What Helene might tell us about hurricanes of the future

Every year on Aug. 20, meteorologists at Colorado State University ring a bell to signal the start of peak hurricane season - a weeks-long stretch when hot ocean temperatures tend to generate frequent and destructive storms. But this year, the tradition gave way to an eerie, echoing quiet, with storm activity in the Atlantic at its lowest level in 30 years despite projections of a historic season.

That lull came to a decisive end this week, when Hurricane Helene slammed into Florida’s Big Bend with violent, deadly force. Fueled by exceptionally warm Caribbean waters, the Category 4 storm is one of the biggest to ever make landfall in the United States - and forecasters are already warning that additional cyclones are hot on its heels.

This lopsided hurricane season illustrates the challenges facing forecasters as climate change makes extreme weather less predictable and more intense. Even as some scientists say that Helene’s rapid growth and historic rainfall are signatures of a storm influenced by human-caused warming, they are still striving to understand whether this year’s unusual storm activity is a fluke or a sign of things to come.

“Is every season going to be like this? It’s hard to say,” said Phil Klotzbach, a meteorologist at Colorado State University. “We’ll just have to keep our eyes to the sky.”

[Dozens dead and millions without power after Helene’s deadly march across southeastern US]

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Helene’s climate fingerprints

Scientists have growing evidence that major hurricanes - those at or above Category 3 - are increasing in frequency, and many point to climate change as a cause. But at this point, the data is not definitive on whether the overall number of storms will increase.

In a study published in Proceedings of the National Academy of Sciences in 2020, researchers found that the number of major hurricanes globally increased between 1979 and 2017. Later studies found that the trend could not be easily explained by natural variability.

Hurricane Helene is the eighth Category 4 or 5 storm to make landfall in the United States since 2017 - the same number of Category 4 or 5 storms that struck the country in the previous 57 years. There was, however, a similar surge of powerful storms in the late 1940s.

“Certainly the very, very warm ocean temperatures are playing the major role in what we’re seeing,” said James Kossin, a science adviser at the First Street Foundation. “There’s just a whole lot more fuel.”

All of those large storms also had another factor in common - they all experienced what is known as “rapid intensification,” or a surge in peak wind speeds of greater than 35 miles per hour in less than 24 hours. Passing over a record-hot Caribbean, Helene transformed from a Category 1 to a Category 4 storm in the space of a single day.

Those rapidly intensifying storms have increased dramatically since 1990, according to a recent study.

Rising global temperatures help accelerate a feedback loop between water evaporation and wind speed, said Kerry Emanuel, an atmospheric scientist at MIT who has studied the phenomenon. This shortens the time frame in which hurricanes can develop and raises the ceiling on how bad they can become.

“It’s really stomping your foot on the accelerator,” Emanuel said, particularly for the fastest-intensifying storms.

Other researchers warn that it may be too early to say if those major storms are definitively increasing.

“It’s a little bit murky,” said Tom Knutson, a senior scientist at the National Oceanic and Atmospheric Administration. Knutson pointed out that when researchers look at the last century of major hurricanes making landfall in the United States, there isn’t an obvious trend. “There’s still some questions,” he said.

There is clearer evidence that human-caused warming has heightened the destructiveness of hurricanes by amplifying their rainfall, researchers said. Scientists have long known that air can hold twice as much moisture for every 10 degree Celsius (18 degrees Fahrenheit) increase in temperature.

“Even if hurricanes themselves didn’t change their structure or how fast they were moving, it would rain more,” Emanuel said.

Even before Helene made landfall, its moisture was sucked into a storm front that ran ahead of the hurricane, dumping almost a foot of rain on parts of the Southeast. This is called a predecessor rain event and is more common with high-intensity storms, studies suggest, because of the way severe hurricanes interact with the jet stream.

That early rain left soils saturated and filled rivers to their banks, creating conditions for catastrophic flooding as Helene marched inland. By late Friday afternoon, more than 150 river gauges from Florida to Virginia were at flood stage. For many communities in western North Carolina, it was a rainfall event that should be expected to happen only once in 1,000 years.

“The most impacts come from situations like this,” said Russ Schumacher, a Colorado State University professor of atmospheric science.

Schumacher said that while more study is needed to assess the impact of climate change, “it’s not that hard to connect the dots” between warmer oceans, more moisture in the air and more prolific predecessor rain events.

“Small increases in the moisture can lead to very big increases in the amount of rainfall,” he said.

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A wide storm in a weird season

Though Helene bore some trademarks of a climate-change-influenced storm, scientists aren’t sure what one of its most distinctive traits - its size - heralds for future hurricanes.

At some 420 miles in diameter, Helene is among the biggest storms to strike the United States - on par with behemoths like Sandy, Katrina and Andrew. Research shows that the biggest storms are often disproportionately destructive. But because officials didn’t record the size of hurricanes until relatively recently, Emanuel said, studies are inconclusive about whether large-diameter storms will become more common with climate change.

That is one of many questions scientists must now ponder as they look back on Helene and the season that led up to it.

With the world shifting into a La Niña weather pattern, which is typically associated with severe hurricanes, and “off the charts” water temperatures in the Atlantic, experts projected that this season would be among the worst in decades. But after experiencing Beryl in July, its earliest-ever Category 5 hurricane, the ocean basin saw the longest stretch in more than 50 years without a single late-summer cyclone.

“The season wasn’t the way we expected it to play out,” Klotzbach said. “And we’re still trying to figure out why.”

Though hurricanes are primarily fueled by heat from the oceans, it takes more than just warm water to brew up a storm. Some of those necessary ingredients - such as moisture-laden air - were missing from the Atlantic this summer, which helped quash cyclone activity.

This year also saw exceptionally high temperatures in the upper atmosphere. The combination of hot air over a hot ocean may have had a stabilizing influence, Klotzbach said - raising the question of whether human-caused warming may have pushed the atmosphere to a point where it starts to suppress storm formation.

It’s too soon to tell whether the dry, hot atmospheric conditions were just quirks of this particular season or signs of longer-term change, Klotzbach said.

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An uncertain outlook

Some meteorologists expect the next month of hurricane season to heat up. A set of thunderstorms is set to shift east toward Africa, leaving the Atlantic Ocean with favorable conditions for hurricane formation.

“Late-season storms bear a lot of watching,” Klotzbach said, because they tend to form in the western part of the Atlantic basin and don’t have far to travel before they hit land. Like Helene, which formed in the Caribbean, such storms can quickly go from unnamed tropical depressions to billion-dollar disasters.

Despite this flurry of activity so late in the year, Klotzbach said there isn’t any conclusive evidence that climate change may be shifting the peak of hurricane season or causing it to last longer. But he and his colleagues will be carefully studying this season to understand why it behaved so unexpectedly.

Still, he added, “it’s also important when you have a seasonal forecast model built on 40 or 50 years of historical data to not throw everything out the window if one season doesn’t go perfectly.”

For Emanuel, Helene’s rapid intensification - and the quiet months that preceded it - highlight the challenges facing meteorologists as people continue to warm the planet. Even as weather models and forecasting technology improve, the unprecedented rate of human-caused warming is causing storms to behave in new and unpredictable ways.

“This is one thing that scares me, if these things can intensify more rapidly,” Emanuel said. “We’re going to have cases where forecasters go to bed with a tropical storm and wake up with a Category 5 when it’s too late to evacuate people.”

“How difficult will it be to forecast the weather in the future?” he added. “I think it’s a wide-open and important question which science has not yet answered.”