With $175 billion in potential losses, Tampa Bay is the most vulnerable metro area in the nation to storm surge floods caused by a once-in-a-century hurricane.

That's the conclusion from a recent study by Karen Clark & Co. (KCC), a Boston-based firm that specializes in modeling the likely property damage and losses from windstorms, earthquakes and other catastrophes for the insurance industry.

Analysts estimate Tampa could suffer higher losses than New Orleans or New York City, the next two most vulnerable cities on the KCC list.

Sobering news, but no surprise to local officials familiar with similar assessments, including from the National Hurricane Center.

"If we have a Cat 3 storm come up through Hillsborough Bay, where you're standing now would be 15 feet underwater," Tampa Mayor Bob Buckhorn said in an interview on the plaza outside City Hall. "We all know that. …

"We as a city can train to prepare for evacuation, and we can train to prepare for the cleanup, but we can't control anything in the middle."

Of the eight most vulnerable cities in the United States, KCC's analysis put four in Florida, three on the Gulf Coast: Tampa; Miami (No. 4, with $80 billion in potential losses); Fort Myers (No. 5, with $70 billion in potential losses); and Sarasota (No. 7, with $50 billion in potential losses).

KCC analysts looked at the potential damage from a 100-year hurricane for the study. That's a storm for which there's a 1 percent probability in any given year. Thus, this type of storm would be expected to take place once every 100 years on average (though it could occur more often, too). Also, the firm noted, the 100-year hurricane does not equal the worst-case scenario. Even more catastrophic hurricanes could occur, though the chances that they will are smaller.

Several factors make Tampa especially vulnerable to a major hurricane, according to the analysis.

First, the continental shelf off the west coast of Florida is wide, shallow and gently sloping toward the shore. As a hurricane spins above the ocean, its cyclonic winds push the water. Their effect is not as great in deep water, but when a hurricane moves over shallower areas closer to shore, it piles up a huge surge.

Also, KCC said, Tampa Bay creates a "large funnel" for storm surge, especially if the hurricane's maximum winds are near the mouth of the bay. When strong winds force water into narrow channels, it has nowhere to go but up. Storm surge models for a major storm show 10 feet or more of water covering much of the South Tampa peninsula, as well as large parts of northwest Hillsborough and Pinellas County.

"A severe storm with the right track orientation will cause an enormous buildup of water that will become trapped in the bay and inundate large areas of Tampa and St. Petersburg," the KCC study said. "Fifty percent of the population lives on ground elevations less than 10 feet."

• • •

For Tampa, the 100-year hurricane would be expected to be a strong Category 4 storm with peak winds of 150 mph — much worse than anything the bay area has seen in living memory.

The region's last direct hit from any hurricane was in 1946, when a Category 1 storm came up through the bay.

The last time a major hurricane hit the Tampa Bay area was 1921. In that Category 3 storm, waters rose 10 feet, flooding mansions on Tampa's Bayshore Boulevard.

To find the kind of storm surge that KCC models for Tampa from a once-in-a-century storm, you have to go back more than 150 years to Sept. 25, 1848.

That day, what became known as the Great Gale of '48 arrived at Tampa's Fort Brooke about 4 p.m. In 15 minutes, the tide rose 5 feet.

Soon, a storm surge 15 feet above the mean low-water mark submerged what is now downtown. A house floated across Tampa Street, lodging in some trees. Another house rolled end over end into the Hillsborough River and floated into the bay.

At 6 p.m., a man named Ferris climbed a tree near what is now Lykes Gaslight Square Park. He saw the tops of trees and ruined buildings, but no dry land.

Writing to her sister after the storm, one pioneer said, "if you could see, you might well say Tampa is no more."

• • •

Today, officials said, the damage would be just as profound. In downtown, Buckhorn said, too many office buildings still have data processing, electrical and air conditioning on lower floors where they would be knocked out in a storm-driven flood.

That threat is why Tampa General Hospital moved its electrical, air-conditioning and other critical systems to floors 25 feet above ground and installed "submarine doors" with inflatable bladders that create water-tight seals below.

During Hurricane Elena in 1985, the hospital on Davis Islands had to evacuate patients. It ended up turning to the 301 Truck Stop for tractor-trailers to help with the move, hospital spokesman John Dunn said.

Today the hospital is much bigger, with 800 or more patients on a given day, and they're sicker. So the hospital has invested in hardening the facility and creating emergency staffing plans so that a storm surge of even 10 or 15 feet would not knock it out of commission.

"We've gone so far as to have MREs stored in our warehouse to feed people," Dunn said. "We're trying to make this as invulnerable as we can."

But, he added: "I don't want to have to test that."

Emergency management officials says the enormity of the region's storm surge risk makes it all the more important for bay area residents to plan and be ready every year.

While mobile homes get evacuated because of wind, evacuation for virtually everyone else is all about storm surge.

"Storm surge equals evacuation," Pinellas County emergency management director Sally Bishop said. To help residents have a clear and personal understanding of the potential for storm surge, the county last year launched a website — egis.pinellascounty.org/apps/stormsurgeprotector/index.html — that can give residents a look at how vulnerable their homes and neighborhoods would be to rising water.

In a Category 4 storm — the magnitude KCC's analysts considered — Pinellas would be looking at evacuating 528,000 out of 948,000 residents. Hillsborough's numbers would be similar.

"We want our citizens to understand that when we tell them to prepare, we're serious," Hillsborough emergency management director Preston Cook said.

That means knowing where you're going to go — in Pinellas, that might entail going no further than a high non-evacuation zone like Countryside — and not waiting too long to leave, Bishop said.

Still, she is shocked to hear residents of barrier islands say they have storm shutters, so they won't be going anywhere.

"If your evacuation plan is to get plucked off your roof," she said, "you don't have an evacuation plan."

Contact Richard Danielson at rdanielson@tampabay.com or (813) 226-3403. Follow @Danielson_Times