Inland flooding from tropical cyclones, even at high altitudes, is a major worry—and one that scientists don’t know enough about.

Hurricane Helene hit Florida’s western coast as a Category 4 hurricane on September 26 and was accompanied by serious storm surges—but the damage didn’t end there.

Still, a Category 2 hurricane when it swept into Georgia, Helene dumped staggering amounts of rain over eastern Tennessee and western North Carolina, far inland and at much higher elevations in the Appalachian Mountains than people often consider to be at risk from hurricanes. All told, Helene is known to have killed more than 100 people, predominantly in North and South Carolina and Georgia—and that count will likely rise. Because the communities most affected are difficult to reach, simply understanding the storm’s total damage is likely to take weeks, says Janey Camp, a civil engineer at the University of Memphis.

“These are historic flooding levels in an area where the terrain is not conducive to being able to withstand those levels of precipitation,” Camp adds. “Unfortunately, it’s a perfect storm for one of the worst-case situations you could have.”

To be clear, Helene would have been devastating no matter where it hit, given that it dropped a truly enormous amount of rain—more than 18 inches across swathes of western North Carolina, with three-day totals that were well above 20 inches at multiple stations. For context, a three-day-long precipitation event in Asheville, N.C., the largest city in the most-affected region, is considered to be a once-in-1,000-year occurrence if it produces 8.4 inches of rain. (A once-in-1,000-year flood is one that has a 0.1 chance of happening in any given year.) The longest period that the National Oceanic and Atmospheric Administration calculates that out to is 60 days, for which a rainfall event in Asheville is considered to be a once-in-1,000-year occurrence if it produces 19.3 inches.

The only place that can endure that sort of rainfall without serious consequences is the ocean, Camp says.

The rain in the days prior to Helene’s arrival also contributed to the extent of flooding. “There was a tremendous amount of rain before the tropical cyclone got very close to North Carolina,” says James Smith, a hydrologist at Princeton University. And when the ground is already saturated, any further rainwater will flow off right away.

The most devastated areas are also predominantly rural and lower-income, Camp notes, increasing their vulnerability. “These are not areas that get a lot of attention and investment for resilience and planning and improved infrastructure,” she says. It’s likely that some local infrastructure wasn’t designed to be resilient even under once-in-100-year or once-in-500-year circumstances, much less the type of flooding Helene produced. Because of the sheer magnitude of the rains, “those design guidelines and standards kind of got thrown out the window; they wouldn’t have really helped,” Camp says. The scale of the events we’re now seeing suggests that even stronger building codes are necessary to protect people and infrastructure.

Then there’s the terrain. In terms of response, mountains mean there are fewer roads to any given town, hampering both evacuation and response efforts, Camp says.

Water will always flow downhill, no matter what, but mountainous terrain constrains where it can go. That means water cascading down slopes will more quickly accumulate in lower-elevation areas, worsening effects—and it will pick up speed as it travels, potentially making the flood even more dangerous.

Although tropical storm systems don’t often reach inland mountains, they can be particularly vicious when they do because of these sorts of factors. “This is a common way of producing catastrophic flooding,” Smith says. “There’ve been a number of these from the southern Appalachians all the way up into New England.” In particular, he points to 1916, when Asheville itself saw horrific flooding after consecutive tropical storms arrived in June and July. Helene was able to reach this area and dump so much rain in part because it was so strong at landfall, extremely large, and moving quickly, which meant it kept more of its energy farther inland than storms often do.

Despite the known risk of these storms reaching Appalachia, scientists don’t know a whole lot about how they behave once they get to the mountains. For example, high-elevation terrain often forces storm systems to drop more rain on the mountains’ windward side but scientists aren’t sure whether that phenomenon might play a role in cases like Helene’s Appalachian deluge. “The way tropical cyclones behave over land has received only a fraction of the attention that tropical cyclones over open ocean have received,” Smith says.

And of course, as climate change unfolds, it could make this type of situation worse—perhaps not directly but certainly in terms of how often the groundwork is set. Atmospheric and sea-surface temperatures are rising, feeding more extreme rainfall and a higher proportion of more intense tropical storms. “Those are all bad things for inland rainfall,” Smith says. “In general, you don’t want a major hurricane making landfall and then moving inland.”

In the case of Helene, emergency response personnel are still evaluating the damage done, but what we know so far bodes ill. The North Carolina Department of Transportation has said that all roads in the west of the state are effectively closed, with nonemergency travel prohibited and evacuations from Asheville funneled through two eastbound highways. About 1.5 million people remain without power across the Carolinas and Georgia. Such lack of power can in turn take out communication and water supply infrastructure, among other consequences.

The effects will also be long-lasting, she says. Recovery from such a disaster can be difficult to measure: When does life truly go back to normal? But given the scale and challenges at play here, Camp says, “it may take years and maybe as much as a decade.”

Editor’s Note (10/1/24): This story has been edited after posting to reflect updated comments from Janey Camp.

https://www.scientificamerican.com/article/why-appalachia-flooded-so-severely-from-helenes-remnants/