In July 2022, intense thunderstorms traversed the central United States, inundating the area and leading to unprecedented flash flooding. The most significant rainfall occurred in the larger St. Louis metropolitan region on July 26, subsequently moving to eastern Kentucky on July 28. In both locations, the storm established all-time records for cumulative rainfall within a 24-hour timeframe.

A rainfall gauge at St. Louis Lambert International Airport noted 8.64 inches of precipitation on July 26, with a substantial portion occurring within a few hours. This extraordinary rainfall resulted in two fatalities and damages exceeding $1 billion.

It marked the nation’s most expensive flooding incident in 2022. However, was it truly a 1-in-1,000-year occurrence? An evaluation conducted by researchers from Washington University in St. Louis indicates it likely wasn’t.

“Everyone in St. Louis during the storm recalls it vividly, as their basements were inundated — or worse,” stated Bronwen Konecky, an assistant professor of earth, environmental, and planetary sciences in Arts & Sciences. “In the media, meteorologists were proclaiming this as a 1,000-year flood.”

However, a recent investigation from the Konecky lab indicates that this might not have been the truth, and a storm of such nature is more akin to a 1-in-500-year event.

“In both St. Louis and eastern Kentucky, we discovered that the storms which occurred that week and the quantity of rainfall that fell were indeed not as extraordinary as we had thought at the time,” Konecky remarked. “And storms of that intensity will likely become increasingly frequent in the future.”

Contextualizing extreme rainfall

Climate change is elevating the frequency of exceptional precipitation events, including those categorized as 1-in-1,000-year storms or floods — events with a 0.1% probability of occurring in any given year.

Such events are critical in the design and infrastructure planning across the nation. Nonetheless, estimating the frequency of these extreme storms is complicated by considerable uncertainty. This uncertainty hampers efforts to place a storm in its appropriate context.

Regarding the classification of an event as a 1,000-year occurrence, how much uncertainty is involved? Researchers explained that it varies, depending on the storm’s location and other variables.

“In St. Louis, the anticipated range had error margins of 7 inches” of rain within a 24-hour timeframe, stated Alexander Thompson, the principal author of the recent study, referencing the precipitation frequency estimates from the National Oceanic and Atmospheric Administration. “This means daily rainfall could vary from around 7.5 inches to 15 inches (designated as a 1,000-year event), which is an extremely broad range.”

This range of error is partly a consequence of the limited number of weather station observations, as explained by Thompson.

Modern rain gauge data extends back a century or so at most. Additionally, in certain geographical regions, rain gauges are sparse. In other locales, historical records from rain gauges may be inconsistent or exhibit significant gaps for years when no measurements were recorded.

“With the assistance of statistical methods, it’s possible to classify something as a 1,000-year event without having 1,000 years of data,” Thompson noted. “However, a primary reason for the expansive uncertainty range is that we are basing that classification on only a few decades of observational data.”

Exploring climate models of Earth’s history

Thompson and Konecky recognized the necessity for a more effective approach to analyze the data.

As paleoclimatologists, they were familiar with utilizing alternative records of historical climate conditions that predate human measurement. These records, often referred to as climate proxies, encompass climate information gleaned from analyses of tree rings, corals, glaciers, and sediments from lakes and oceans. Scientists can use these environmental archives to reconstruct past conditions, extending our comprehension of climate back hundreds to millions of years. Climate proxies, in turn, enhance the validation of climate models which forecast both historical and anticipated climate changes.

For this investigation, Thompson and Konecky devised a novel method to integrate actual historical observations — the genuine rain gauge data from numerous local weather stations, compiled in NOAA’s Applied Climate Information System — with the Last Millennium Ensemble, a series of simulations executed using the well-established Community Earth System Model starting from the year 850 and extending through to 2100.

“The late 20th and early 21st centuries represent a unique period wherein weather station data coincides with climate model simulations,” Konecky stated. “We leveraged that overlap to create a blended data set that represents the accuracy of the station data while incorporating the long-term statistics of the climate model simulations.”

Subsequently, Thompson and Konecky employed this integrated dataset to investigate the record-setting rainfall event that occurred in July 2022.

Examining the occurrence through their updated perspective, the WashU investigators discovered that the recurrence interval for the storm’s 24-hour rainfall was approximately 530 years (90% confidence interval: 370-700 years) for the wider St. Louis area and roughly 280 years (90% confidence interval: 115-340 years) for eastern Kentucky. This interval is considerably shorter than the 1,000-year return timeframe that was disseminated in the media at that moment.

“Our methodology yields a more accurate assessment of this event’s occurrence probability and indicates that a rainfall volume akin to that of the July 2022 incident is two to four times more probable to take place in the future, compared to the previous millennium,” stated Thompson, currently a research scientist at the University of Colorado’s Cooperative Institute for Research in Environmental Sciences, who collaborates closely with NOAA on evaluations of extreme precipitation.

Bracing for additional significant storms

Beyond the tragic loss of life, surges in extreme precipitation exert an economic burden through escalating expenses related to flood destruction. Adaptation strategies for essential infrastructure, such as stormwater systems, depend on evaluations of precipitation and flood occurrence frequency. These strategies are projected to incur costs amounting to hundreds of millions of dollars annually by this century’s conclusion.

Considering the devastating potential of extreme rainfall events, it is essential to accurately evaluate their historical precedence to gain a better understanding of how frequently these storms may occur in the future, asserted the researchers.

The outcomes of this investigation can be utilized to enhance societal and infrastructural preparedness for the current and forthcoming threats posed by extreme precipitation. Other laboratories are actively working on developing improved models for forecasting when and where extreme rainfall events are likely to materialize. “There’s a surge of research focused on new and more effective methodologies for these types of analyses,” Thompson remarked.

“Extreme storms and flooding are not disappearing,” Konecky emphasized. “St. Louis must be equipped.”

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