Dueling Cyclones

It’s hard to think of a Category-5 hurricane as a good thing. But in 2025, Hurricane Humberto helped save the East Coast from a direct hit by a smaller hurricane, Imelda.

The deflection was an example of the Fujiwhara effect. It’s named for the Japanese scientist who first described the effect, in 1921. It’s an interaction between two or more storms that pass close together. It applies to both tropical and non-tropical cyclones.

Such storms are big and powerful. But they’re influenced by the conditions around them. And the stronger the influence, the more the storms can change.

As two storms approach each other, they can change direction, for example. They might move closer, with both of them spinning around a point between them. If there’s a big difference in the sizes of the storms, the bigger one might deflect the smaller one, or even absorb it. But if they’re about the same size, they might loop around each other, then be shot out in opposite directions.

Tropical storms and hurricanes begin to interact at separations of about 900 miles. As they get closer, they may spin faster. And at less than 200 miles, they’re likely to merge. The exact process depends on the size and intensity of the storms and many other factors, so it’s tough to forecast.

The Fujiwhara effect is seen more often in the Pacific Ocean. But it does play out in the Atlantic as well. The Humberto-Imelda interaction is the most recent—a dance of giant storms that helped coastal residents—this time.

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