Hurricane Irma in Atlantic Ocean
An image of the 600-kilometer-wide Hurricane Irma on 5 September. Credits: NASA/NOAA GOES Project

As residents in Houston set out to recover from last week’s category 4 hurricane Harvey, which dumped more than a meter of rain onto southeastern Texas, a new storm slowly emerged in the eastern Atlantic: Hurricane Irma. The storm formed off the northwestern coast of Africa and drifted across the Atlantic, and by the time it started to threaten the Leeward Islands on 5 September it had strengthened to a category 5 hurricane, with sustained wind speeds at 185 miles per hour (about 298 kilometers per hour).

Scientists have called Irma one of the strongest hurricane ever recorded for an Atlantic Ocean storm outside the Gulf of Mexico or north of the Caribbean. By the time Irma strengthened to a category 5 storm, President Trump had declared states of emergency in Florida, the U.S. Virgin Islands, and Puerto Rico.

Irma’s Path

In the early hours of 6 September, Hurricane Irma’s eye enveloped the Caribbean island Barbuda. The storm brought with it winds approaching 298 kilometers per hour, knocking out phone lines, disrupting communications between islands, and ripping the roofs off buildings. Soon after landfall, barometers and wind speed instruments stopped recording measurements—evidence that they’d been destroyed by the storm. Nearly every building on Barbuda was damaged, and about 60% of its 1,400 residents are now homeless, Antigua and Barbuda Prime Minister Gaston Browne told the Associated Press.

As Wednesday stretched on, Irma moved northwest, causing widespread damage on the islands of Saint Martin, Saint Barthélemy (also known as St. Barts), and Anguilla.

As of Wednesday afternoon, the National Weather Service’s Global Ensemble Forecast System forecast that Irma will most likely make landfall in southern Florida on Sunday. Another model, developed by the European Centre for Medium-Range Weather Forecasts, gave the same trajectory and timing. Some residents have already been ordered to evacuate, especially from the Florida Keys. Governor Rick Scott suspended highway tolls to help expedite evacuation.

By Wednesday night, Irma slammed into Puerto Rico, leaving 900,000 people without power. Fortunately, the island seemed to avoid the most destructive eye wall winds as the hurricane passed by to the north. However, much of Puerto Rico’s capital, San Juan, may be without power for months, its mayor Carmen Yulín Cruz told the Guardian.

Hurricane warnings were in effect for the northwest Bahamas and Cuba since 5:00 a.m. Eastern Daylight Time Thursday, with hurricane watches likely to be issued for the Florida Keys and the Florida Peninsula later today.

The strength of the storm as it moves toward Florida, however, depends on where its swirling eye lands, said Jill Trepanier, a hurricane expert at Louisiana State University in Baton Rouge. Hurricanes gain and maintain strength from the availability of warm water—that’s what keeps the cycling of warm and cool air going that sustains a storm. Irma grew and strengthened as it moved across the warm Atlantic Ocean, where calm winds kept the cyclone spinning and symmetrical. If the eye moves across land, it could weaken the storm by cutting off its access to warm water.

Irma is expected to somewhat weaken in the next couple days but may remain a dangerous category 4 or 5 storm. One thing is clear: Regardless of what track the category 5 hurricane takes, “Florida is going to take a hit,” Trepanier said.

Two Hurricanes, Two Tracks

Texas residents had only 2 days to prepare for Harvey, but Florida residents will have at least twice that. Why did Harvey come as a surprise? Because once the storm that formed Harvey got to the Gulf of Mexico, it had weakened to a “cluster of thunderstorms,” Trepanier said.

Scientists weren’t sure what the cluster was going to do. By 24 August, the Gulf’s warm water managed to fuel the thunderstorms into a full-blown hurricane. Although its status was later downgraded to a tropical storm, hazards from high rainfall persisted, particularly because high-pressure systems surrounding the storm forced it to linger over southeastern Texas. In total, it dumped one sixth the volume of Lake Erie onto Houston and other Texas cities, including Galveston and Rockport.

Irma, meanwhile, is “kind of like this textbook hurricane,” although one of behemoth size, Trepanier said. The storm formed near Africa and drifted across the relatively warm Atlantic Ocean. Vigorous, vertical cycling of warm and cool air creates a tropical depression, which grows as the storm encounters more warm-air fuel. This vigorous cycling creates a tropical storm, then a hurricane once winds exceed about 120 kilometers per hour.

Watch Irma develop as it approaches Saint Martin from 3 September to the morning of 6 September:

YouTube video

—JoAnna Wendel (@JoAnnaScience), Staff Writer


Wendel, J. (2017), Hurricane Irma tears across Caribbean, heads to south Florida, Eos, 98, Published on 07 September 2017.

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