Hurricane Helene’s 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, North Carolina, 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 cut off the drinking water supply. 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 dropping 37 percent more rain since 1958, according to a recent study. As the climate keeps changing, that could increase by 20 percent or more.
“We’ve had these shocking amounts of rain,” says Bill Hunt, a professor at NC State University who works on stormwater infrastructure. “It’s hard to imagine where you’re safe.”
The infrastructure in most cities, including roads, bridges and buildings, isn’t set up to handle increasingly intense storms. That’s because engineers design it using old rainfall records, sometimes decades old. That means even recently built infrastructure is only adequate for last century’s storms.
“The situation is just getting worse,” says Chad Berginnis, executive director of Association of State Floodplain Managers. “Every decade, the average annual flood losses in the U.S. is roughly doubling. It’s unsustainable.”
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.
<b>Hurricane Helene is not a one-off</b>
As Hurricane Helene approached the U.S. coast, forecasters sent out alerts that it had reached a Category 4 storm. That’s the rating system for a hurricane’s severity, 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, a storm dropped 20 inches of rain on Wilmington, North Carolina, 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.
“It’s not isolated to Hurricane Helene and it’s not isolated to North Carolina,” Hunt says. “It’s no longer that if you’re on the river, it’s a problem. You may be miles away and have a problem.
Most of the country is already experiencing heavier rainstorms, a trend that’s 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 historical rainfall records that are maintained by NOAA, known as Atlas 14.
But those rainfall records are only sporadically updated, which means they don’t reflect the increasing severity of storms. Some cities use records that are more than 60 years old. That means billions of dollars of infrastructure spending is going toward projects that may not be able to handle climate change.
“We’re flying blind right now,” Berginnis says. “We don’t know what the appropriate standard is because we have outdated data that we’re making those assumptions on.”
After a new federal law was passed in 2022, NOAA began updating rainfall records nationwide. Atlas 15, as it’s known, will also take climate change into account, helping city engineers design instructure 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.
“I think we’re going to be having a much different conversation five years from now than we are today,” Berginnis says.
North Carolina is also joining a growing number of states in doing cutting-edge flood planning. The North Carolina Flood Resiliency Blueprint 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.
“It is a big undertaking,” says Will McDow is senior director for Climate Resilient Coasts and Watersheds at the Environmental Defense Fund. “It 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.”
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.
“We're never going to eliminate all the risk,” McDow says. “But 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.”
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