Dangers of Idalia's storm surge projected to extend into Wednesday afternoon's king tide

A rare blue supermoon could raise tides above normal, exasperating the flooding dangers of Hurricane Idalia’s storm surge in the greater Tampa Bay region and the rest of Florida’s Gulf coast.

The moon will be closest to the Earth on Wednesday as Idalia is expected to make landfall as a powerful Category 4 storm in the Big Bend of Florida. While a supermoon can make for a spectacular backdrop in photos of landmarks around the world, its intensified gravitational pull also makes tides higher.

The effect is known as a king tide, and Gulf coast will experience two on Wednesday as Idalia’s storm surge pounds the coast. The first occurs overnight while Idalia is still churning in the Gulf. But officials are warning residents from the Big Bend to Tampa Bay that the biggest danger could come from the afternoon high tide.

In Citrus County, officials are expecting significant flooding, particularly in Homosassa and Kings Bay in Crystal River, just before 5 p.m.

Citrus County Sheriff Mike Prendergast told Bay News 9 that there will be major waterway intrusion during this time. Deputies already are planning to shut down multiple trails heading toward the beach.

“We're going to see, you know, not only the potential for tornadoes, but that water being stacked up in our bays and along our rivers for a very long time into the evening hours,” Prendergast said. “And that's going to be the most disastrous time for the people to be back out there.”

He said some Citrus County residents will remember fast-rising flood waters after Hurricane Hermine in 2016. He is hoping newer residents take these warnings seriously.

This king tide will have similar effects up and down the coast, with record-setting storm surge a possibility. For example, the peak high king time is expected in St. Petersburg about 2 p.m.

“I would say the timing is pretty bad for this one,” said Brian Haines, the meteorologist in charge at the National Weather Service office in Charleston, South Carolina.

The storm surge is often the greatest killer when hurricanes strike. The ocean water pouring onto land could be up to 15 feet (4.6 meters) in the Big Bend, the National Hurricane Center projected. Farther south, up to 7 feet (2.1 meters) of storm surge is expected in the Tampa Bay area.

Storm surge that can be taller than a person is a concern with any major hurricane. The tides and the influence of a supermoon can increase that.

"There’s a saying that you hide from the wind and run from the water, and hopefully people are heeding that advice,” said Brian Tang, associate professor of atmospheric science at University at Albany in New York.

The part of northwest Florida that could be hit by Idalia is especially vulnerable to storm surge because of the region's geography. The continental shelf extends so far out from the coast and has a gradual slope, allowing the ocean to grow higher as hurricane winds drive the water onto land, Tang said. The shape of the coast in that region is also curved inward, which can focus the storm surge to make it even more dangerous, he said.

When the moon is full, the sun and the moon are pulling in the same direction, which has the effect of increasing tides above normal ranges, said Kerry Emanuel, professor emeritus of atmospheric science at the Massachusetts Institute of Technology.

The moon's gravitational pulls are even stronger when it's closer to Earth, so the tides are even higher.

In South Carolina, there’s concern that Idalia’s path will take it near the historic city of Charleston and the surrounding area known as the Low Country. That would add water to the high tide that’s in the forecast, Haines said.

“Wednesday evening looks really nasty for coastal flooding here,” he said.

The weather service is forecasting an 8.2-foot (2.5 meter) tide in Charleston Wednesday evening, which could produce widespread flooding in downtown Charleston, Haines said. Even with a 7.5 foot tide (2.3 meters), some roads in the city flood and become impassible, he said.