Why Irma drained the water from Tampa Bay

Nobody could believe it. As Hurricane Irma approached Florida, Tampa Bay suddenly went dry. People hopped down onto the bay bottom, now a vast sandy expanse, and walked around, stunned.

There are different terms for what happened: "a negative surge," "a blowout tide," a "water level set-down." Whatever you call it, what occurred in Tampa Bay was one of the five biggest ones ever, according to Texas storm surge expert Hal Needham.

"Tampa has three of the top five," said Needham, who runs what he says is the world's only storm surge blog. The other two Tampa Bay record-setters occurred when enormous hurricanes hit South Florida in 1926 and 1935, he said.

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During the 1910 hurricane made famous by Peter Matthiesson's book Killing Mr. Watson, the water in the Hillsborough River fell 9 feet below mean tide, leaving 40 vessels stranded, noted Evan Bennett, a Florida Atlantic University professor who's writing an environmental history of Tampa Bay.

The counter-clockwise rotation of the strong winds on Irma's leading edge pushed the water out, not just in Tampa Bay but all over the state's gulf coast, according to William Sweet, an oceanographer with the National Oceanic and Atmospheric Administration.

"The Big Blowout," as he called it, happened down in Naples and Fort Myers too, he said. When Sarasota Bay emptied out, it stranded a pair of manatees. It continued up into the Panhandle, where the Pensacola News Journal posted the headline, "Pensacola Bay Is A Giant Sandbox."

To make this happen, a storm has to travel up the Florida peninsula parallel to the gulf coast, Sweet said. Usually, the blowout would be followed, after the eye has passed, by a huge storm surge as the winds switched directions, pushing all the water back in.

But Irma's eye had weakened so much by the time its easterly winds hit the Tampa Bay area that it caused no more than a 2 foot surge above the regular water level, Sweet said. The whole thing played out in just 12 hours' time.

"This is a rare phenomenon," he said, "and it could have swung the other way."

Contact Craig Pittman at craig@tampabay.com. Follow @craigtimes.

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