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The Tropics | Hurricane Irma

How Hurricane Irma became such a deadly, devastating storm

Jaclyn Whittal

Tuesday, March 13, 2018, 2:33 PM - The hurricane season of 2017 lived up to its aggressive forecast with above-average activity and the storms that did develop made major landfalls in several Caribbean locations and right here in the U.S.

In total, there were seven major hurricanes, including three that slammed the U.S. – Harvey, Irma and Maria.

The National Hurricane Center has published its Tropical Cyclone Report for 2017's Hurricane Irma - a long-lived Cape Verde hurricane that reached Category 5 intensity. It caused widespread devastation and is one of the strongest and costliest hurricanes on record in the Atlantic basin. We break down the report below.

Irma: Seven landfalls

Hurricane Irma started off the coast of Africa (as many hurricanes do) on August 27th as a piece of energy known as an easterly wave. It didn't take long for it to intensify and track across the Atlantic. When it finally got close to North America, it reared its ugly head. 

The catastrophic hurricane made seven landfalls, four of which occurred at Category 5 strength across the northern Caribbean Islands. 

Irma made landfall as a Category 4 hurricane in the Florida Keys and struck southwestern Florida at Category 3 strength.

Later on August 31st, Irma turned west-northwestward as the ridge to the north of the cyclone weakened a little. Meanwhile, Irma continued to rapidly strengthen, and it reached major hurricane status (winds greater than 185 km/h) by September 1st, only two days later, a substantial increase only achieved by about 1 in 30 tropical systems each season.

Irma fluctuated between Category 2 and 3 strength from September 1st to September 4th. The main causes for the intensity fluctuations were likely eyewall replacement cycles and intrusions of dry air. This happens when hurricanes are well developed and strengthening.

The hurricane reached its maximum intensity of 286 km/h by September 5th, when it was located about 112 km east/southeast of Barbuda. When it finally made landfall it was deadly. A second landfall was not far behind in St. Martin that day, with the same wind speed and pressure.

Irma made its third landfall on the island of Virgin Gorda in the British Virgin Islands on September 6th still as a Category 5 hurricane. The storm started to weaken and the eye of Irma passed just south of the Turks and Caicos Islands on September 8th, and it made landfall on Little Inagua Island in the Bahamas that day at Category 4 intensity. This was the storm's fourth, but not final, landfall.

Why it somewhat weakened when it hit the U.S. Coast

Irma intensified a little more and made its fifth landfall near Cayo Romano, Cuba, on September 9th as a Category 5 storm. It was this interaction over Cuba that spared the U.S. coast from a devastating Category 5 landfall.

It weakened to a Category 4 storm and made it's sixth landfall at Cudjoe Key, Florida on September 10th and a final landfall near Marco Island, Florida, that day with estimated maximum winds of 185 km/h as a Category 3 hurricane.

WATCH BELOW: Hurricane Irma creates new island off Georgia coast

Death and destruction

Irma caused 44 direct deaths in total. Three deaths were a result of its strong winds, heavy rains, and high surf across the Caribbean Islands and the southeastern United States. 

The majority of the causalities were in the Caribbean Islands, where Irma’s winds were the strongest. Eleven direct deaths were reported combined in St. Martin and St. Barthelemy, nine in Cuba, four in Sint Maarten, four in the British Virgin Islands, three in the U.S. Virgin Islands, three in Barbuda, one in Barbados, one in Haiti, and one in Anguilla. 

In the United States, seven direct deaths were reported, and an additional 85 indirect deaths occurred, 80 of which were in Florida. Hundreds more were injured before, during, or after the hurricane. About six million residents in Florida were evacuated from coastal areas.

The 2017 Hurricane season was a busy one with a total of 17 named storms, ranking alongside 1936 as the fifth-most active season since records began in 1851. The season also featured both the highest total accumulated cyclone energy (ACE) and the highest number of major hurricanes since 2005.

BELOW: Science Behind The Weather: Most dangerous part of a hurricane

Stay tuned to The Weather Network for the latest on the 2018 Hurricane Season that begins June 1.

For information on the Hurricane Harvey Summary, click here.

SOURCE: National Hurricane Center

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