Delta went from tropical storm to Cat 4 hurricane in 24 hours. How did that happen?

Mary Perez
·4 min read

Hurricane Delta strengthened from a tropical storm to a Category 4 hurricane with 140 mph in 30 hours, making it the most rapidly strengthening tropical storm in Atlantic basin history since Hurricane Wilma.

The process is called rapid intensification, and Delta could get stronger yet.

NOAA defines rapid intensification as an increase in the maximum sustained winds of a tropical cyclone by at least 34.5 mph in a 24-hour period. Hurricane Delta increased by 63 mph in 24 hours, according to the National Hurricane Center, going from a Category 1 on Monday to a Category 3 on Tuesday morning, with wind speeds of 115 mph. By 4 p.m. Tuesday, the wind speed was 145 mph.

“Once Hurricane Delta moves into the Gulf of Mexico, we will see rapid intensification. A brief period of Category 5 may not be out of the question,” said longtime Gulf Coast meteorologist Rocco Calaci said in his daily weather newsletter.

The Weather Channel’s Jim Cantore and others compared it to Hurricane Wilma in 2005. He tweeted Tuesday, “This is Wilma type rapid intensification!!”

In one day, Oct. 19, 2005, Hurricane Wilma strengthened from a Category 2 to the most intense Category 5 hurricane on record, according to the NHC.

Cantore also said Monday that Gulfport has been in the cone of probability for hurricanes six times this year.

Delta gains strength

“Yesterday, the models expected Delta to become a hurricane by Wednesday and only reach Category 2 status,” said Calaci. “Well, that was definitely wrong! Delta became a hurricane last night and is already a strong Category 2 hurricane and will definitely hit category 4 by tomorrow afternoon.”

On Tuesday, he said anxiety is building along the Gulf Coast as Hurricane Delta moves into the Gulf of Mexico — “and it’s going to be a monster.”

Powerful thunderstorms circled the eye of the hurricane and southern quadrant as it moved through the Caribbean Sea on Tuesday.

Environmental conditions are prime for intensification, with low vertical wind shear, deep warm waters and sufficient mid-level moisture that are expected to support additional rapid intensification through today, the NHC report said.

It’s the earliest landing for a 25th named storm of the season and the first landfall of a storm named from the Greek alphabet.

Hurricane Delta timeline

Oct 4 — The cyclone formed over the Central Caribbean at 3 p.m. Sunday. Six hours later it was classified as Tropical Depression #26.

Oct. 5 — By 7 a.m. the depression strengthened into Tropical Storm Delta. It becomes a hurricane Monday night.

Oct. 6 — Delta had wind speeds of 63 mph at 5 a.m. that increased to 115 mph by the 10 a.m. report from the National Hurricane Center. Delta became a Category 4 storm, according to a 10:20 a.m. update from the National Weather service in New Orleans, with wind speeds of 130 mph. The speed had intensified to 140 mph by the 2 p.m. report.

Forecast — It’s first target is forecast to be the Yucatan Peninsula in Mexico.

“Some reduction in intensity is likely when Delta moves over land, but the environmental conditions over the southern Gulf of Mexico are expected to support re-strengthening, and the NHC intensity forecast shows a second peak in 48-72 hours,” the report said.

Delta is forecast to make a second landfall Friday or Saturday along the northern Gulf Coast. Wind speeds are expected to weaken when Delta meets increasing southwesterly shear and cooler shelf waters near the coastline, but it is still expected to be a dangerous hurricane when it makes landfall.

Accoridng to the Saffir-Simpson Hurricane Wind Scale:

  • Category 1 - 74 to 95 mph winds

  • Category 2 - 96 to 110 mph winds

  • Category 3 - 111 to 129 mph winds

  • Category 4 - 130 to 156 mph winds

  • Category 5 - winds of 157 mph or higher

Delta compared to Laura and Sally

South Mississippi Coast saw Hurricane Laura go to the west into Louisiana, and Hurricane Sally last month make landfall near the Alabama-Florida border.

Hurricane Laura made landfall in Louisiana on Aug. 29 — 15 years to the day after Hurricane Katrina — and was the last storm to see rapid intensification.

Laura went from a Category 1 with winds of 75 mph to a category 4 with winds of 140 mph in 24 hours.

In 2018 Hurricane Michael jumped from Category 2 to Category 5 in one day before hitting the Florida Panhandle.

The major difference between Hurricane Delta and Hurricane Sally that dumped more than two feet of rain on parts of Florida is Delta is speed. Delta is moving much faster, with the latest report showing Delta has increased forward speed to 16 mph, while Sally was barely moving at 2 mph before it made landfall.

Forecasters say a faster moving storm will bring less rain and less damage when it makes landfall compared to Sally.