Hurricane Milton delivered surprises at almost every turn.
What started as a small, tightly wound hurricane has grown into a sprawling monster that has strengthened at one of the fastest rates in history. The storm threatens to send a dangerous wave of water to parts of both the west and east coasts of Florida, with the flood-prone urban area of Tampa Bay – home to more than three million people – particularly at risk.
As the storm developed, record warm seas in the Gulf of Mexico contributed to its intensification. Later it became larger as it underwent a process of eye wall replacement.
Here’s how Milton developed into such a major threat.
A Pacific influence
Hurricanes approaching the U.S. typically follow a similar path: Tropical storms form on the west coast of Africa, cross the Atlantic Ocean, and strengthen as they enter the warm waters of the Caribbean Sea.
But part of Milton’s origin story lies in the Eastern Pacific. The hurricane formed when the remnants of a tropical depression in the Pacific Ocean swept eastward over the Yucatán Peninsula and encountered a stalled front in the Gulf of Mexico. The most recent storm to hit Florida after forming in the same area – Mexico’s Bay of Campeche – did so in 1867.
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When the tropical depression entered the Gulf, it provided “a little bit of vortex, that amount of spin,” to a system of thunderstorms there, said Chris Slocum, a physicist at the National Oceanic and Atmospheric Administration’s Center for Satellite Applications.
Then Milton was organized and pushed away from other systems.
“It’s that isolation from other thunderstorms that allows the pressure to build and the winds to develop,” Slocum said. Milton began to draw air into the center and draw energy from the warm ocean.
Small but strong
Milton started out as an extremely small storm, allowing him to conserve his angular momentum and spin tightly and quickly around a narrow eye.
In the Gulf, it faced record high ocean temperatures and moist, warm air – the ingredients needed to intensify. On Monday, central pressure in Milton’s core dropped at a rate one scientist described as “insane” as Milton grew stronger. Values of central pressure are closely related to a storm’s strength and wind speeds.
“This is just horrific,” said John Morales, a hurricane specialist for NBC South Florida, who choked on air while discussing the significance of the pressure drop.
According to Climate Central, a nonprofit research group, Milton’s wind speeds increased by 90 miles per hour in about 24 hours. That easily surpassed the milestone for what scientists consider rapid intensification: a gain of 55 km/h in 24 hours.
“In less than two days, it went from a tropical storm to a Category 5 hurricane, which is off the charts,” said Karthik Balaguru, a climate scientist at the Pacific Northwest National Laboratory.
Jonathan Lin, an atmospheric scientist at Cornell University who predicts and models hurricanes, called Milton “one of the fastest intensifying hurricanes we’ve ever seen in the Atlantic Ocean.”
The hurricane’s wind speeds, which reached more than 240 miles per hour, were unprecedented for an October storm. Milton is the strongest hurricane in the Gulf of Mexico since Hurricane Rita in 2005.
A new eye wall
In the Northern Hemisphere, hurricanes spin counterclockwise around mostly cloud-free eyes at their centers.
Rain bands developed on the outer edge of Milton overnight Monday through Tuesday. Those storms merged to form a second ring, creating a replacement eyewall and tripling the radius where maximum wind speeds were recorded, Slocum said.
That phenomenon, known as eyewall replacement, typically causes storms to increase in width but lose some wind speed, which is what happened to Milton. It can happen several times as a storm develops. Once the process is over, the hurricane may regain intensity, conditions permitting.
“You can think of it as his skin peeling off. Once it sheds its skin, it can intensify again. That’s exactly what we saw at Milton,” Lin said.
A wobble
According to the National Hurricane Center, Milton “wobbled” Tuesday afternoon, changing the expected track and shifting the estimated landfall to the south.
Wobbles result from instability due to complicated dynamics within the eyewall.
Lin explained the dynamics by comparing a hurricane to a spinning top, or dreidel.
“Sometimes you see a spinning top – you push it a little bit, give it a push, and it wobbles a little bit and then starts spinning again,” Lin said. “It reorients itself.”
A significant fluctuation can change a storm’s track and determine which locations bear the brunt of the hurricane.
Forecasters expect a storm surge of as much as 4 meters. If the storm turns slightly south, the worst of the flooding could be kept out of Tampa Bay, which is particularly vulnerable. Hurricane Irma wobbled eastward in 2017, preventing Tampa Bay from experiencing an expected storm surge of 12 feet or more.
When the storm reaches the coast, areas south of Milton’s eye should see the heavy winds that push water onto the coast – and the resulting storm surge.
That’s because of the angle at which the storm approaches the Florida peninsula and the way the wind spins counterclockwise around the core.
What comes next
Milton is weakened during his final approach to the Florida Peninsula. That’s mainly because there was vertical wind shear, which refers to changes in wind speed or direction at higher levels of the atmosphere.
But Lin said, “That doesn’t make it any less dangerous.”
Even with slower winds, Milton is expected to remain a major hurricane until landfall Wednesday evening.
After landfall, it is expected to cross the peninsula and head out to sea. The time on land will deprive the storm of the energy it gets from the heat in the ocean, and it will weaken quickly — like a crest slowing its speed, Lin said.
Breaking an eerie silence
A new report from the World Weather Attribution Group on Hurricane Helene – which made landfall in Florida on September 26 – found that climate change most likely made the storm’s wind speeds 11% more intense and rainfall totals about 10% higher is.
The researchers said they expect Milton to behave similarly and also be worse because of climate change.
Due to warmer-than-normal ocean conditions in the North Atlantic and Gulf of Mexico and the emergence of the La Niña climate pattern, which is associated with hurricanes, forecasters predicted an extremely active hurricane season this spring.
But in mid-September, the typical peak of hurricane activity, the season remained ominously quiet, confusing researchers and making them wonder whether their aggressive forecasts had failed despite the ocean’s heat – hurricane fuel.
The eerie silence broke in late September, when Hurricanes Helene and Milton came into view. After Milton makes landfall, the Gulf Coast will have recorded the second-highest number of hurricane landfalls in a year.
“That would tie 2024 with 2005 and 2020 for the second-most Gulf hurricane landfalls on record, behind 1886,” wrote meteorologist Philip Klotzbach on X.
This article was originally published on NBCNews.com