Why Hurricane Helene was so devastating

An attractive storm of climate, geologic and geographic conditions have combined to make Hurricane Helene one of many very important 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 aim Helene came onshore as a Category four storm, its wind speeds surpassed 209 kilometers per hour (100 thirty 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 Sea coast, Fla., the storm surge turned into estimated to have reached at the very least 4.5 meters (15 feet) high.

And that turned into just the start. After making landfall, Helene barreled north through Georgia, turning in to Atlanta a record-breaking 28 centimeters (eleven inches) of rain in Forty eight 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 is termed 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 in particular tough, with some locations like Jeter Mountain and Busick reporting more than seventy six centimeters (30 inches) of rainfall. Washed-out roads and downed power lines caused outages that isolated town of Asheville, home to nearly 100,000 residents.

As of October 1, the death toll from Hurricane Helene has surpassed 100 thirty people across six states, and that figure may perhaps rise over the coming days as hundreds are still reported missing. What’s more, the associated economic damages are estimated to be somewhere around $100 and fifty billion.

To learn the way Helene turned into in a position to depart in the back of one of these devastating trail of harm a long way into the mountains, Science News spoke with four experts. Charles Konrad is a climatologist on 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 turned into this hurricane’s storm surge so destructive?

Luettich: The thing about Helene turned into it turned into in actuality large, and that implies it should perhaps push a sort of water together with it. [Tropical storm force winds reached over 480 kilometers (300 miles) from its center.] Our models predicted that virtually all the barrier islands, from Estero Island south of Fort Myers all the way wherein up around Tampa Bay, would go underwater. To the appropriate of our current working out, that turned into fairly accurate. The 2nd thing turned into that as Helene moved over the Gulf [of Mexico], and in particular because it began to make landfall, it turned into over very warm water. That helped it rapidly develop an exceptionally strong core.

The west Florida shelf is likewise fairly wide and shallow, and that makes it amenable to storm surge. Deep water is tough to percent out up. And naturally, the Big Bend of Florida is C-shaped, and as you push water up into that area, water tends to acquire in the hook.

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

Luettich: Our barrier islands, which are normally manufactured from sand dunes, are a first-rate 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 will otherwise be protected.

There’s absolute self belief that Helene has made the west Florida coast more in danger of flooding from lesser events, should they occur over the following month. There’s a storm of some sort brewing in the Gulf at present. We’re no longer very certain of what it’s going to appear to be. But something is likely to occur there.

Paerl: All that rain that has fallen, it becomes runoff and it carries all sorts of contaminants. That that you simply may 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.

After which there’s also the nutrients that get washed out of fertilizers on farmland. When a storm comes it should perhaps wash these nutrients into our estuarine and coastal areas and can result in algal blooms. These blooms can occasionally produce toxins that can perchance be harmful to fish, invertebrates, domestic pets and humans, and that they are in a position to last anyplace from days to months.

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

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

That that you simply may factor in it as a head start on the rainfall. There turned into 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 should rise over an important and steep landform in the mountains is termed 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 type of precipitation. As Helene began pushing air over the escarpment, it caused massive enhancement of rainfall in that area.

Johnson: It’s no longer surprising to get landslides and debris flows in these situations.

The established threshold for landslides in North Carolina is 5 inches of rain. While you take a look into every set of landslides which have came about, it’s usually always in an event where you get at the very least that so much rain.

When the storm started hitting, every rain gauge I had get entry to to in the mountains turned into over eight inches of rain, some were at 10 inches, and the hurricane turned into still 100 miles out in the Gulf. I just thought, I will’t see a method 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 height risk of flooding, landslides and debris flows is one day of 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: With any good fortune it’s going to dry out, but the soils are in actuality wet. I’m sure there’s a whole lot of places where the rainfall has set the stage for landslides and debris flows, so that it wouldn’t take as so much rainfall to trigger now. Rock slides, too.

More than some people in these communities aren’t going as a strategy to get entry to medications or health care as a consequence of road damage, and so I suspect there’s going to be a whole lot of what we call indirect deaths. It’s a public health disaster or not it could possibly be still unfolding.