It took Hurricane Irma just two days to whip from a ring of angry thunderstorms into a major storm, a “remarkable” strengthening that would come to define its wrathful path through the tropical Atlantic.
In a postmortem released Monday on the fourth hurricane of 2017’s impressive season, National Hurricane Center experts outline the storm’s unpretentious beginning as a tropical wave off Africa to its disastrous Sept. 10 collision with Florida.
It’s a routine reanalysis conducted on all tropical cyclones, a chance to look more closely at details that may get lost in the fog of breakneck forecasting and bustle of an evacuation that had 6 million Floridians looking for an Irma escape route.
On its 13-day journey, Irma made seven landfalls, four of which were at Cat 5 power. It hit Florida twice, first at Cudjoe Key as a 134-mph Cat 4, before skipping up to Marco Island where it landed as a 115-mph Cat 3.
Orlando, where many people fled, felt a Cat 1 hurricane, while tropical storm-force winds extended 360 nautical miles from Irma’s center.
John Cangialosi, a hurricane specialist with the NHC in Miami who co-authored Monday’s report, said no significant forecast changes were made in the 50-page review on Irma, although its maximum wind speed fell from 184 mph to 178 mph.
Still the report reinforces the power Irma held in a red eye that was watched by millions. The National Hurricane Center’s website had more than 7 billion hits attributed to Irma — three times the traffic ever handled by the NHC website during a single storm event.
“Irma was in an ideal environment as far as hurricanes go and it stayed away from major land masses,” said Cangialosi, who slept in a supply closet at the hurricane center while on duty for Irma. “It went over some little islands but they didn’t impact the intensity so it was able to gather strength until it hit Cuba.”
Irma’s 80-mph gain in wind speed over 48 hours is a rate achieved by only about 1 in 30 Atlantic tropical cyclones, according to the report.
Another notable storm stat — Irma maintained Category 5 strength for a stunning 60 consecutive hours.
“Irma didn’t break records in terms of maximum strength, but it’s in second place for how long it sustained Category 5 strength,” Cangialosi said. “Most hurricanes will oscillate more, and while they can get to this intensity, they don’t hold it very long.”
The 1932 Cuba Hurricane, or Santa Cruz del Sur hurricanen holds the top spot for maintaining Cat 5 status at 72 consecutive hours.
Irma also produced 25 confirmed tornadoes, hit Fort Pierce with nearly 22 inches of rain in three days, was directly responsible for 44 deaths, including four in Florida, and sent storm surge as high as 10 feet into southwest Florida.
The strongest wind speed recorded in the Florida Keys was a 120-mph gust on Big Pine Key. A gust of 129 mph was recorded near Marco Island, while a gauge at the Naples Pier registered a 141-mph gust.
While many official National Weather Service gauges failed during the storm, Palm Beach International Airport’s remained operational, recording a 91-mph gust at about 7 p.m. on Sunday, Sept. 10.
Monday’s report also includes a critique of forecast accuracy. It noted that intensity forecast errors were larger than the 5-year average, but that meteorologists did better with the track forecast where errors were 30 to 40 percent lower than average.
“We always call this the worst case scenario for Florida because it’s coming up the state and people were focused on whether it was going to hit Miami or Naples,” Cangialosi said. “It’s a skinny peninsula, and the system was so big, really both sides were going to feel it.”
Warm water, low wind shear and an abundance of moist air helped feed 2017’s storms, three of which are now in the top 5 costliest storms on record. Those include Harvey ($125 billion), Maria ($90 billion), and Irma ($50 billion) .