How Hurricane Helene Became a Monster Storm

The southeastern United States is grappling with Hurricane Helene, a monster storm that made landfall in Florida on Thursday and then cut a terrifying path into Tennessee. How did it get so bad?

The storm killed more than 100 people and hundreds more are still missing. Millions of people lack electricity. Residents of Asheville, North Carolina – one of the hardest-hit areas – are reportedly struggling to find food, water and cell phone access. We don’t know the full effects of the storm yet; Search and rescue missions are still ongoing and scientists are finalizing data on the storm’s strength.

What is clear, however, is that the storm was catastrophic due to its extraordinary size, intensity and speed. There were perfect conditions to supercharge the storm.

“He had all the weapons at his disposal that a hurricane (can have),” says John Knox, distinguished teaching professor and undergraduate coordinator in the atmospheric sciences program at the University of Georgia. “Everything we thought a hurricane could do, Helene actually did.”

While Helene was still boiling in the Gulf of Mexico, forecasters were already warning that the storm would be “unusually large.” At its maximum, tropical storm winds extended nearly 550 miles from downtown Helene. This massive scope placed Helene in the 90th percentile for storm size, according to the National Hurricane Center. This means that on the ground, the effects of the storm – wind, storm surge and heavy rainfall – were felt over an extremely wide area.

The storm was not only powerful, but stronger than most. Storm systems this large don’t always have a small inner core that allows them to strengthen quickly. Helene, however, was able to form a relatively small eye and then quickly intensify – a term used to describe tropical storms with sustained wind speeds that increase by at least 30 knots (about 35 miles per hour) in a 24-hour period.

It made landfall with winds reaching 140 mph, making it a major storm rated 4 out of 5 on the Saffir-Simpson scale.

Helene also poured water over the punch. When it hit the Big Bend region of Florida, it brought a massive storm surge, flooding the coast with up to 5 feet of seawater. The underwater topography of Florida’s west coast, with its more gradual slope, acted as a ramp, making it easier for the storm to carry a higher wall of water with it. The sheer size of the hurricane also meant that the storm surge flooded a larger area.

Heavy rainfall dumped more water on communities, leading to historic flooding in western North Carolina. Over a three-day period, from September 25 to 27, almost 5 inches of rain fell at the Asheville airport. The highest preliminary rainfall total was over 31 inches and was recorded in Busick, North Carolina.

“This was certainly a very catastrophic event for parts of the southeastern United States, especially the southern Appalachians, where they experienced tremendous amounts of rainfall and flooding,” said Daniel Brown, division chief of the National Hurricane Center’s hurricane unit. 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.

Moreover, the storm was fast moving, with forward speeds ranging from 20 to 30 miles per hour. By comparison, storms that make landfall in the Gulf of Mexico typically only move forward at about 16 to 25 miles per hour, Brown said. Tropical storms usually weaken as they move over land because they draw strength from thermal energy from warm waters at the sea surface. Helene’s speed, however, allowed her to maintain greater strength as she moved inland.

“Therefore, the effects were felt much further inland than (people) are usually used to,” says Karthik Balaguru, a climate scientist at the Pacific Northwest National Laboratory. “I mean, the further inland you go, the more people are going to be exposed to this threat.” 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 these types of disasters.

Climate change is changing the way storms like Helene are calculated. Rising global temperatures create favorable conditions more intense storms that can quickly gain strength and remain stronger over land. Helene developed in the face of rapidly rising sea surface temperatures in the Caribbean Sea and Gulf of Mexico. Water levels rose to 31 degrees Celsius (87.8 degrees Fahrenheit) at the beginning of the storm, providing ample fuel. The atmosphere’s ability to retain moisture is increasing due to greenhouse gas emissions from fossil fuels, causing more severe downpours.

To find out how much of a role climate change has played in Helene’s case in particular, scientists will need to conduct more research. But Balaguru compares the effects of climate change to the world’s weakened immune system. “It doesn’t mean you’ll get sick. This only increases your tendency to get sick,” says Balaguru.

All in all, everything was in place to create the perfect storm with Helene. “The storm started out big, which was bad, came through hot water, which was bad, hit a place prone to heavy storm surge, and then accelerated, moved into populated areas, and took wind and rainwater into those populated areas,” Knox said. . “You don’t want to see anything worse.”