DISASTERSWhy Are So Many Historically Rare Storms Hitting the Carolinas? Geography Puts These States at Risk, and Climate Change Is Loading the Dice
Why have so many storms that, historically and statistically, should be exceedingly rare, struck the Carolinas in just a few years? In regions near the coasts, the frequency of heavy rainfall has increased as a result of human-caused climate change. Warmer air can hold more moisture, and warmer oceans provide that moisture as the fuel for heavy rainfall.
Hurricane Helene caused deadly and destructive flooding when it swept through the Southeast on Sept. 26-29, 2024. Across a broad swath of western North Carolina, where the worst flooding occurred, the amount of rainfall exceeded levels that would be expected on average only once every 1,000 years.
But this wasn’t the first 1,000-year rainstorm in North Carolina this year. In mid-September, an unnamed slow-moving storm produced more than a foot of rainfall closer to the Atlantic coast. This storm inundated areas that had already been drenched by Tropical Storm Debby in August.
As atmospheric scientists and state climatologists, we believe it’s important for the public to understand the risk that extreme events may occur. That’s especially true as climate change alters the conditions that create and feed storms. Here’s how scientists calculate storm probabilities, and why events like a 1,000-year storm can happen much more frequently in some places than that term suggests.
Forecasting the Future Based on the Past
Estimates of rainfall return periods – how long it will be, on average, between storms of a given size – come from the U.S. National Oceanic and Atmospheric Administration, the home of the National Weather Service. NOAA publishes these projections in a series of reports called Atlas 14. Architects and engineers use them to design buildings, dams, bridges and other facilities to withstand heavy rainfall.
The estimates use past rainfall data to calculate how frequently rainstorms of various sizes occur at given locations. In places where historical rainfall observations have been collected for decades, it’s possible to calculate the amount of rain that is exceeded, on average, one or two times per year with very high confidence.
Experts then use statistical methods to estimate how frequently larger rain amounts would occur. As the amounts get bigger, the calculations become less precise. But it’s still possible to make reasonable estimates of very rare rain events.