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Fred, like storms before it, peaked in the Gulf of Mexico. Why is that?

Tropical Storm Fred had sustained winds of 65 mph, strengthening rapidly in a short time period in the Gulf of Mexico.
Infrared view of Tropical Storm Fred as it neared the Gulf Coast of the United States on Monday.
Infrared view of Tropical Storm Fred as it neared the Gulf Coast of the United States on Monday.
Published Aug. 18, 2021|Updated Aug. 19, 2021

High sea-surface temperatures are often what makes the Gulf of Mexico a hotspot for tropical systems to rapidly intensify.

It was the culprit a year ago, when Hurricane Laura rapidly intensified from a Category 1 hurricane to a Category 4 monster storm within 24 hours. It also happened with Hurricane Katrina in 2005. But abnormally high sea-surface temperatures weren’t the main cause for Tropical Storm Fred reaching its peak strength while off the Florida coast.

Instead, USF oceanography professor Robert Weisberg says that the Gulf’s sea-surface temperatures were mostly at or below normal for this time of year — levels that may have even hindered Fred as opposed to strengthening it.

“We tend to key in on sea-surface temperatures because it’s what the media speaks about the most, but it’s far from the only factor,” said Weisberg. “There is nothing special this year about sea-surface temperatures in the tropical Atlantic and Gulf of Mexico.”

Related: One killed in Florida as Fred spawns twisters and flooding in US

So what caused Fred to strengthen by 25 mph in a 24-hour span in the Gulf of Mexico? First off, it was because it was the first time since the open Atlantic Ocean the storm had extended time over open water without land interaction.

Secondly, Weisberg said it was largely because of Fred’s interaction with the Gulf of Mexico Loop Current, which brought water that’s warm beyond just the surface deep into the Gulf from the Caribbean Sea.

The loop current is a flow of warm water that travels through the Gulf of Mexico, past the Florida Keys and eventually up the Atlantic Seaboard. It feeds a current of warm water from the Caribbean, which enters the Gulf between Mexico’s Yucatan Peninsula and Cuba, and tracks north at varying latitudes.

A loop current is partially to blame for Tropical Storm Fred's intensification just before landfall with the Florida Panhandle.
A loop current is partially to blame for Tropical Storm Fred's intensification just before landfall with the Florida Panhandle. [ National Oceanic and Atmospheric Administration ]

As Fred approached over the weekend, Weisberg said the current was as far north as the Mississippi River Delta. This is partly why Fred didn’t begin intensifying until its final 24 hours ahead of landfall despite being in the Gulf of Mexico since Saturday night.

It’s also why Fred jumped from having sustained winds of 40 mph Sunday morning — which was its highest wind speed since forming on Aug. 4 at the time — to have sustained winds of 65 mph when it made landfall Monday afternoon. And, while the 25-mph increase was significant, it’s still not as much as it could have been, according to Weisberg.

“Fred passed to the east of the loop, so it wasn’t even going over the warmest water,” Weisberg said. “We’re fortunate Fred made its turn when it did so it didn’t go directly over the loop current, or else we would have seen even more dramatic intensification probably.”

Also working against Fred was moderate wind shear and dry air, Weisberg said. If it weren’t for these factors, he predicts that Fred could have easily intensified and reached the 75 mph winds needed to be classified as a Category 1 hurricane.

As for how the loop current will affect future storms this season, Weisberg says it’s hard to tell. Unlike the tracking of El Niño and La Niña phenomena in the Pacific Ocean, research into projecting the future strength and location of loop currents is still ongoing. As for now, there is no real way to know weeks in advance where the loop current will be.

“The loop current is always there, the question is, is it following a fairly direct pathway through the Yucatan and out the Straits of Florida, or is it extending some distance into the Gulf of Mexico?” Weisberg said. “What actually controls the penetration of the loop current and when it may cease, that’s something we have no skill at all to predict, and it’s really a shame.”


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