Coastal communities were relieved in 2019 when Hurricane Dorian, like so many storms before it, stayed out at sea and spared the Southeast from the brunt of its winds. It's a common pattern for tropical storms, but strong hurricanes driven by a rare alignment of winds have the capacity to get turned around and head back toward land.
Once hurricanes pass north of Florida, their paths are affected by jet stream winds — narrow currents of wind speeding east and west several miles above the Earth's surface. The streams' paths and strengths vary, as do their effects on different storms.
If jet streams are weak enough and a tropical storm floats north, the wild card that dictates its path forward is the Bermuda High, an offshore merry-go-round of air that can snag storms propelled from Africa and send them clockwise: west across the Bermuda Triangle, and up the United States coast.
The Bermuda High shifts, expands and contracts with the seasons. In 2004, for example, it swelled over the Southeast, forcing tropical storms up the Gulf of Mexico rather than skirting along the Atlantic coast.
That's the year that experts debated whether Hurricane Ivan made a huge loop, blowing west across the Atlantic, veering north into the Gulf of Mexico, slowing slightly before it hit Alabama and drenched the Southeast. As it weakened, jet streams swiped the remnants east across the Appalachians and back into the Atlantic.
And in 1989, National Weather Service Charleston meteorologist Douglas Berry said, the high had extended just north of Charleston, effectively sling-shoting the storm along the high's southern edge and into the coast.
In normal years, when the Bermuda High sat squarely off the coast and spun storms north, that might've been Ivan's end. But the high, larger and set farther southwest than usual, collected what remained of Ivan and sent it south instead, to warmer waters off the Florida coast where — though experts debate whether it should count as Ivan or a separate system — the storm gained traction and swept back west across southern Florida and up into Louisiana and East Texas.
"The wind patterns have to be at the right place, 20 to 30 miles off can make the biggest difference," University of South Carolina climatologist Dr. Cary Mock said. "They can forecast the stalling pretty well, they just can't forecast the loop."
While the strength, size and placement of the Bermuda High and jet streams that help direct many storms vary by year, the intensity of the storms that meet them has been on the rise.
"It's not on our Top 10 list of climate change impacts for storms," said Tom Knutson, division leader of the National Oceanic and Atmospheric Administration's Weather and Climate Dynamics Division.
The good news for those hit by the hurricane's second strike is that, at least in theory, it should be less damaging.
Hurricanes tend to weaken when they sit on a single spot for too long because the storm pulls cold water from the ocean's depths and cools the overall system. The same principle would hold true for a hurricane returning to a spot it's already ravaged, Mock said, as the chillier waters wouldn't fuel the cell as quickly as the warm water it encountered on its first contact.
And the phenomenon remains rare. Most of the time, jet streams and the Bermuda High are one of the Lowcountry's best defenses against storms, Berry said. When the high develops in the summer, it generally deflects storms south, he said.
"Generally the stronger the Bermuda High, the better chance of it deflecting to the south," Berry said.
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