By Aatish Bhatia
By the time Hurricane Helene reached western North Carolina a week ago, causing devastating floods, it was no longer a hurricane.
This hints at the challenges in alerting the public about the risks that storms can bring inland, and also to the changing understanding of what makes these storms so dangerous.
“We’re realizing more than ever how deadly inland flooding is,” said Carl Schreck, a hurricane researcher at North Carolina State University, who was in his home near Asheville, North Carolina, when Helene struck.
Before a hurricane hits, news headlines usually highlight the category of a storm and its wind speeds, he said. But after the storm hits, the headlines tend to be about the rain.
That familiar five-point category rating, known to experts as the Saffir-Simpson scale, is the main way the public understands a hurricane’s strength.
But the category rating is based only on wind speeds. It leaves out all the other ways hurricanes can do damage, including storm surges, tornadoes and rain-caused flooding.
These other dangers can be more deadly than wind: Of 455 deaths directly caused by hurricanes from 2013 to 2023, 12% were caused by wind and 11% by storm surges (where wind pushes seawater toward the shore).
But 55% — 252 deaths — were caused by flooding due to rain, according to the National Hurricane Center.
As the climate warms, Americans can expect to see wetter, stronger and slower-moving storms that may be more likely to intrude farther inland. Inland populations are also generally poorer and less likely to have flood insurance compared with their coastal counterparts.
Yet there is no simple, widely used numerical scale to communicate a hurricane’s potential to cause flooding, as there is to communicate its wind speed.
Some scientists have developed new metrics to try to capture a hurricane’s rain risk. But Schreck, who recently developed one such measure, said the hurricane center understandably does not want to confuse the public with multiple rating scales.
Experts still hope to grow public awareness beyond the category rating. “We need to rethink how we name, categorize and talk about these things because too many people are dying,” said Kathie Dello, the state climatologist of North Carolina.
Even “downgraded” to a tropical storm, Helene was still deadly: Its unusually large size meant that it took longer for the storm to pass overhead, and so it rained longer. As the mountain slopes drove the storm’s moisture upward, it condensed into even more rain that funneled down valleys, leading to record-breaking flooding.
Officials warned ahead of time about the risk of extreme rainfall and flash flooding. The National Oceanic and Atmospheric Administration put out a rare news release warning of “catastrophic, life-threatening inland flooding.” And the hurricane center supplements its category scale with a lot more information about different hurricane hazards, including rainfall.
But despite all these warnings, the human toll remained catastrophic.
A Storm By Any Other Name
About 10 days before Helene struck, a different tropical storm caused extensive flooding in southeastern North Carolina. That storm fell short of the wind speeds required of a hurricane. It didn’t receive an official name or a category rating, and was termed Potential Tropical Cyclone Eight.
“Not having a name was a problem,” said Dello, because it made it harder to communicate its risks to the public.
But the unnamed storm had severe effects. Some locations saw about 20 inches of rain. There were flash floods on Carolina Beach. Parts of Brunswick County saw extreme flooding that lasted for days. Some roads were washed out. One river had its second-highest levels on record, exceeded only by Hurricane Florence’s flood levels.
Weather experts have gotten much better over the years at forecasting major storms — hurricanes or not. But they can’t easily overcome how entrenched the hurricane category system is when it comes to the public’s sense of risk.
The National Weather Service does provide an excessive rainfall outlook, which predicts the risk of extreme rainfall in a given location with four ratings: marginal, slight, moderate and high risk.
The high risk ratings are issued on only about 16 days annually across the U.S., said Alex Lamers, who leads forecasting at the weather service’s Weather Prediction Center. But those days account for 36% of the deaths and 80% of the damages caused by flooding in the U.S., he said.
“It’s about the strongest bell that we can ring,” he said.
For Helene, the weather service issued a high risk rating around Asheville about two days before the worst flooding started.
“It was a really remarkable forecast,” Dello said of the warning’s accuracy. But she said the forecast alone isn’t sufficient. To be more actionable, it needs to be translated into how much damage might happen on the ground — the threat to lives and infrastructure.
A False Sense of Security
Reports have referred to Helene as a “1 in 1,000-year storm.” This term is essentially a statistical estimate by NOAA based on past rainfall in a location. It’s used by city planners, engineers and floodplain managers to design for future floods.
But these estimates may provide a false sense of security. They are based on the past, and as the climate changes, the past becomes a poorer predictor of the future.
As a 2021 study in Europe found, climate change can increase the intensity of the most extreme flooding while also decreasing the intensity of moderate flooding. This means that recent historical experience may downplay the risk of what is to come.
In Asheville, a storm that occurs on average once in 1,000 years is predicted to bring 11.4 inches of rain in three days. But during Helene, the airport’s weather station recorded nearly 14 inches of rain in that duration, according to the North Carolina State Climate Office.
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