SC has seen 20 inches of rain before. What can that tell us about Debby's potential impacts? (2024)

Tropical Storm Debby is heading toward South Carolina laden with rain bombs that forecasters say will unleash a massive amount of water and disruption. Past storms offer insights into what we might expect.

In 2015, tropical systems converged over the Charleston area, dumping 23 inches over peninsular Charleston alone. That's roughly equivalent to 3.2 billion gallons, or what normally flows over Niagara Falls in an hour.

Floods covered a good part of the downtown peninsula, but some of the worst damage happened inland as at least 18 small dams burst under the pressure.

At the time, Gov. Nikki Haley called that 2015 event a “thousand-year storm.”

But storm after storm would soon challenge that statistical yardstick: Matthew in 2016, which fortunately dumped its worst in South Carolina's spongy Santee Delta; Irma in 2017, which cut through Florida as Debby is doing now; and then Florence in 2018, which may be our most helpful gauge.

That storm slammed into North Carolina and then sat on the Carolinas for four days, with the Atlantic serving as a conveyor belt of moisture to keep it fueled. The result was a13-trillon-gallon assault. The pummeling continued long after the rain drops ceased, as swollen inland rivers raced toward the coast, putting vast swaths of Horry County under water for weeks.

"Florence was a big hose, and that's what we're about to get with Debby," said Norman Levine, director of the Lowcountry Hazards Center at the College of Charleston.

Much depends on Debby's track through Florida and Georgia, Levine said.

If it spins out of Georgia into the warm waters of the Gulf Stream, Debby could strengthen from a tropical storm into a weak hurricane.

Something like this happened in 2004 with Hurricane Charley, which made landfall near Cape Romain.

More likely is that it parks over the region like Florence. "And, if it stays over water longer, it will put more rain over us," Levine said.

Scientists later studied how much rain fell during Florence and found that an average of 17.5 inches fell in a “core area” measuring 14,000 square miles, land equivalent to about half of South Carolina.

When it comes to concentrated rain dumps, that multi-day deluge was second only to Hurricane Harvey, which unloaded more than 50 inches on Texas in 2017.

"Florence was absolutely unprecedented," said Joshua Bregy, a Clemson University climate paleontologist who used tree rings to analyze changes in tropical storms over long periods of time.

Florence not only stood in records compiled by instruments, which reach back to about the 1850s, it was worse than storms hundreds of years before, he said.

A rapidly warming planet is supercharging these already powerful rainmakers, Bregy said. Warmer air holds more water, which means more rain when clouds let go of that moisture. With every 1 degree Celsius increase in the planet's temperature, the atmosphere can hold about 7 percent more moisture.

"Climate change is a threat-multiplier, making storms rainier, making them stronger and slowing them down," Bregy said.

Although Florence's frog-drowning rains were brutal, that storm still packed less overall than the rain bombs from Irma and Matthew, which both dumped about 18 trillion gallons, according to estimates by Ryan Maue, a private meteorologist.

But those storms dumped their rain over wider areas. Irma hit an area from Florida to North Carolina, while Matthew's reach was even greater — from Florida to New York.

Alicia Wilson, a hydrology professor at the University of South Carolina’s School of the Earth, Ocean & Environment, said persistent rain over days will likely drive visible flooding throughout the region, even in places that don’t normally swell with water during storm events. But hidden risks exist as well.

As the water table rises below the surface, water can inundate septic systems and cause them to fail or release contaminating nitrates, Wilson said. Floodwaters can also push mud and other debris into septic tanks, leaving them unable to filter waste material, she said.

With these threats coming from below, it can be a challenge for homeowners to see a problem coming, she said.

All of this is forcing scientists, insurers and policymakers to reassess what it means for a place to experience an extreme event. Data used to calculate historically relevant storms is based on what happened over the previous 50 years, said Levine of the Lowcountry Hazards Center.

But a rapidly warming planet has changed those equations over the past 20 years. "We're reassessing what it means to have a 500- or 1,000-year storm," he said. Those terms are "simply less meaningful" because changes in the climate "are moving so fast."