Hurricane Helene鈥檚 destructive path tore across several states, causing the ocean surge on the Florida coast and cutting off power supplies in Georgia. But the heaviest rainfall, and some of the worst damage, was hundreds of miles from where the storm made landfall.
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In the area around Asheville, N.C., rain swelled streams and tributaries in the almost 1,000-square-mile watershed above the city. More than 15 inches of rain fell in the area, running off mountainous terrain that was already saturated from recent storms. The swollen French Broad River crumbled interstate highways, flooded homes with mud, and . The flooding has killed dozens of people so far.
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The catastrophic damage is a sign of what climate scientists have been warning about: as the Earth heats up, rainfall is becoming increasingly extreme and deadly. And torrential rain can occur anywhere, including far from coastlines.
The heaviest storms in the southeastern U.S. today are already since 1958, according to a recent study. As the climate keeps changing, that could .
鈥淲e鈥檝e had these shocking amounts of rain,鈥� says Bill Hunt, a professor at NC State University who works on stormwater infrastructure. 鈥淚t鈥檚 hard to imagine where you鈥檙e safe.鈥�
The infrastructure in most cities, including roads, bridges and buildings, isn鈥檛 set up to handle increasingly intense storms. That鈥檚 because engineers design it , sometimes decades old. That means even recently built infrastructure is only adequate for last century鈥檚 storms.
鈥淭he situation is just getting worse,鈥� says Chad Berginnis, executive director of Association of State Floodplain Managers. 鈥淓very decade, the average annual flood losses in the U.S. is roughly doubling. It鈥檚 unsustainable.鈥�
Still, cities could soon have new tools to make themselves safer. The National Oceanic and Atmospheric Administration (NOAA) is currently updating the rainfall records for the whole country, including projecting how much worse storms could be. North Carolina, like some other states, is also working on in-depth flood planning to help communities prepare for the risks ahead.
Hurricane Helene is not a one-off
As Hurricane Helene approached the U.S. coast, forecasters sent out alerts that it had reached a Category 4 storm. That鈥檚 the , which is based entirely on wind speed.
But that masks the hidden danger hurricanes bring: rainfall. In 2018, Hurricane Florence hit North Carolina as only a Category 1, but the slow-moving storm dropped up to 30 inches of rain, causing severe flooding. Just in mid-September this year, on Wilmington, N.C., causing flooding there.
In Asheville, the steep mountain terrain funneled the runoff into the river valley, where much of the city is built. Most cities are also largely paved over, preventing the rainfall from soaking into the ground. As a result, flooding can happen far from any water body.
鈥淚t鈥檚 not isolated to Hurricane Helene and it鈥檚 not isolated to North Carolina,鈥� Hunt says. 鈥淚t鈥檚 no longer that if you鈥檙e on the river, it鈥檚 a problem. You may be miles away and have a problem.
Most of the country is , a trend that鈥檚 only expected to continue. As humans add more heat-trapping emissions to the atmosphere, temperatures are getting hotter. Warmer air is able to hold more water vapor, meaning that storms have more potential rainfall to release.
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New tools for future storms
While it may sound wonky, human lives can depend on dusty volumes of weather data.
All of the infrastructure in a city is designed to handle water. Bridges and highways are constructed to withstand large floods. Roadways and sidewalks funnel water into storm drains, which prevent rainfall from pooling in the streets and flooding buildings.
When all of that is built, engineers need to know how much rainfall the infrastructure should be able to handle. For that, they turn to that are maintained by NOAA, known as Atlas 14.
But those rainfall records are only sporadically updated, which means they don鈥檛 reflect the increasing severity of storms. Some cities use records that are . That means billions of dollars of infrastructure spending is going toward projects that may not be able to handle climate change.
鈥淲e鈥檙e flying blind right now,鈥� Berginnis says. 鈥淲e don鈥檛 know what the appropriate standard is because we have outdated data that we鈥檙e making those assumptions on.鈥�
After a new in 2022, NOAA began updating rainfall records nationwide. , as it鈥檚 known, will also take climate change into account, helping city engineers design infrastructure that will be adequate in the decades ahead. The records are expected to be released in 2026 for the Lower 48 states, with the rest of the country in 2027.
鈥淚 think we鈥檙e going to be having a much different conversation five years from now than we are today,鈥� Berginnis says.
North Carolina is also joining a growing number of states in doing cutting-edge flood planning. The is a new initiative to use advanced computer modeling to help communities understand how different flood projects could improve their safety. The effort is now being piloted for one community.
鈥淚t is a big undertaking,鈥� says Will McDow, who is senior director for Climate Resilient Coasts and Watersheds at the Environmental Defense Fund. 鈥淚t is not happening as fast as any of us would like, but I'm really excited that this will be a chance for communities to really understand their risk in a new way and to design solutions that could meet those risks.鈥�
For communities like Asheville that face rebuilding after a disaster, having the tools to plan for future floods and storms could be the difference in saving lives in the future.
鈥淲e're never going to eliminate all the risk,鈥� McDow says. 鈥淏ut we can do better as we rebuild from these storms, as communities invest going forward in new infrastructure to make sure that we're reducing risk for the people who live there.鈥�
Daniel Wood contributed to this story.
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