Tropical Tangle as Hurricane Humberto and Brewing Storm Stir Uncertainty
Hurricane Humberto and a system that may become Tropical Storm Imelda in the coming days are swirling quite close to each other in the western Atlantic Ocean
The uncertain hurricane forecast hinges on a storm system that is currently located over Cuba and the Bahamas. Dubbed Potential Tropical Cyclone Nine, it will become known as Tropical Storm Imelda if the system’s peak wind speeds reach 39 miles per hour. And as of 2 P.M. EDT on September 26, National Hurricane Center forecasters placed the probability of this occurring within the next 48 hours at 90 percent.
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That complexity is in part because of a second storm roiling the western Atlantic Ocean: Hurricane Humberto. With peak sustained wind speeds of 90 miles per hour, Humberto is currently a strong Category 2 hurricane and is expected to explode into a Category 3 hurricane overnight. This storm is currently moving west toward the Bahamas, although it is expected to veer north toward Bermuda before reaching the archipelago.
Having two concerning storm systems so close to each other isn’t common in the region. “It’s pretty unusual,” Gerard says. “It’s something you see more often in the West Pacific, where you just have more storms.”
Because of the two storms’ proximity, experts are watching for the possible occurrence of an unusual phenomenon called the Fujiwhara effect. This phenomenon is named for Sakuhei Fujiwhara, a scientist who studied how vortices in fluid interact. The effect occurs when tropical cyclones come within about 850 miles of each other, although the distance at which it kicks in—and its end result—depends on the size of each storm.
“They could dance around each other, and if one hurricane is a lot stronger than the other, then the smaller one will orbit around the stronger one and eventually crash into the stronger one,” says Haiyan Jiang, an atmospheric scientist at Florida International University. “If the two hurricanes are closer in strength, they can just rotate around a common point.”
A recent example of the Fujiwhara effect occurred between Hurricanes Hilary and Irwin in the eastern Pacific in 2017; the storms eventually combined. And a rare Caribbean occurrence happened between Hurricanes Connie and Diane in 1955.
Whether Humberto and the possible storm Imelda experience the Fujiwhara effect remains to be seen. First, of course, Imelda must develop into a true cyclone rather than the mess of thunderstorms that it currently is. Then it’s a matter of the comparative speeds and directions of that storm and Humberto, Jiang says.
Even if the storms remain too distant for the Fujiwhara effect to occur, they have already interacted, Gerard notes. On September 25 winds churned up by the developing system were caught up and blown toward Hurricane Humberto, where it became wind shear, a mess of wind that can tear apart a storm or slow its strengthening.
The two storms could also influence each others’ paths, even without the Fujiwhara effect coming to pass, Gerard says. That’s because, as Humberto shifts to steer northeast, its effect on the atmosphere around it may leave a trail that could attract the would-be storm Imelda, keeping the latter away from the East Coast.
A key challenge of forecasting what will happen in the next couple of days is that the storm that could become Imelda does not yet have a clearly defined center around which winds rotate. Where the center develops will affect how the storm moves and reacts to the larger atmospheric environment around it. “Until we get a better handle on that, it’s hard to know which of these scenarios is more likely to occur,” Gerard says, noting that the system could develop a center by midday on September 27.
“All of that combined should help us get a better picture,” Gerard says. “Hopefully, by the end of the weekend, we’ll have a much better expectation of what’s going to happen with all of this.”
“It could be a much more disruptive situation” than Gerard experienced during one active hurricane season he worked during a shutdown.
Meghan Bartels is a science journalist based in New York City. She joined Scientific American in 2023 and is now a senior news reporter there. Previously, she spent more than four years as a writer and editor at Space.com, as well as nearly a year as a science reporter at Newsweek, where she focused on space and Earth science. Her writing has also appeared in Audubon, Nautilus, Astronomy and Smithsonian, among other publications. She attended Georgetown University and earned a master’s degree in journalism at New York University’s Science, Health and Environmental Reporting Program.
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