Treacherous Hurricane Helene is expected come ashore on Thursday evening on the northwestern coast of Florida and then continue to pummel parts of Georgia, the Carolinas and Tennessee with heavy rains, flash flooding and gusty winds.

Although Helene will likely weaken as it moves inland, its “high speed will allow strong, damaging winds, especially gusts, to penetrate well inland across the southeastern United States,” including the southern Appalachians, the hurricane center of the National Weather Service. said Thursday. Warnings for minor tropical storms were posted as far north as North Carolina.

The unusual range as far north and inland as forecasters expect – and the possible consequences – raise questions about the Fujiwhara effect, a rare weather event.

What is the Fujiwhara Effect?

The National Weather Service defines the Fujiwhara effect as “a binary interaction in which tropical cyclones within a certain distance… of each other begin to rotate around a common center.”

This means that the two storms interact and are formed by each other, sometimes even combining into one storm.

The concept arose from the interaction between typhoons in the Pacific Ocean, says Peter Mullinax, acting Warning Coordination Meteorologist at the National Oceanic and Atmospheric Administration’s Weather Prediction Center.

It was first identified over a century ago by Sakuhei Fujiwhara, a meteorologist in Tokyo, who published his findings on the “tendency toward symmetry of motion” in 1921.

Is that what happens to Helene?

Helene “is going to do a little dance,” but not with another hurricane or tropical storm, said Gus Alaka, director of the Hurricane Research Division of NOAA’s Atlantic Oceanographic & Meteorological Lab.

Instead, Helene responds to the effects of a low-pressure weather system in the northwest.

That interaction takes place in the upper atmosphere, where commercial aircraft fly, and not at surface level. That means it technically doesn’t undergo the Fujiwhara effect.

The combination of that weather event in the northwest and a high-pressure system in the northeast creates a fast-moving “conveyor belt” for Helene, steering and eventually forcing it to a standstill over Tennessee, northern Georgia and lower Appalachia. Alaka said.

Has this type of weather happened before?

The interaction between a tropical storm and an atmospheric weather system is more common than the Fujiwhara effect. Weather systems are common, moving regularly across the country and causing weather changes, Alaka said.

An example is Hurricane Sandy, which ravaged the mid-Atlantic and Northeast in 2012.

There was a weather system over the Great Lakes at the time that was “burying itself” in the mid-Atlantic states, Mullinax said. “As Sandy came down the East Coast, she felt the pull of that higher low, as Helene will feel and be drawn into today and tonight,” he said.

What does that mean for the southeastern US?

The speed at which Helene is moving and the sheer size of the storm, along with interactions with the pressure systems, are leading to severe weather warnings miles from the Florida coastline.

Mullinax said there is the potential for catastrophic and life-threatening flash flooding, including in north and northwest Florida and the metro Atlanta area, and significant landslides in the southern Appalachians.

“They are not so used to seeing not only the tropical rainfall, but also the winds that in some cases can exceed 70 to 80 kilometers per hour,” he said of the inland areas. “And that’s helped by this interaction at the higher levels, which is moving the storm inland more quickly.”

Alaka warned that gusty winds could still be dangerous — even if they are not at hurricane speeds by the time Helene is further inland — and could potentially down trees and power lines.

The hurricane center has warned that much of the southeastern U.S. could experience extended power outages and dangerous flooding. The governors of Florida, Georgia, the Carolinas and Virginia have all declared states of emergency in their states.

When and where does Helene first come to the US?

Helene could bring a “nightmare scenario” of catastrophic storm surge if it hits northwest Florida on Thursday evening. The storm was upgraded to a Category 2 hurricane Thursday morning and is expected to be a major hurricane — meaning a Category 3 hurricane or higher — when it makes landfall.

The National Weather Service office in Tallahassee predicts storm surges up to 6 meters (20 feet).

The storm formed Tuesday in the Caribbean Sea.

Helene had flooded parts of Mexico’s Yucatán Peninsula on Wednesday, flooding streets and toppling trees as it passed offshore and hit the resort town of Cancun.

In western Cuba, Helene knocked out power to more than 200,000 homes and businesses as they passed over the island.