The southeastern United States is being ravaged by Hurricane Helene, a monstrous storm that made landfall in Florida on Thursday before carving a terrifying path all the way to Tennessee. How did it get so bad?

The storm killed more than a hundred people, and hundreds of others are still missing. The power has gone out for millions of people. Residents of Asheville, North Carolina – one of the hardest-hit areas – are reportedly struggling to find food, water and cell phone service. We don’t yet know the full impact of the storm; search and rescue missions are still underway and scientists are finalizing data on how powerful the storm was.

But it is clear that the storm was disastrous because of its unusual size, intensity and speed. The perfect conditions were in place for the storm to gain momentum.

“It had all the different weapons that a hurricane (could have),” said John Knox, professor of education and undergraduate coordinator of the Atmospheric Sciences Program at the University of Georgia. “Everything we say a hurricane can do, Helene has done.”

With Helene still churning in the Gulf of Mexico, forecasters were already warning that the storm would become “unusually large.” At its maximum, tropical storm force winds extended nearly 350 miles (560 kilometers) away from central Helene. That enormous range placed Helene in the 90th percentile for storm size, according to the National Hurricane Center. On the ground, this means that the storm’s effects – wind, storm surge and heavy rainfall – were felt over an unusually large area.

The storm was not only huge, but stronger than most. Storm systems of this size do not always develop a small inner core that allows them to strengthen quickly. But Helene was able to form a relatively small eye and then rapidly intensify, a term used to describe tropical storms with sustained winds that increase by at least 30 knots (about 55 miles per hour) in a 24-hour period.

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

Helene also hit her with water. When it hit the Big Bend region of Florida, it caused a massive storm surge, flooding the coastline with as much as 15 feet of seawater. The underwater topography off Florida’s west coast, with a more gradual slope, acted as a slope, making it easier for the storm to bring with it a larger wall of water. The sheer size of the hurricane also caused the storm surge to inundate a larger area.

Heavy rains dumped more water on communities, leading to historic flooding in western North Carolina. The Asheville airport recorded nearly 40 inches of rain over three days between September 25 and 27. The highest preliminary total was over 31 inches of rain, recorded in Busick, North Carolina.

“It’s certainly been a very catastrophic event in parts of the southeastern U.S., especially in the southern Appalachians, where they’ve seen tremendous amounts of rain and flooding,” said Daniel Brown, division chief of hurricane specialist at the National Hurricane Center. But with reports of damage and fatalities still coming in, it’s probably too early to know how Helene compares to other storms, he said.

Furthermore, the storm was fast, with a forward speed of between 20 and 30 miles per hour. By comparison, storms that make landfall along the Gulf of Mexico typically only move at a speed of about 10 to 15 miles per hour, Brown said. Tropical storms tend to weaken as they move over land because they draw strength from heat energy from warm water at the sea surface. However, Helene’s speed allowed it to maintain more power as it moved inland.

“That’s why the impacts were felt much further inland than people are typically used to,” said Karthik Balaguru, a climate scientist at the Pacific Northwest National Laboratory. “The further inland it goes, I mean, the more people would be exposed to this danger.” Another risk factor is that inland communities may not have as much experience preparing for hurricanes as coastal areas that are more accustomed to dealing with these types of disasters.

Climate change changes the calculus for storms like Helene. Rising global temperatures are creating conditions conducive to… more intense storms that can quickly gain strength and remain more powerful on land. Helene developed amid rising sea surface temperatures in the Caribbean Sea and the Gulf of Mexico. Water along the storm’s early path rose to 31 degrees Celsius (87.8 degrees Fahrenheit), providing plenty of fuel. The atmosphere’s ability to retain moisture increases due to greenhouse gas emissions from fossil fuels, allowing for heavier rainfall.

To know what major role climate change has played specifically on Helene, scientists will have to do more research. But Balaguru compares the effect of climate change to a weakened immune system in the world. “It doesn’t mean you’ll get sick. It just increases the tendency to get sick,” says Balaguru.

All together, the pieces were in place for the perfect storm with Helene. “The storm started out big, which was bad, it went over hot water, which was bad, it hit a place prone to high storm surge, and then it accelerated and went into populated areas and brought wind and rainwater to those populated areas. ,” says Knox. “You don’t want to see much worse.”