Retreating water alarms observers.

Suddenly, the water went away. In the Bahamas, in Tampa Bay and in Naples, observers were shocked to see the waters that usually lap against the shore recede into the distance.

On social media, people reacted with incredulity, noting that the water had disappeared where whitecaps were just hours before on Sunday in Tampa Bay. James Spann, an Alabama meteorologist and weather blogger, reacted sternly to a photograph on Twitter of people playing in the sand exposed by the retreating water.

“The water will come rushing back with a vengeance,” Mr. Spann said on Twitter. “They won’t have time to get out when it begins.”

On Twitter, Gov. Rick Scott issued an urgent warning to stay away from the water. “DO NOT GO IN. The water will surge back & could overtake you.”

Chris O’Donnell, a reporter with The Tampa Bay Times, later reported that the police had cleared people off of the shore well before the water came back.

The phenomenon of water being drawn off by the power of Hurricane Irma is known as a negative surge. As Mr. Spann warned, that odd condition will not last — and will become dangerous. Michael R. Lowry, a scientist with the University Corporation for Atmospheric Research, a nonprofit education consortium, explained in a series of tweets that staying away from the water in this storm is important because the hurricane is sending a dangerous surge ashore: 10 to 15 feet, for instance, in Naples, Fla.

And as the National Hurricane Center explained, the water will come rushing back to Naples after the eye passes. On Twitter, the center warned: “MOVE AWAY FROM THE WATER!”

One reporter writes from the center of the storm.

Henry Fountain is a Times reporter who was in Fort Myers on Sunday.

Irma’s center passed over Naples at about 5 p.m., the hurricane center said, and continued up the Gulf Coast. For the better part of an hour as it neared Fort Myers, winds that earlier had seemed to fluctuate wildly became far steadier and stronger, spitting rain violently.

Only the sound varied — at times a muffled rattling, other times a rumbling roar.

Then shortly after 7 p.m. the winds began to wind down and the rain stopped hitting with such force. Within about 10 minutes there was almost no rain at all, and the palm fronds that moments before were streaming horizontally were now waving in a light breeze. Even though dusk was approaching, the sky lightened considerably.

The center of Irma had arrived.

It stayed that way for about 20 minutes, long enough for a few people at one Fort Myers hotel to sneak outside, marvel at the calm, and breathe in the tropical air.

Then the wind started to pick up again, blowing in the opposite direction.

Authorities in the Tampa Bay area brace for the storm’s arrival.

The city of Tampa had called its police officers off the streets shortly before 5 p.m. as consistent winds of 40 miles per hour started to pummel the city. “Let’s hope the wind gets to a point where we can be out,” Mr. Buckhorn said. The Tampa Bay area is the largest metropolitan region in Hurricane Irma’s path in Florida.

Officials closed the westbound lanes on two of the three bridges over Old Tampa Bay that connect Tampa with Pinellas County, the Gandy Bridge and the Courtney Campbell Causeway. Pinellas County includes St. Petersburg and Clearwater.

By Sunday afternoon, more than half of the 45 shelters in Hillsborough County, which includes Tampa, were full, including a shelter for people with special medical needs that had sprung at the USF Sun Dome, an arena at the University of South Florida.