Why was Hurricane Helene so devastating?

Powerful eruptions pushed the sea ashore, generating record floods that inundated coastal communities in meters of seawater. Near Keaton Beach, Fla., the storm was estimated to have reached at least 4.5 meters (15 feet) in height.

Preliminary Post-Landfall Modeling of Hurricane Storm Surge #Helen shows areas within Florida’s Big Bend region near Keaton Beach, Steinhatchee and Horseshoe Beach where water levels reached 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 moved north through Georgia, giving Atlanta a record 28 centimeters (11 in) of rain in 48 hours, beating the previous record of 24 centimeters (9.6 in) set in 1886. While Helene moved into the Appalachian Mountains, its rainfall causing widespread flooding and rapid landslides called debris flows, deadly and unstoppable slush of water, soil and rock that can creep downhill for miles.

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

Since Oct. 1, the death toll from Hurricane Helene has surpassed 130 people in six states, and that number could rise over the coming days as hundreds are still reported missing. Additionally, the associated economic damages are estimated to be somewhere around $150 billion.

To find out how Helena was able to leave such a devastating trail of damage far in the mountains, Scientific news spoke to 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 landscape evolution in the southeastern United States. Their responses have been edited for clarity and brevity.

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

Luettich: The thing about Helene was that it was really big, and that meant it could push a lot of water along with it. [Tropical storm force winds reached over 480 kilometers (300 miles) from its center.] Our models predicted that almost all of the barrier islands, from Estero Island south of Fort Myers to Tampa Bay, would go under water. In our present understanding, this was quite accurate. The second thing was that as Helena moved over the Gulf [of Mexico]and especially when it began to fall to land, it was on very warm water. This helped him quickly develop a very strong core.

The West Florida shelf is also quite wide and shallow, making it suitable for hurricanes. Deep water is hard to catch. And of course, Florida’s Big Bend is C-shaped, and as you push water up into that area, water tends to pool in the hook.

SN: Are there ongoing effects or risks to coastal areas from this storm?

Luettich: Our barrier islands, which usually consist of sand dunes, are the main defense against flooding. When a storm like Helene comes through and damages or covers them, then a later, smaller storm can flood areas that would otherwise be protected.

There is no doubt that Helene has made the west coast of Florida more susceptible to flooding from smaller events, should they occur over the next month. There’s some kind of storm brewing in the Gulf right now. We’re not quite sure what it will look like. But something is likely to happen there.

Paerl: All that rain that has fallen becomes runoff and carries all kinds of pollutants. You can just imagine a gas station flooding and all the pollutants that come out of it. Or a sewage treatment plant. There are pesticides, herbicides, PFAS, a whole soup of chemicals in those waters.

And then there are the nutrients that leach from manure into farmland. When a storm comes, it can spread these nutrients into our estuary and coastal areas and can lead to algal blooms. These blooms can sometimes produce toxins that can be harmful to fish, invertebrates, pets, and humans, and they can last anywhere from days to months.

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

Konrad: In the mountains, there was what meteorologists call an antecedent event, which happened just before the hurricane came in. I think the Asheville airport got six or seven inches of rain before Helena’s rainfall got there.

You can think of it as the beginning of rain. There was already considerable flooding. The soils were saturated and the streams were already in minor to moderate flood stage.

To make matters worse, the winds were blowing from the southeast and east, and that air must rise over a large, steep landform in the mountains called the Blue Ridge Escarpment. When air rises to higher altitudes, it encounters lower pressure, causing it to expand, cool, and release moisture as precipitation. As Helena began to push air over the escarpment, it caused massive increases in rainfall in the area.

Johnson: It is not surprising that there are landslides and debris in these situations.

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

When the storm began to hit, every rain gauge I had access to in the mountains was over eight inches of rain, some were 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 and hundreds of landslides.

SN: Is there any ongoing danger in the mountains from this hurricane?

Johnson: The maximum risk of flooding, landslides and debris flows is during the rainfall event. In my experience, once that precipitation event is over, you’re pretty much in the clear. But there are other dangers moving around, with people going out in the rain with downed power lines, and inevitably there is flooding at the bottom of the valley.

Konrad: Hopefully it will dry out, but the grounds are really wet. I’m sure there are many places where rainfall has set the stage for landslides and debris flows, so it wouldn’t take that much rainfall to cause one now. Rock slide too.

Many people in these communities will not be able to access medication or health care because of road damage, and so I think there will be a lot of what we call indirect deaths. It is a public health disaster that is still unfolding.