Once in a thousand years? Extreme storms are becoming much more frequent

In July 2022, powerful thunderstorms pummeled the central United States, bringing extreme flooding to the St. Louis metropolitan area and then dumping vast amounts of water on eastern Kentucky.

St. Louis set a new benchmark for daily rainfall, with a gauge at Lambert International Airport recording 8.64 inches of precipitation. 

Though media reports labeled it a 1-in-1,000-year event, a new study from Washington University in St. Louis suggests otherwise.

Researchers led by assistant professor Bronwen Konecky analyzed the storm’s intensity using a fresh approach that blends real-world measurements with climate model data.

The team found that this extreme rainfall was more likely a 1-in-500-year storm – still significant, but not as rare as early estimates suggested.

A record-breaking deluge

The downpour in St. Louis, which occurred on July 26, caused extreme floods that claimed two lives and led to more than $1 billion in damages. It was the most expensive flood disaster in the nation that year.

Eastern Kentucky saw a similarly devastating impact only two days later. Preliminary coverage described the rainfall as having a 0.1% chance of happening in any given year, hence the “1,000-year” label.

But the Washington University researchers discovered that official frequency estimates carry huge error bars – potentially up to 7 inches of rainfall in some cases – which complicates claims that the storm was a once-in-a-millennium event.

Instead, they say a storm of that magnitude is more likely on the order of a 1-in-500-year scenario for St. Louis and closer to a 1-in-300-year event for eastern Kentucky.

Why estimates are so uncertain

Alexander Thompson, first author on the study and a recent PhD graduate, explains that short observation records are partly to blame.

Rain gauges have only been recording data in some U.S. locations for about a century, and the coverage of these stations varies widely. Some areas have robust data, while others are spotty or have large historical gaps.

With such limited sets of observations, scientists must rely heavily on statistical methods to guess the likelihood of big rainfall events. But if the data only covers a few decades, it will show a broad range of possible return periods for any major storm.

The term “return period” is how scientists express the probable timing of an event. A 1-in-500-year storm has a 0.2% chance of occurring in any given year, whereas a 1-in-1,000-year storm has a 0.1% chance.

As global warming intensifies, storms with these high return periods could become more frequent in many regions.

A fresh blend of data and models

Because standard measurements produce such wide error ranges, Thompson and Konecky – both paleoclimatologists by training – turned to an unusual strategy.

The experts combined traditional rain gauge data from the late 20th and early 21st centuries with outputs from a large set of climate model simulations known as the Last Millennium Ensemble.

These simulations span back to the year 850 and run forward to 2100, covering both past and projected future climates.

“The late 20th and early 21st century is a unique period of time when we have weather station data that overlap with climate model simulations,” Konecky said.

“We used that period of overlap to figure out how to come up with a blended data set that reflects the reality of the station data, but with the long-term statistics of the climate model simulations.”

This blended dataset allowed the researchers to recast the July 2022 storm in historical context.

Future risk of extreme storms

The results show the 24-hour rainfall that drenched St. Louis has a likely return period of around 530 years, while the eastern Kentucky event clocks in at about 280 years.

In real terms, that means these extreme storms, though unusual, are more probable than the initial 1-in-1,000-year assessments indicated.

Moreover, the data suggest that such events will become even more common in decades ahead as global warming accelerates.

For communities like St. Louis, where basement flooding turned into a major crisis last year, this conclusion carries an urgent message. “Extreme storms and flooding aren’t going away,” Konecky said. “St. Louis needs to be ready.” 

The same holds for other cities facing similar precipitation extremes. Planning decisions on stormwater systems, road design, and new construction all hinge on understanding how often severe rainstorms could happen.

A new era of extreme storms

With more intense storms looming, adaptation becomes critical. The researchers noted that economic losses from extreme floods continue to soar, and preparing now can save both money and lives.

“There’s an explosion of research into new and better ways to analyze these kinds of events,” Thompson said, adding that emerging methods – like the one used in this study – will help refine how we talk about storm frequency.

In the meantime, the takeaway is that an extreme storm like the one that slammed St. Louis is already too frequent for comfort – and only likely to get more so.

As climate change reshapes historical weather patterns, societies must lean on better data, climate models, and proactive infrastructure planning to help keep damage in check.

The study is published in the Journal of Climate.

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