What does a king tide in Tampa Bay mean for Idalia’s storm surge?

Tampa Bay will see the highest tides of the month just as Tropical Storm Idalia, which is projected to develop into a major hurricane, makes landfall.

Wednesday is expected to bring the next full moon and a king tide, which only comes once or twice a year, according to the National Weather Service.

That means storm surge from Idalia could reach about 1 or 2 feet higher than under normal tide conditions.

King tides occur when the Earth, moon and sun align to generate the greatest tidal effects of the year, according to the U.S. Environmental Protection Agency. These peak-high tides also offer a glimpse into how sea level rise will affect coastal areas in the future.

Tides are just one factor in storm surge, but researchers say this especially high tide could mean the Tampa Bay area will see increased flooding brought by Idalia.

The hour that Idalia delivers its peak storm surge to the Tampa Bay area could signal just how severe flooding will be, according to Jeff Masters, a hurricane scientist formerly with NOAA. If the timing of peak surge overlaps with the king tide, storm surge levels could reach a record high.

”If peak surge happens to come in at low tide, and you get a 4-foot surge, maybe it’s only going to be a foot above normal,” Masters said. “But if it comes in at a high tide, 4-foot surge now is going to be record-setting.”

The current highest surge record for St. Petersburg was set during Hurricane Elena in 1985 — 4 feet above high tide. On its current track, Idalia’s storm surge is poised to break this record, experts say.

For instance: The difference between high and low tide in St. Petersburg is roughly 3 feet, 2½ inches. In East Bay, that difference is about 3 feet, 6 inches. The National Hurricane Center is now predicting more than 4 feet of surge.

The peak-high king tide is expected to arrive in St. Petersburg just before 2 p.m. Wednesday. Forecasters predict Idalia will arrive sometime that same day.

The Tampa Bay area risks “dangerous storm surge and heavy rainfall,” regardless of the storm’s projected landfall, according to Ali Davis, a meteorologist with the NWS office in Tampa Bay.

Storm surge is an abnormal rise of water brought ashore by strong winds from a hurricane or tropical storm. The highest surge typically occurs near the strongest hurricane winds, according to the National Oceanic and Atmospheric Administration.

A 12-foot surge was responsible for the majority of deaths during Hurricane Ian last year.

But storm surge is notoriously difficult to predict, according to Brian McNoldy, a senior research associate at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.

“It’s very easy to be wrong,” he said.

The storm’s angle of approach, the shape of a coastline and the speed of a storm are all important factors, according to NOAA.

A storm that meets the shore head-on, perpendicular to the coast, can produce a higher storm surge than one that moves parallel to the coast.

Idalia is currently moving at 8 mph and is expected to double in speed as it approaches land.

This increasing speed of the storm may keep storm surge in check, said Gary Mitchum, a marine science professor at the University of South Florida.

“It’s looking to me that even with the high tide that we’re not going to be talking about catastrophic storm surge,” he said.

Mitchum said he is more concerned by what the tides could mean for rainfall and flooding. Idalia could drop up to 10 inches of rain in the Tampa Bay area. High sea levels would hinder rainwater draining out into the bay.

“So, you have a double effect there,” he said.

Experts warn storm surge brings a much higher risk of prolonged flooding on the mainland than on barrier islands.

“Storm surge can get a lot worse when it’s piled up onto land,” McNoldy said. “It has nowhere else to go, so it just keeps getting higher and higher.”

Meteorologists are already taking into account these high-tide values in their forecasts.

Davis said the public should know that a surge prediction like “4 to 7 feet” includes the added effects of tide levels.

“People need to stay alert, listen and listen to any evacuation orders that come from their local officials,” Davis said. “People still need to pay attention, even if this is making landfall north.”

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