Will merging storms cause Atlantic coast havoc?  

So far, the Atlantic hurricane season has seemed eerily mild, despite predictions 2025 would be an active year. To date, there have been eight named storms, compared to 10 by Sept. 25 of 2024.

But things may be about to change, due to a trio of events The Weather Channel is calling a “parade of Atlantic storms.”

The network is forecasting the current Tropical Storm Humberto will gain strength and become the season’s third named hurricane over the coming weekend.

The Weather Channel’s current tracking forecast calls for Humberto to stay south, where it would pose some threat to Bermuda by early next week, “but is unlikely to be a U.S. threat,” the network adds.

However, as of Thursday afternoon, AccuWeather placed Humberto roughly 772 km east-northeast of the northern Leeward Islands with maximum sustained winds of 72 kph. “Humberto is forecast to take a curved path between Bermuda and the East Coast of the U.S.,” says AccuWeather. The Canadian Hurricane Centre’s tracking map shows a similar path.

A third Atlantic storm, Hurricane Gabrielle, is moving East toward Spain.

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Merging storms?

More worrisome, both networks say, is the development a new tropical storm — Imelda — in relatively close proximity to Hurricane Humberto.

And that’s where concerns could mount for the North American Atlantic seaboard, says Glenn McGillivray, managing director of the Institute for Catastrophic Loss Reduction.

“It is quite possible, though rare, for two North Atlantic basin storms to merge into one,” he tells Canadian Underwriter.

“But it is possible enough that there is a name for the phenomenon. The ‘Fujiwhara Effect’ outlines how two nearby storms can merge, sometimes with the larger, more powerful storm absorbing the other, and sometimes with two storms of equal size combining into one. There is also the possibility that the two can spin around each other for a time before going their own separate ways. Again, rare, but possible.”

In September 1995, he notes, two Atlantic hurricanes – Iris and Humberto – “moved in lockstep with each other” before Iris absorbed Humberto to create a stronger storm. The Fujiwhara Effect is more common in the Pacific.

Related: Why Atlantic Canada is at increased risk of getting hit by a hurricane this year

“There are several consequences that can surround storms subjected to the [Fujiwhara] Effect, including increased storm strength, longer lasting storms, unpredictable storm paths and unexpected landfall locations,” McGillivray tells CU. “In short, when two storms merge, it may throw models out the window.

“It’s too early to say what this all means for Canada,” he says.

McGillivray notes the three currently active storms in the North Atlantic are more in line with expectations for an active 2025 season, adding the record for the most tropical depressions and storms in the North Atlantic at one time stands at six, and was set in 1971.

Phil Porado

Phil, an award-winning journalist with over 30 years of experience in financial topics, has been managing editor of Canadian Underwriter for more than three years.