Extreme rainfall is becoming an increasing threat as the climate gets hotter, even for non-hurricane storms.

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Melissa Sue Gerrits/Getty Images North America

The destructive path of Hurricane Helene devastated several states, caused a sea surge on the Florida coast and disrupted power in Georgia. But the heaviest rainfall and some of the worst damage occurred hundreds of miles from where the storm hit land.

In the Asheville, N.C., area, the rain caused streams and tributaries to swell in the nearly 1,000-square-mile watershed above the city. More than 15 inches (38 centimeters) of rain fell in the area, running off mountainous terrain already drenched from recent storms. The swelling French Broad River collapsed highways, flooded homes with mud and cut off drinking water supplies. Dozens of people have died in the floods so far.

The catastrophic damage is a sign of what climate researchers have warned about: as the Earth warms, precipitation will become more extreme and deadly. And torrential rains can occur anywhere, even far from the coast.

According to a recent study, the most severe storms in the southeastern United States are already dropping 37% more rain since 1958. As the climate continues to change, that number could increase by 20% or more.

“We had shocking amounts of rain,” said 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, is not designed to cope with increasingly severe storms. That's because engineers design it based on old rainfall records, sometimes decades old. This means that even newly built infrastructure is only sufficient for the storms of the last century.

“The situation is getting worse,” said Chad Berginnis, executive director of the Association of State Floodplain Managers. “Every decade, average annual flood damage in the U.S. roughly doubles. It’s not sustainable.”

Nevertheless, cities could soon have new tools to make themselves safer. The National Oceanic and Atmospheric Administration (NOAA) is currently updating rainfall records for the entire country and is also forecasting how much worse the storms could be. North Carolina, like several other states, is also working on detailed flood planning to help communities prepare for the risks ahead.

Hurricane Helene is not an isolated incident

As Hurricane Helene approached the US coast, meteorologists sent out warnings that it had reached a Category 4 storm. This is the hurricane severity rating system based solely on wind speed.

But that obscures the hidden danger that hurricanes bring: rain. In 2018, Hurricane Florence only hit North Carolina as a Category 1, but the slow-moving storm dropped up to 30 inches of rain and caused severe flooding. Just in mid-September of this year, a storm dropped 20 inches of rain on Wilmington, North Carolina, causing flooding there.

In Asheville, the steep mountain terrain directed runoff into the river valley where much of the city is built. Most cities are also largely paved to prevent rain from seeping into the ground. This can cause flooding far from any body of water.

“It’s not limited to Hurricane Helene and it’s not limited to North Carolina,” Hunt said. “It's no longer like being on the river is a problem. You may be miles away and have a problem.

Most of the country is already experiencing heavier rainfall, a trend that is expected to continue. As humans release more heat-trapping emissions into the atmosphere, temperatures are rising. Warmer air can hold more water vapor, which means storms can produce more precipitation.

New tools for future storms

As strange as it sounds, human lives can depend on dusty amounts of weather data.

The entire infrastructure of a city is designed to handle water. Bridges and highways are built to withstand major floods. Streets and sidewalks direct water into storm drains, which prevent rain from collecting on streets and flooding buildings.

When all of this is finished, engineers will need to know how much rainfall the infrastructure can handle. To do this, they rely on historical precipitation records maintained by NOAA, known as Atlas 14.

However, these rainfall records are only updated sporadically, meaning they do not reflect the increasing severity of storms. Some cities use records that are more than 60 years old. That means billions of dollars in infrastructure spending is going toward projects that may not be able to address climate change.

“We’re flying blind right now,” says Berginnis. “We don’t know what standard is appropriate because we have outdated data on which to make these assumptions.”

After new federal law was passed in 2022, NOAA began updating precipitation records nationwide. Atlas 15, as it is called, will also take climate change into account and help city engineers design infrastructure that will be adequate in the coming decades. The records are expected to be released in 2026 for the lower 48 states and in 2027 for the rest of the country.

“I think that in five years we will be having a very different conversation than we are having today,” Berginnis says.

North Carolina also joins a growing number of states implementing state-of-the-art flood planning. The North Carolina Flood Resiliency Blueprint is a new initiative that uses advanced computer modeling to help communities understand how various flood projects could improve their safety. The experiment is currently being tested for a community.

“It’s a major undertaking,” said Will McDow, senior director of climate-resilient coasts and watersheds at the Environmental Defense Fund. “It's not happening as quickly as any of us would like, but I'm really excited that this will be an opportunity for communities to really understand their risks in a new way and develop solutions to address those risks could.”

For communities like Asheville that are rebuilding after a disaster, having tools to plan for future floods and storms could make all the difference in saving lives in the future.

“We will never eliminate all risk,” says McDow. “But we can do better in rebuilding from these storms if communities invest in new infrastructure in the future to ensure we reduce the risk to the people who live there.”

Daniel Wood contributed to this story.