Why Hurricane Helene was so devastating

A perfect storm of climate, geologic and geographic conditions have combined to make Hurricane Helene one of the most devastating storms to ever hit the United States. Days after it slammed into Florida’s Big Bend region on September 26 and traveled hundreds of kilometers inland, Helene’s destructive impact has continued to grow.

Fueled by warm water in the Gulf of Mexico, which climate change made hundreds of times more likely, the tempest rapidly intensified offshore. By the time Helene came onshore as a Category 4 storm, its wind speeds surpassed 209 kilometers per hour (130 miles per hour) (SN: 9/27/24).

The powerful gusts pushed the sea onto the shore, generating record-breaking floods that inundated coastal communities in meters of seawater. Near Keaton Beach, Fla., the storm surge was estimated to have reached at least 4.5 meters (15 feet) high.

Preliminary post-landfall modeling of storm surge from Hurricane #Helene indicates areas within the Big Bend region of Florida near Keaton Beach, Steinhatchee, and Horseshoe Beach had water levels reach more than 15 ft above ground level.

— NHC Storm Surge (@NHC_Surge) September 27, 2024

And that was just the beginning. After making landfall, Helene barreled north through Georgia, delivering to Atlanta a record-breaking 28 centimeters (11 inches) of rain in 48 hours, besting the previous record of 24 centimeters (9.6 inches) set in 1886. As Helene moved into the Appalachian Mountains, its rainfall triggered widespread flooding and fast-moving landslides called debris flows, deadly and unstoppable slurries of water, soil and rock that can surge downhill for kilometers.

The mountainous western parts of North Carolina were hit especially hard, with some locations like Jeter Mountain and Busick reporting more than 76 centimeters (30 inches) of rainfall. Washed-out roads and downed power lines caused outages that isolated the city of Asheville, home to nearly 100,000 residents.

As of October 1, the death toll from Hurricane Helene has surpassed 130 people across six states, and that figure could rise over the coming days as hundreds are still reported missing. What’s more, the associated economic damages are estimated to be somewhere around $150 billion.

To find out how Helene was able to leave behind such a devastating trail of damage far into the mountains, Science News spoke with four experts. Charles Konrad is a climatologist at the University of North Carolina at Chapel Hill. Coastal oceanographer Rick Luettich and aquatic ecologist Hans Paerl are both with UNC, based in Morehead City. And geologist Brad Johnson of Davidson College in North Carolina studies landslides, erosion and the evolution of the Southeastern United States’ landscape. Their answers have been edited for clarity and brevity.

SN: Why was this hurricane’s storm surge so destructive?

Luettich: The thing about Helene was it was really large, and that means it can push an awful lot of water along with it. [Tropical storm force winds reached over 480 kilometers (300 miles) from its center.] Our models predicted that virtually all of the barrier islands, from Estero Island south of Fort Myers all the way up around Tampa Bay, would go underwater. To the best of our current understanding, that was fairly accurate. The second thing was that as Helene moved over the Gulf [of Mexico], and particularly as it started to make landfall, it was over very warm water. That helped it rapidly develop a very strong core.

The west Florida shelf is also fairly wide and shallow, and that makes it amenable to storm surge. Deep water is hard to pick up. And of course, the Big Bend of Florida is C-shaped, and as you push water up into that area, water tends to accumulate in the hook.

SN: Are there any lingering effects or risks in coastal areas from this storm?

Luettich: Our barrier islands, which are typically made of sand dunes, are a primary defense against flooding. When a storm like Helene comes along and damages or overwashes them, then a later, lesser storm event can flood areas that would otherwise be protected.

There’s no question that Helene has made the west Florida coast more susceptible to flooding from lesser events, should they occur over the next month. There’s a storm of some sort brewing in the Gulf right now. We’re not very certain of what it’s going to look like. But something is likely to occur there.

Paerl: All that rain that has fallen, it becomes runoff and it carries all sorts of contaminants. You can just imagine a gas station being flooded and all the contaminants coming out of that. Or a wastewater treatment plant. There are pesticides, herbicides, PFAS, a whole soup of chemicals in those floodwaters.

And then there’s also the nutrients that get washed out of fertilizers on farmland. When a storm comes it can wash these nutrients into our estuarine and coastal areas and can lead to algal blooms. These blooms can sometimes produce toxins that can be harmful to fish, invertebrates, domestic pets and humans, and they can last anywhere from days to months.

SN: Why did Helene hit the Appalachian Mountains so hard?

Konrad: In the mountains, there was what meteorologists call a predecessor event, which occurred right before the hurricane moved in. I think the Asheville airport got six or seven inches of rain before Helene’s rainfall even got there.

You can think of it as a head start on the rainfall. There was already significant flooding. Soils were saturated and streams were already in minor to moderate flood stage.

To make matters worse, the winds were blowing out of the southeast and east, and that air must rise over a large and steep landform in the mountains called the Blue Ridge escarpment. When air rises into higher elevations it encounters lower pressure, causing it to expand, cool and release moisture as in the form of precipitation. As Helene began pushing air over the escarpment, it caused massive enhancement of rainfall in that area.

Johnson: It’s not surprising to get landslides and debris flows in these situations.

The established threshold for landslides in North Carolina is five inches of rain. If you look at every set of landslides that have happened, it’s basically always in an event where you get at least that much rain.

When the storm started hitting, every rain gauge I had access to in the mountains was over eight inches of rain, some were at 10 inches, and the hurricane was still 100 miles out in the Gulf. I just thought, I can’t see a way out of this that doesn’t have dozens to hundreds of landslides.

SN: Are there any lingering hazards in the mountains from this hurricane?

Johnson: The peak risk of flooding, landslides and debris flows is during the precipitation event. In my experience, once that precipitation event has ended, you’re fairly well in the clear. But there’s other hazards moving around, with people walking out in the rain with power lines down, and inevitably there’s flooding in the valley bottoms.

Konrad: Hopefully it’s going to dry out, but the soils are really wet. I’m sure there’s lots of places where the rainfall has set the stage for landslides and debris flows, so that it wouldn’t take as much rainfall to trigger now. Rock slides, too.

A lot of people in these communities aren’t going to be able to access medications or health care because of the road damage, and so I think there’s going to be a lot of what we call indirect deaths. It’s a public health disaster that is still unfolding.