Why did Hurricane Ian drain Tampa Bay ahead of storm surge?

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As Hurricane Ian arrived in the Tampa Bay area, the water drained from the bay and created the eerie sight of an empty basin amid the normally bustling harbour.

Officials warned people not to explore the bay, as the waters would come back even higher than before as the storm surge begins later today.

“Do not walk out into receding water in Tampa Bay or Charlotte Harbor - the water WILL return through storm surge and poses a life-threatening risk,” the Florida Division of Emergency Management tweeted.

Photos from the Tampa Police show a muddy bay floor stretching out from the coast, clumps of seaweed piled up on dry land and water levels were clearly well below normal.

A meteorologist with the National Weather Service told the Tampa Bay Times that this is called a “reverse storm surge.”

Just as strong winds can push waters in towards land, creating dangerous storm surges that flood the coast, winds in the opposite direction can pull waters out from land, draining the coast.

A similar phenomenon was also spotted south of Tampa this morning, in the city of Venice, Florida.

The hurricane is forecast to push those waters back with a vengeance. The ocean is forecast to rise up to six feet (two metres) above normal levels in Tampa Bay, including around the cities of Tampa and St Peterburg, as Hurricane Ian sweeps through the area.

Originally, the storm was predicted to have a much higher surge in the area — but as the forecast moved south, the peak surge forecast for Tampa Bay dropped.

Now, areas south of Tampa are expected to see up to an 18 ft (5.5 m) storm surge during the peak of the hurricane, creating “life-threatening” conditions, according to the National Hurricane Centre (NHC).

Anything below high water levels will be inundated along the coast near Port Charlotte, Cape Coral and Fort Myers.

A similar “reverse storm surge” was seen during 2017’s Hurricane Irma, the Times reports. That storm pulled tons of water offshore to empty the bay but had weakened enough by the time the waters would have surged back, so it only created a two-foot (0.6 m) surge, the paper adds.

Hurricane Ian has strengthened to a near-Category 5 storm. Officials have warned people to prepare or evacuate if they live in vulnerable areas — but in many places, evacuation may no longer be safe as hurricane conditions set in.

The storm is the most dangerous hurricane to hit the mainland US this year, and the second hurricane to damage the country this month after Hurricane Fiona smashed into Puerto Rico.

As the climate crisis grows, these kinds of storms may only get more common. Warmer ocean waters can supercharge a storm, adding tons of rain and wind power to a hurricane — sometimes very quickly, as happened this week with Hurricane Ian.

As of Tuesday, the storm was a Category 3 but has reached near Category 5 on its approach to land.

A United Nations climate science panel has found that the percentage of tropical cyclones reaching Category 3 or higher has been increasing over the past 40 years.