Late Monday night, Hurricane Florence -- the Category 4 storm barreling toward the East Coast of the U.S. -- decided to replace its eyewall. And it's threatening to do so again.
This dramatic-sounding event tends to make major hurricanes even stronger, and larger, explained Chris Slocum, a storm researcher specializing in the inner-core processes of hurricanes at Colorado State University.
But how does such an atmospheric event unfold for any storm, let alone the ominous Hurricane Florence with extreme winds forecast to blow just off the coast of the Carolinas late Thursday?
"Eye replacement is something that major hurricanes naturally do," Jason Dunion, an atmospheric scientist at the National Oceanic and Atmospheric Administration's (NOAA) Atlantic Oceanographic and Meteorological Laboratory, said in an interview. (A major storm has wind speeds of at least 111 mph, or a Category 3 storm).
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Such powerful storms typically have a prominent wall of clouds, made up of a band of thunderstorms, surrounding the tranquil eye.
And for reasons still largely uncertain but are being actively researched, another eyewall will sometimes begin to form around the original, inner eyewall.
"Suddenly you have two of them," said Dunion. "Eventually it just replaces the inner eyewall."
In the 1950s and 1960s, before the age of satellites, hurricane hunting pilots sometimes spotted this happening and called it "an eye within an eye," said Dunion.
Today, these replacements are of great significance to storm scientists and forecasters. Because once a new eyewall replaces the collapsed, original eyewall, the storm is prepped to intensify.
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Growing more powerful
Hurricanes of all persuasions feed off warm bathtub-like water of at least 80 degrees Fahrenheit.
They suck up evaporated seawater, adding to their girth, and when this water vapor is converted to both liquid and then ice particles (high up in the clouds), energy is released, which hurricanes convert into wind, explained Slocum.
Given enough fuel, the process repeats and repeats, and the storms grow larger and more powerful.
Bands of spiraling thunderstorms, which give hurricanes that "buzzsaw" type of appearance, form outside the eyewall and can eventually become their own ring, or second eyewall. Once this is accomplished, the thick ring of storms starve the inner wall of moisture-rich air, and eventually, the inner ring simply dissipates away, often leaving a larger, gaping eye, explained Slocum.
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Then, "you have a temporary weakening of the storm," he said, as that original eyewall was home to the hurricane's strongest winds.
But, no matter, for the grounds have been set for a reawakening of the hurricane.
Now with a larger eye, the storm can contract in on itself.
"It's like an ice skater throwing their arms inwards," said Slocum. "If it is able to contract more, you’ll see a spin up."
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After Florence completed its eyewall replacement, the eye grew to about 40 miles in diameter, as of Tuesday afternoon.
It's also possible that Florence could go through another eyewall replacement cycle before getting to the coast.
"It's just such a hard thing to forecast," said Dunion, Tuesday afternoon.
On Wednesday, meteorologists spotted the cycle happening again.
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Some hurricanes replace their eyewall every day, said Dunion. Others, every few days.
We'll see if Florence completes this second replacement. But overall, the storm is running out of time. Florence is forecast to lose steam once it interacts with the coast.
The drama, however, won't be over. The stalled storm is forecast to sit over land, dumping deluges of rain inland, similar to Hurricane Harvey.
But if Florence were to undergo another replacement today, and experience a temporary weakening, the storm's overall effect likely wouldn't be lessened.
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The process of building another eyewall creates a bigger storm, as the new, outer ring of thunderstorms creates a larger eye and expands the storm's diameter.
This means the storm can spread over more ocean, and push more water inland in the form of violent storm surges.
"So, while a hurricane may weaken during eyewall replacement, the impacts from the storm are not necessarily diminished," said Slocum.
And this storm won't be a walk in the park for anyone in its path.
More eyewall replacements or not, at this juncture Florence will impact the evacuating Southeastern coast with extreme effects including high winds, flooding, and torrential rain.
