The Tampa Bay area in west-central Florida has been spared a major hurricane for nearly a century, but the mayor of the region's largest city, Tampa, predicted Sunday that its time had run out.

"We are about to get punched in the face by this storm. We need to be prepared," Mayor Bob Buckhorn tweeted Sunday. Later he said, "I think our day has come."

The region is flat and flood-prone, and by some estimates it is the nation's most vulnerable area in the event of a hurricane strike. Pinellas County, home to St. Petersburg, is a peninsula off the Florida peninsula and the most densely populated county in the state, with nearly 1 million people surrounded by water. Hillsborough County, home to Tampa, has billions of dollars of development in a watery downtown and artificial islands.

When it was churning toward Florida as a Category 5 hurricane, Irma threatened to bring to life the fictitious hurricane named Phoenix that was modeled by the Tampa Bay Regional Planning Commission to understand the potential impact of a major hurricane. Irma weakened as it approached Tampa Bay and was expected to pass over the area as a Category 1 storm — still dangerous enough to cause flooding and wind damage, and a powerful reminder of the area's risk of catastrophe.

When the last big one hit Tampa Bay in the 1920s, hardly anyone lived there. Now the area has a highly developed coastline and 4 million residents.

Hillsborough County initiated mandatory evacuations for residents of trailer parks on Thursday and stepped them up as Irma approached Florida's Gulf Coast. An estimated 72,000 people are in that county's shelters alone.

People on Davis Islands, two spits of lands created from muck dredged from the bottom of Tampa Bay, were ordered to evacuate, although dozens of residents said they would stay. Tampa General Hospital, one of the largest in the state, sits on Davis Islands, in the middle of the bay, but it did not evacuate.

Hotels on the water near the airport were ordered to shut their doors and move occupants out.

In St. Petersburg, residents of wealthy Shore Acres and other communities on the bay were ordered out, and Pinellas County ordered every single beach community to evacuate. That county is operating 34 shelters, and more than half were full.

At least 60 percent of Tampa homes and businesses will be without power after the hurricane strikes, according to an estimate by Tampa Electric. Tampa issued a 6 p.m. curfew Sunday; St. Petersburg's was an hour earlier.

By then, the safe window to evacuate the city had long passed and officials from the Emergency Operations Center were sending information to residents about the best ways to shelter in place.

"We've asked people to get to know their neighbors, if you don't already," said Jason Penny, spokesman for Tampa Fire Rescue. "We're trying to put out a message of community; we're all in this together. We could use help from about anyone right now."

Just last week, a couple of members of the region's swift water rescue team dispatched to Beaumont, Tex., to help those underwater after Hurricane Harvey. Now, Penny said, they're preparing to do the same at home.

Isabella Sumner was riding out the storm in a condominium in the Channelside section of downtown Tampa. A Colorado native, she has lived in Tampa for two years. Irma is her first hurricane. For nearly a week, the 23-year-old had been intently tracking the hurricane's path, her safety plans shifting each time the storm did.

"If anything, it's just been the anticipation that has been the hardest," Sumner said.

The building is on the edge of an evacuation zone. Sumner said she and her boyfriend and six other people sheltering there were hopeful that the condo's fourth-floor location would keep them away from the storm surge and that the windows, which were built to sustain 130-mph winds, would keep out debris.

She never intended to evacuate because, according to earlier forecasts, Tampa Bay would be touched by Irma's outer bands. But when the hurricane's path began tracking west on Friday, right toward her home, Sumner said she felt it was too late to leave. Roadways in the region are jammed on good days. Evacuating north along with millions of people with little gasoline would be too risky, she said.

"In my opinion, no matter where you are in the state, you're about to get hit pretty hard," she said.

In St. Petersburg, 26-year-old Max Tuten was hunkered down with friends in a condo building just four blocks west of Tampa Bay. They shut themselves inside at 10 a.m. Sunday, after saying goodbye to their favorite haunts — the pizza joint downtown, Vinoy Park on the bay, the marina filled with fancy yachts. They all took pictures, just in case.

"It was a little emotional," Tuten, who works for the Interior Department, said Sunday afternoon.

He contemplated evacuating but said he was too concerned about traffic. The storm's shifting path made preparing to leave more challenging.

"It wasn't worth it to get on the road with people nervous and panicking," Tuten said.

The St. Petersburg City Council chair, Darden Rice, said weeks before the storm that there was only one response to a major hurricane: "Get out of Dodge."

Rice is one of the figures who took heed of chronic flooding in her city and led a drive to adapt to climate change. Pinellas County planners pushed an effort that led to the creation of a climate change advisory panel that issues reports to local governments and a One Bay Resilient Community where governments work together on responding to sea-level rise and other impacts.

"Even when we don't take a direct hit, even when it's a tropical storm or a category 1, the rain it delivers to our city puts enormous stress on our rainwater and sewer collection system," Rice said.

Flooding in Florida will eventually cost the state regardless of whether a hurricane hits it, according to an analysis by a group called Risky Business. In 12 years, the value of property that will be lost to sinking land and rising water will amount to $15 billion. By midcentury, that figure is likely to increase to $23 billion, the report said.

The Phoenix scenario that modeled a Category 5 strike on Tampa Bay projected that wind damage would destroy nearly half a million homes and businesses. About 2 million residents would require medical treatment, and the estimated death toll, more than 2,000, would top the number of people who perished from Hurricane Katrina in Louisiana and Mississippi.

Pinellas County could be sliced in half by a wave of water, the 2010 report said. The main roads and an interstate connecting it to Tampa get clogged with traffic even on a clear day, making mass evacuations a nightmare.

Karen Clark & Co., a Boston firm that analyzes risk, said flooding alone from a major hurricane could cause $175 billion in damage. A World Bank study called the bay area one of the most at-risk places on the planet.

As weather forecast models predicted that Irma would track toward the Gulf of Mexico, putting its eye in position to brush Tampa Bay on the Gulf Coast, Mark Luther packed his bags.

Luther, an oceanographer who knows that a surge from a Category 5 storm could send 15 feet of water into the living room of his house with a dock on the bay, thought his condo 19 stories above downtown was safer. "I spent a whole lot of money to put in storm windows," he said. "We'll see if they work."

Storm surge in Tampa Bay should put fear in anyone's heart. Sea level had risen naturally for centuries, about an inch per decade. But in the mid-1990s, it accelerated, and suddenly residents were noticing street flooding after moderate rains.

It was a combination of things. There were rains, of course, which were getting harder and longer as the skies grew warmer. There were also higher tides. And unseen, there was the creep of water inching into the storm drains, carrying barnacles and junk that clogged them, so that street flooding couldn't easily flow out, leaving water to pool in the streetsfor days.

"You live in a paradise, and that's wonderful, but it has storms," Eugene Henry, a disaster mitigation manager for Hillsborough County, said in an interview two months ago. He said public officials and planners can't help people when a storm such as Irma looms. "If the inevitable monster storm comes, it's not going to keep you safe from 30-foot storm surge," Henry said.

Mark Hafen, a University of South Florida instructor who specializes in planning, said public officials can do more: Prepare for climate change by adapting. To develop their coastlines, municipalities pulled out much of the natural barriers, such as mangroves, that soak up storm surge and reduce flooding.

"The bay's getting higher and the bay needs to go somewhere else. But there's nowhere for the water to go," Hafen said.

Mettler reported from Washington.