How Hurricane Helene Became a Monster Storm

The southeastern United States is reeling from Hurricane Helene, a monstrous storm that made landfall in Florida on Thursday and then cut a terrible path into Tennessee. How could it get so bad?

The storm killed more than 100 people and hundreds more are still missing. The power is out for millions of people. Residents around Asheville, North Carolina — one of the hardest-hit areas — are reportedly struggling to find food, water and cell service. We don’t yet know what the full impact of the storm will be. Search and rescue missions are still underway and scientists are finalizing data on the storm’s strength.

But it is clear that the storm was catastrophic due to its unusual size, intensity and speed. The conditions were perfect for the storm to intensify.

“It had all the different weapons that a hurricane can have,” says John Knox, distinguished teaching professor and undergraduate coordinator of the Atmospheric Sciences Program at the University of Georgia. “Everything we say a hurricane can do, Helene did.”

While Helene was still raging in the Gulf of Mexico, meteorologists were already warning that the storm would be “unusually large.” At their maximum, tropical storm winds extended nearly 350 miles from the center of Helene. According to the National Hurricane Center, this enormous range placed Helene in the 90th percentile of storm strength. On the ground, the effects of the storm – wind, storm surge and heavy rain – were felt over an unusually large area.

The storm was not only huge, but also stronger than most. Storm systems of this size do not always develop a small inner core that allows them to intensify quickly. But Helene was able to form a relatively small eye and then quickly intensify, a term that describes tropical storms with sustained wind speeds that increase by at least 30 knots (about 35 miles per hour) in 24 hours.

It made landfall with winds of 140 miles per hour, making it a severe storm and a category 4 out of 5 on the Saffir-Simpson hurricane scale.

Helene also packed a punch full of water. When it hit the Big Bend region of Florida, it unleashed a massive storm surge that inundated the coast with up to 15 feet of seawater. The underwater topography off the west coast of Florida acted like a ramp with a lower gradient, making it easier for the storm to bring a higher wall of water. The sheer size of the hurricane also caused the storm surge to inundate a larger area.

Heavy rains dropped more water on communities, causing historic flooding in western North Carolina. Between September 25th and 27th, almost 14 inches of rain was recorded at Asheville Airport over three days. The highest preliminary total was more than 31 inches of rain, measured in Busick, North Carolina.

“It was certainly a very catastrophic event in parts of the southeastern United States, particularly in the southern Appalachians, where there were tremendous amounts of rain and flooding,” said Daniel Brown, branch chief of the hurricane specialist unit at the National Hurricane Center. But with reports of damage and deaths still coming in, it’s probably too early to know how Helene compares to other storms, he says.

In addition to its magnitude, the storm was fast, reaching a forward speed of 20 to 30 miles per hour. By comparison, storms that make landfall along the Gulf of Mexico typically only move forward at about 10 to 15 miles per hour, Brown said. Tropical storms tend to weaken once they move over land because they derive their strength from the thermal energy of warm water at the ocean’s surface. However, Helene’s speed allowed her to retain more of her strength as she moved inland.

“That’s why the impacts were felt much further inland than people are normally used to,” says Karthik Balaguru, a climate scientist at the Pacific Northwest National Laboratory. “The further inland you go, the more people would be at risk.” Another risk factor is that inland communities may not have as much experience preparing for hurricanes as coastal areas, which are more accustomed to dealing with disasters of this type are.

Climate change is changing the calculation for storms like Helene. Rising global temperatures are creating favorable conditions More intense storms that can quickly gain strength and remain stronger on land. Helene formed amid rising sea surface temperatures in the Caribbean Sea and Gulf of Mexico. Waters in the storm’s early path reached as high as 31 degrees Celsius (87.8 degrees Fahrenheit), providing plenty of fuel. The atmosphere’s ability to retain moisture is increasing due to greenhouse gas emissions from fossil fuels, leading to increased rainfall.

In order to find out how big a role climate change played in Helene specifically, scientists need to carry out further research. But Balaguru likens the effects of climate change to the world having a weakened immune system. “That doesn’t mean you’ll get sick. It just increases your propensity to get sick,” says Balaguru.

All in all, the conditions for the perfect storm with Helene were in place. “The storm started big, which was bad, it went over hot water, which was bad, it hit a location prone to high storm surges, and then it accelerated, moving into populated areas and carrying wind and rainwater into them populated areas,” says Knox. “You don’t want to see much worse.”