"We know that we are ground zero for Hurricane Irma," Tampa Mayor Bob Buckhorn said at a Sunday morning news conference. "We have for 90 years avoided this day, but I think our day has come."
With about 3 million people, the Tampa Bay metropolitan area is the second-most populous in the state.
Buckhorn said areas along the Tampa Bay shoreline could expect a dangerous storm surge
anywhere from 3 to 8 feet, adding that it would depend on where Irma went.
"What we really fear more than anything is that storm surge," Buckhorn told CNN's Anderson Cooper on Sunday.
CNN meteorologist Judson Jones said the severity of the storm surge depended on which side of the storm Tampa was on.
"If Tampa stays on the east side of the eye, we expect the storm surge to be worse than if the eye moves inland and the west side of the storm hits it," he said.
That's because wind from the east side of Irma would be blowing toward Tampa, pushing water from the bay inland and raising the height of the storm surge, he said.
Late Sunday, as the storm moved up Florida's west coast, officials said 28,000 people had taken refuge in 45 county shelters in Hillsborough County, which includes Tampa.
There's room for more, County Administrator Mike Merrill said, but he urged people to stay put for now.
"We're not saying get in your car and go," Merrill said. "This is the time to stay where you are. If you're in shelter or other safe location -- stay there. If you're outside, get inside and shelter in place."
He said first responders and law enforcement officers have been pulled in until the weather lets up. St. Petersburg Fire and Rescue has also suspended emergency services due to dangerous weather conditions.
As of Sunday night, 112,759 customers for Tampa Electric had lost power, the utility said on its website.
Storm shifted course
As anxious Florida residents watched weather forecasts this week, it seemed clear Hurricane Irma
was on track to strike Miami, Fort Lauderdale and other Atlantic coast cities, prompting a massive evacuation
But on Saturday, all that changed when Irma's course shifted.
By Sunday morning, the Tampa-St. Petersburg area, once thought to be relatively safe from harm, suddenly found itself in the storm's crosshairs.
Forecast models had anticipated Irma turning north earlier than it did, Jones said. That would have put the hurricane over the east coast of Florida.
"What happened was the storm continued -- after interacting with Cuba -- to move west-northwest," Jones said. "It took longer to make that turn to the north that we started to see more over the last 12 hours."
Jones warned that some Floridians "may not have power for a number of days, if not weeks."
Irma's threat prompted Tampa officials to order a 6 p.m. curfew for Sunday evening. Neighboring St. Petersburg announced a 5 p.m. curfew.
While considering how the city would react, Buckhorn quoted boxer Mike Tyson, saying, "Everybody has a plan until they get punched in the face."
"Well," he added, "we're about to get punched in the face."
"I will tell you in no uncertain terms -- and I am not going to sugarcoat it -- this is going to be a difficult storm," Buckhorn said at the news conference. But he emphasized that Tampa is prepared.
"So look out for your neighbors, take care of each other," he said. "This is when we are Tampa strong. This is what we do."