One chart shows how Hurricane Helene turned into a monstrous storm

Hurricane Helene has quickly intensified into a massive Category 4 storm, with hurricane-force winds extending up to 60 miles outward from the eye. Forecasters warn that Helene — which has wind speeds of near 120 miles per hour — could be deadly for those living in coastal Florida, where it’s expected to make landfall this evening.

The National Hurricane Center predicts storm surge as high as 20 feet in some parts of Florida’s Big Bend, a region between the panhandle and the peninsula. Storm surge, which describes a rise in sea level, is the most dangerous part of tropical storms and has a deadly track record: In 2022, storm surge killed more than 40 people during Hurricane Ian. The storm is also expected to inundate inland regions across much of the southeastern US with rain, dumping a foot or more in parts of southern Appalachia.

“This rainfall will likely result in catastrophic and potentially life-threatening flash and urban flooding,” the National Hurricane Center said early Thursday afternoon.

Helene could also disrupt part of the epic monarch butterfly migration, which typically passes through the Big Bend’s St. Marks National Wildlife Refuge in early October.

Helene is the eighth named storm in what has so far amounted to a somewhat puzzling hurricane season. It started with a bang — June’s Hurricane Beryl became the earliest Category 5 storm on record — and then much of August and September was unexpectedly quiet.

Many meteorologists, though, have been warning not to be fooled by this late-summer lull.

“Having multi-week periods of quiet and then multi-week periods of activity is very normal throughout a hurricane season,” Brian McNoldy, a climatologist at the University of Miami, told me earlier this month. “I definitely would not read too much into it.”

Plus, McNoldy said, the ocean in the Gulf of Mexico has been — and still is — exceptionally hot, and hot water fuels hurricanes. Ocean heat content, a measure of how much heat energy the ocean stores, is at a record high for this time of year.

Take a look at the chart below. The red line is 2024 and the blue line is the average over the last decade.

This record ocean heat is a clear reason why Hurricane Helene — which has been traveling through the Gulf on its way to Florida — has intensified so quickly. Put simply, hotter water evaporates more readily, and rising columns of warm, moist air from that evaporation are ultimately what drive hurricanes and their rapid intensification.

“The sea surface temperature and the ocean heat content are both record high in the Gulf,” McNoldy, who produced the chart above, told me. “That heat at the surface and available through a depth will give Helene all the fuel it needs to rapidly intensify today and into tomorrow.”

The record Gulf temperatures are just one signal of a more widespread bout of warming across the North Atlantic that ramped up last year.

It’s not entirely clear what’s causing this warming, though scientists suspect a combination of factors including climate change — which raises the baseline ocean temperature — as well as lingering effects of El Niño, natural climate variability, and perhaps even a volcanic eruption.

“This is out of bounds from the kinds of variability that we’ve seen in [at least] the last 75 years or so,” Ben Kirtman, director of the Cooperative Institute for Marine and Atmospheric Studies, a joint initiative of the University of Miami and the National Oceanic and Atmospheric Administration (NOAA), told Vox in August. “That can be scary stuff.”

Update, September 26, 6:50 pm ET: This story, originally published September 25, has been updated with new information as Hurricane Helene approaches the Florida coast.