It didn't take long for Tampa-area residents to flock out onto the bay after it went dry Sunday afternoon. Some thought it would be neat to forgo the use of a bridge. Others were left scratching their heads at the eerie phenomenon. Other instances of drained beaches were reported up and down the Gulf Coast, particularly near Marco Island, Naples and Fort Myers, but also as far away as Mobile Bay.
Many people were wary of the sudden tidal shift, thinking back to the Christmas Day 2004 Indonesian tsunami, when an offshore magnitude 8.7 earthquake caused seas to virtually disappear within a matter of minutes. But the curious spectacle was short-lived as an endless surge of water raced onto land.
Seas towered to 30 feet above normal in that tsunami, wiping out entire coasts and killing 150,000 people. Experts have long used this event to offer a stark warning: If the water suddenly disappears from a shoreline, run — fast.
Sunday's storm surge in reverse was not the result of plate tectonics, but rather caused by the exceptional pull that Irma had on the oceans. Areas near its center that first saw an offshore wind watched as all their water was gobbled into Irma's middle, before returning in a damaging storm surge when the winds shifted with the passage of the eye.
Just how dramatic was the effect? The tidal gauge in Naples — where the wind gusted to 142 mph — fell six feet between sunrise and 2 p.m. Sunday. Within one hour, the water climbed 5.5 feet (approximately one inch per minute) and continued to rise.
The acting director of the National Hurricane Center, Ed Rappaport, released a statement at 2:15 p.m. reading "urgent… water levels from storm surge to rise rapidly in the Naples and Marco Island area." He stated that these effects would occur "in a matter of minutes."
A storm surge warning had already been in place, but the National Weather Service office in Miami made the unusual decision to issue a flash flood emergency tailored specifically for storm surge, which triggered the Wireless Emergency Alert System at 3:12 p.m.
This set people's cellphones into a frenzy, prompting action. The alert, hoisted for Southwestern Collier County, demanded that residents "evacuate vertically NOW!"
This isn't the first time that weather has caused unusual tidal motions. The unusual ocean activity Sunday was due not only to Irma's wind field, but also to a consequence of the pressure difference. In regimes of high pressure, the atmosphere above weighs more than beneath a low pressure center like a tropical system. The lesser force under Irma let water bulge upward, since there was less downward force than elsewhere along the Gulf Coast.
Pressure differences can sometimes result in such tidal seiches. The term is used to describe weather-driven fluid motions that can affect bays, lakes, rivers, or other smaller bodies of water. They're relatively common on the Great Lakes, but normally occur on the order of a few inches or a foot in magnitude.
Sunday's backward surge was rare, but strangely enough, they've happened in Naples before – but due to a separate set of circumstances on a smaller scale which created a phenomenon known as a meteotsunami – in which rapid changes in barometric pressure cause the displacement of a body of water.
In the early morning of Jan. 17, 2016, a line of severe thunderstorms blew across the Florida peninsula, dropping four tornadoes in the Sarasota area and scouring Naples with straight-line winds measured at 84 mph. The leading edge of the line of storms plowed a five-foot-high wall of water ashore just after 5 a.m., catching many beachside residents off-guard. Fortunately, no major damage resulted.
As Sunday's wicked weather taught us, the ocean doesn't play games. Always heed the advice of local officials, and if something about the water ever just doesn't feel quite right, get away — immediately.
Correction: The original version of this post incorrectly called the reverse storm surge that occurred along the southwest and west central coast of Florida a "meteotsunami", whereas that term typically refers to retreat and subsequent advancement of the sea at a much more localized scale.