Irma was the eleventh tropical cyclone to form during the 2017 Atlantic hurricane season. It became one of several very intense and destructive hurricanes that season. It was the third hurricane, second major hurricane, and the first of two Category 5 hurricanes during that extremely active year.
Hurricane Irma's track and intensity (colored by Saffir-Simpson category); dates are labeled along the track at 0000 UTC each day. Click for full-size image.
Tropical Depression Eleven formed from an African easterly wave on August 30 just west of Cabo Verde. It strengthened into Tropical Storm Irma six hours later. It reached Category 1 hurricane intensity on August 31, Category 3 intensity on September 1, and then became a Category 5 hurricane on the morning of September 5. Irma reached its lifetime peak intensity of 180 mph and 914 mb early on the 6th just as it was passing directly over Barbuda and St. Martin... it maintained the 180 mph intensity for an incredible 24 hours. Irma spent a record-setting total of 78 hours at Category 5 status.
The previous Category 5 hurricane anywhere in the Atlantic was Matthew (2016) and the next one was Maria (2017) just a couple weeks later. Over the past century, only 2-3% of Atlantic tropical cyclones reach Category 5 hurricane intensity.
15-day infrared satellite animation from GOES-16 spanning August 27 0000 UTC through September 11 1800 UTC. The tropical wave that would become Irma is on the west coast of Africa at the beginning of the animation. Click for full-size animation.
This next sequence of satellite and radar animations covers Irma's path of destruction from September 5-7. When it passed over Barbuda in the early morning hours of September 6, the result was catastrophic. It was estimated that 95% of the structures on the island were destroyed, and after evacuations for Hurricanes Irma and then Jose, the entire island was uninhabited for the first time in 300 years.
Satellite animation from GOES-16 on September 5. Click for full-size animation.
Radar animation from Martinique shows the eyewall passing directly over Barbuda and then St. Martin 5.5 hours later. Click for full-size animation.
Satellite animation from GOES-16 on September 6. Click for full-size animation.
Radar animation from San Juan, Puerto Rico. Click for full-size animation.
Satellite animation from GOES-16 on September 7. Click for full-size animation.
Finally, early on September 8, Irma "weakened" to a Category 4 hurricane as it passed between eastern Cuba and the southern Bahamas. It regained Category 5 intensity just before making landfall on Cuba's north-central coast. With peak sustained winds of 165 mph at landfall, Irma was the strongest hurricane to ever hit that part of Cuba, and tied the 1924 Cuba Hurricane for the strongest landfall anywhere in Cuba.
Radar composite animation from Cuba. Click for full-size animation.
That scrape with northern Cuba weakened the hurricane considerably... it left Cuba late on September 9 as a Category 2 hurricane with peak winds of 110 mph. However, once over warm water again, Irma quickly strengthened back to a Category 4 hurricane as it made landfall near Cudjoe Key, Florida with peak sustained winds of 130 mph on September 10 at 9:10 am EDT.
31-hour radar animation from Key West FL. Click for full-size animation.
37-hour radar animation from Miami FL. Click for full-size animation.
Irma made a final landfall on mainland Florida near Marco Island as a Category 3 hurricane with 115 mph sustained winds on September 10 at 3:35pm EDT. From there it traveled north along the Florida peninsula and weakened to a tropical storm by the time it reached northern Florida and Georgia on September 11.
Not surprisingly, Hurricane Irma was exceptional for the amount of Accumulated Cyclone Energy (ACE) it generated during its lifetime -- ACE is a common metric that accounts for duration and intensity of a tropical cyclone. During its 13-day stretch as a named storm, Irma accrued 64.9 ACE units which is more than the entire seasons of 2015, 2013, 2009, etc had. The only Atlantic hurricane that generated more ACE than Irma was Ivan in 2004 (70.4). Furthermore, it accrued an incredible 9.61 ACE units in a 24-hour period, the second-highest for any Atlantic hurricane on record. Hurricane Allen (1980) was slightly higher at 9.63 ACE units in 24 hours. Dorian (2019) is up there at 9.47 in a day.
72-hour radar composite animation covering the southeast US. Click for full-size animation.
Irma was also a large hurricane, and that helped it produce significant storm surges along almost the entire Florida coastline, and even up into Georgia and South Carolina. The Big Bend portion of the Florida coast benefitted from having mostly offshore winds, but had the track been slightly further west, the outcome would have been very different there.
The map below shows an estimated surface wind swath. From this, it's easy to see how many areas were impacted by Irma's hurricane-force winds -- and notice that the metropolitan southeast coast of Florida including Miami and Fort Lauderdale narrowly avoided hurricane conditions.
Surface wind swath, colored by Saffir-Simpson category. Hurricane size and intensity parameters come from HURDAT2, and wind speed values over land are a simple 10% reduction of values over water. Click for full-size image.
To illustrate how fortunate the Miami area was with that narrow miss, this was the NWS gridpoint forecast for downtown Miami made on September 7, just 72 hours before landfall. This forecast had an upper-end Category 4 hurricane passing directly over Miami, with gusts to 160 mph at 8am and 11am on September 10 as a large, slow-moving eye moves over the city. The eyewall ended up missing downtown Miami by just 75 miles.
These next two images show the National Hurricane Center forecasts of track and intensity for Irma. The thick black line is the observed position/intensity, and the colored lines are the individual five-day forecasts made every six hours.
This storm caused 52 fatalities and nearly $77 billion in damage (2017 USD). It devastated several Caribbean islands and triggered a large-scale evacuation of over six million people from Florida.