What It Really Means When a 1-in-1,000-Year Rainstorm Hits

Officials called Hurricane Helene’s deadly rainfall and floods “biblical” and “generational.” But weather forecasters used another term: “once in 1,000 years.”

Helene was actually the second once-in-a-millennium storm to strike North Carolina in a matter of days. Less than two weeks before Helene made landfall, an unnamed tropical storm brought 1,000-year rains to communities on the opposite side of the state, inundating homes along the coast.

The idea of two such rainfall events occurring back-to-back might seem confusing. After all, it sounds like they should only occur every 1,000 years. But in reality, it’s all about probability. Understanding the odds — and how climate change is shifting them — is more important than ever for communities and infrastructure managers.

Researchers were able to definitively identify these two extremely rare deluges in North Carolina based on rainfall frequency estimates. Using years of precipitation measurements for a specific place, scientists extrapolate what constitutes a hundred-year storm, for example, for that location.

But it’s a mistake to think that you’d only see one of those within your lifetime in a given place. That’s because these rainfall estimates are just stating the odds: In any given year, there’s a 1% chance of a storm arriving that drops that much rainfall on your area, said Mari Tye, a civil engineer who works on resilience at the National Center for Atmospheric Research in Boulder, Colorado.

The risk also increases the longer you’re in a specific place. So if you wanted to know the likelihood that you’ll someday experience a hundred-year rainfall, Tye said, “you’d be looking at a probability of more like a 1-in-4 chance within the lifetime of your mortgage.”

Add in the effects of climate change — which are not included in current estimates — and the likelihood of catastrophic rains increase, said Daniel Swain, a climatologist at the University of California Los Angeles. A hotter atmosphere can hold additional moisture, which is driving more frequent and intense downpours.

Take Helene’s rains. While they were a 1,000-year event using statistical estimates based on the historic record, a rapid analysis in the wake of the storm found that rains as severe as Helene’s now occur about once every 70 years due to global warming.

“You’re going from an event most people would never experience in generations to something that most people would see at least once in their life,” said Swain. “That’s a huge shift.”

It creates a huge problem for communicating storm hazards to the public, as the risk modeling organization First Street Foundation has pointed out. The shift also obscures whether infrastructure is really up to the task of handling historic rains on a more frequent basis, and it makes it hard for homeowners to gauge their flood risk. Researchers at the US National Oceanic and Atmospheric Administration are working to update precipitation frequency estimates, based on a shifting climate. They’re aiming to issue a new version sometime in 2027.

At the same time, Tye said, engineers are scrambling to produce new building standards and guidance that accounts for the threat of intense rainfall events that occur far more often than once a millennium.

In North Carolina, Helene washed out interstate highways, forcing people to drive for hours to reach an alternate route. It’s critical to start building in redundancies to help shore up aging roads and bridges, Tye said, and to think about flood risk more holistically across an entire neighborhood, instead of only considering “one project at a time.”

“It’s a conversation that’s been needed for a very long time,” she said.