How Irma grew into a monster hurricane so quickly

Hurricane Irma 5 p.m. update

Hurricane Irma grew from a small clump of clouds near Africa 10 days ago to a ferocious, 185-mph monstrosity that's rampaging through the Caribbean — likely destined for the U.S.

The storm began its life just west of Guinea-Bissau, Africa, on Aug. 27 as a non-tropical disturbance, with winds of just 29 mph, the National Hurricane Center said.

It continued to trek west through the Cape Verde islands west of Africa, thus classifying it as a "Cape Verde hurricane," a common type of late-summer hurricane.

These hurricanes form from a tropical depression or tropical wave that passes through or near that chain of islands, and then eventually strengthens into a named system (finally becoming a hurricane at some point) in the mid-Atlantic.

Some of the most notorious hurricanes to ever hit the U.S. have been Cape Verde storms, such as Andrew, Hugo and Ike.

Irma's continued trek west across the ocean was guided by a strong ridge of high atmospheric pressure over the Atlantic, which prevented the storm from curving north and away from North America.

"Irma's path was a bit unusual in that it tracked south of due west for several days," meteorologist Phil Klotzbach of Colorado State University, said. Typically storms tend to track west-northwest across the Atlantic, he said. "The high pressure was just a little extra strong to keep it tracking south of due west."

As it churned west, it picked up steam and finally exploded into a Category 5 beast on Sept. 5 thanks to a perfect combination of weather ingredients: weak wind shear; warm, deep ocean water; and high levels moisture in the air, according to Weather Underground meteorologist Jeff Masters.

In fact, as of late Wednesday, Irma has had 185 mph maximum winds for 24 hours, which is the longest an Atlantic hurricane has maintained that intensity on record.

Now, as the storm nears the U.S., a trough of low pressure along the East Coast —which could have guided the storm out to sea — has shifted too far west, Klotzbach said.

Weather models have been very consistent that Irma will take a sharp turn toward the north-northwest during the weekend, working its way around the west end of the high-pressure ridge that has been steering Irma all week, Masters said.

As for intensity, meteorologist Ryan Maue said Irma is forecast to track over deep, warm water favorable for Category 5 intensity the next four days on its way toward Miami.

If it did maintain Category 5 status at landfall, it would be in rare company: such a ferocious storm has only struck the U.S. three times in recorded history.

The official forecast from the hurricane center has Irma nearing the U.S. as a still-potent Category 4 storm.

Overall, hurricanes this time of year aren't really unusual: We're now right at the climatological peak of the six-month Atlantic hurricane season. On average, 2.4 Atlantic hurricanes form in September each year, the most of any month.

 

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