Fujiwhara Effect: Twin Hurricanes Threaten US East Coast

What is the Fujiwhara Effect?

The wild weather anomaly — named after Japanese meteorologist Sakuhei Fujiwhara in 1921 — sparks when two cyclones come within 900 miles of each other and begin spinning around a shared center in what meteorologists liken to an erratic dance.

“Or simply, they dance around each other. For tropical systems, the two must be within 900 miles of each other. For ‘ordinary,’ non-tropical storms, they need to be within 1,200 miles of each other.”

What happens next depends on each system’s size and strength — but in rare cases, they can smash together and create one giant super-storm, according to the National Weather Service.

However, if the storms are evenly matched, they’ll whirl around a shared point before breaking apart and fading — just like Pacific Hurricanes Hilary and Irwin in 2017.

In other cases — like Typhoon Hinnamnor clashing with a nearby tropical depression in the West Pacific in 2022 — the stronger storm can overpower and absorb the weaker one, Diamond explained.

Each scenario and storm interaction creates a huge forecasting challenge for meteorologists.

The spectacle is rare in the Atlantic but happens more often in the West Pacific, Diamond said, noting there are no recent examples of storms colliding and causing land damage.

“This is mostly due to the West Pacific producing more tropical systems each year than the Atlantic,” he said.

“When it does occur, it’s usually in the open ocean and far away from land.”

He noted the storm could also be shredded by the mountains of the Dominican Republic, take longer to form, and stall over the Bahamas or near South Florida before heading offshore toward Bermuda.

“If this happens, likely-Imelda and Humberto maintain enough distance from each other that they don’t ‘Fujiwhara.'”