, one by one, start to shut down. At wastewater treatment facilities that are flooded, nearby residents could start to see and smell raw sewage emerge from maintenance holes. Flooded manholes, toppled poles, and damaged fiber-optic cables would result in widespread power outages affecting much, if not all, of the county, knocking out telecommunications and internet access for millions.
The supply chain would soon be undermined as well, which would have national implications as hauling goods out of the Port of Los Angeles in San Pedro—the busiest seaport in the Western Hemisphere—would be stalled for who knows how long.
Of course, keep in mind that all of the above are just predictions. There is a chance, however slim, that none of the above will ever materialize. The UCLA report estimates the odds of the ARkStorm happening at about the same as the Big One striking (which, alarmingly, the U.S. Geological Society puts at 70 percent before the year 2030, although the odds are only about 1 percent that the Big One will occur in the next year).
Still, it pays to be prepared. “It’s hard to imagine it because, the more we are afraid of something, the more powerless we feel about stopping it,” offers Dr. Lucy Jones, a seismologist who has been modeling theoretical ARkStorm events since 2010. “But you need to think about it. You have to give people a clear picture that there is something you can do.”
Hopefully, what that something might be—beyond collecting animals two by two—will be the subject of the next UCLA report.
How Bad Can It Get?
Here’s how an ARkstorm weather event could turn l.a. into the biggest, least-fun water park ever.
Moonshadows is Toast (Or More Like Mud)
After the deadly 2018 Montecito mudslide, the 101 freeway was shut down for 12 days. A similar scenario, only worse, will likely play out in Malibu during an ARkStorm as Pacific Coast Highway is prone to cliff failure and mudslides.
Those Row Boats on the Canals Will Come in Handy
Ballona Creek and the Venice Canals are the kind of sea-level basins that are seriously problematic. Not only will they be receiving enormous amounts of runoff but also winds could drive coast surge, quickly turning them into lakes.
A River Runs Through It . . . and All Over It
The Los Angeles River cuts right through Glendale, which puts that community and neighboring areas like Atwater Village and Silver Lake at high risk should the river overflow. Which it almost certainly will, thanks to tons of rain-swept debris clogging the channels.
La Cañada Flintridge
A Mountain Landing in Your Living Room
Any of the communities nestled in the foothills of the San Gabriel Mountains could be severely impacted by flooding and debris flows due to their proximity to alluvial fans. Debris basins are in place to mitigate that but would likely be overwhelmed.
The Key to the City (Disaster Version)
 FOOD SHORTAGES
With highways impassable and the Port of Los Angeles shut down, food supply lines could be interrupted or completely cut off.
Wherever people build multimillion-dollar homes on top of hills—whether in Malibu or Hollywood—there will be mudslides.
 FULLY SUBMERGED
Neighborhoods at or below sea level—like Venice, Marina del Rey, and Long Beach—will likely become lakes.
Large sections of the 405, the 10, and the 5 freeways and other major and minor roadways will be undrivable, thanks to flooding and debris.
 DOWNED CELL TOWERS
Flooding will likely knock down towers all over town, leaving millions without cell or internet service.
 POWER OUTAGES
Electrical plants will be rendered inoperable due to flooding; power lines will also be snapped as poles are toppled by raging waters carrying tons of debris.
This story is featured in the October 2022 issue of Los Angeles