Treacherous Hurricane Helene is expected Landing Thursday evening along the northwest coast of Florida and then plague parts of Georgia, the Carolinas and Tennessee with heavy rains, flash floods and gusty winds.

While Helene is likely to weaken as it moves inland, its “fast forward speed will allow strong, damaging winds, particularly in squalls, to advance well inland across the southeastern United States,” including the southern Appalachians, the hurricane's center National Weather Service said Thursday. Less severe tropical storm warnings were issued as far north as North Carolina.

The unusual reach as far north and inland as forecasters expect and the potential impacts 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 begin at a specific distance…from each other to rotate around a common center.”

This means that the two storms interact and are shaped by each other, sometimes even combining into a single storm.

The concept arose from the interaction between typhoons in the Pacific Ocean, said Peter Mullinax, acting meteorologist for warning coordination 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 motion symmetry” in 1921.

Is this what is happening to Helene?

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

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

This interaction occurs in the upper layers of the atmosphere, where commercial jets fly, rather than at surface level. This means that it is technically not affected by the Fujiwhara effect.

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

Has there ever been a weather event like this before?

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

One example is Hurricane Sandy, which devastated the Mid-Atlantic and Northeast in 2012.

At that time, there was a weather system over the Great Lakes that was “burrowing” into the mid-Atlantic states, Mullinax said. “As Sandy came up the East Coast, she felt the pull of this upper low, just as Helene will feel tonight and be pulled into it,” he said.

What does this mean for the southeastern United States?

The speed at which Helene is moving and the sheer size of the storm, as well as its interactions with the pressure systems, are resulting in severe weather warnings reaching miles from the Florida coast.

Mullinax said there is a risk of catastrophic and life-threatening flash flooding, including in northern and northwestern Florida and the Atlanta metropolitan area, as well as significant landslides in the southern Appalachians.

“They're not as used to seeing not only tropical rains, but also winds that can gust over 45 to 50 miles per hour in some cases,” he said of inland areas. “And that's supported by that interaction in the upper levels, which is pulling the storm inland more quickly.”

Alaka warned that gusty winds can still be dangerous – even if they don't reach hurricane speeds if Helene is further inland – and could potentially topple trees and power lines.

The hurricane center has warned that there could be extended power outages and dangerous flooding across much of the southeastern United States. The governors of Florida, Georgia, the Carolinas and Virginia have all declared states of emergency in their states.

When and where does Helene travel to the USA for the first time?

Helene could trigger a “nightmare” scenario of catastrophic storm surge when 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 – a Category 3 or higher – when it makes landfall.

The National Weather Service office in Tallahassee forecast Storm surges of up to 20 feet (6 meters).

The storm emerged on Tuesday in the Caribbean Sea.

Helene flooded parts of Mexico's Yucatán Peninsula on Wednesday, inundating roads and toppling trees as it passed the coast and hit the resort town of Cancun.

In western Cuba, Helene cut power to more than 200,000 homes and businesses as it passed through the island.