The storm surge is called dangerous and life-threatening, but what exactly is it?
It's not a wall of water or a tsunami. Simply put, hurricane winds push water toward shore. It can happen quickly and far from a storm's center, inundating areas that don't typically flood.
Storm surge doesn't just come from the ocean. It can come from sounds, bays and lakes, sometimes well inland.
Large hurricanes tend to create greater storm surge over a broader area, and coastal features such as bays can act like funnels and back water up into rivers and canals, said Jamie Rhome, head of the U.S. National Hurricane Center's storm surge unit.
"This is going to sneak up on people," Rhome said.
Forecasters say it kills more people than the strong winds. Florida Gov. Rick Scott said it "will cover your house."
The National Hurricane Center forecasts water levels up to 15 feet (4 meters) above ground for the Florida Keys island chain and parts of the state's Gulf coast, along with up to 25 inches (63 centimeters) of rain in the Keys.
The flooding threat extended far beyond the path of Irma's eye. The Atlantic coast from Miami to Isle of Palms, South Carolina, could see up to 6 feet (2 meters) of storm surge.
WHAT'S AT RISK?
About 1,000 miles (1,600 kilometers) of coastline from Tampa Bay to the mid-South Carolina coast could see storm surge. Much of that landscape lies less than 10 feet (3 meters) above sea level, and the surge from Irma could be a few inches higher in some areas.
Much of Florida's southwest coast is uninhabited swampland, including a large section of Everglades National Park.
"The Everglades won't stop the potential flooding to inhabited areas," Rhome said.
North of the Everglades lies Naples, an upscale town of about 22,000 that is also the home of the Florida governor.
The hurricane center's storm surge maps, showing deep inundation for Naples, worried Colorado State University hurricane researcher Phil Klotzbach.
"Look at Naples, the entire town of Naples is underwater," Klotzbach said. "That is horrible. God that looks awful."
Farther north is the Tampa Bay region, with about 3 million people, a Busch Gardens theme park and baseball spring training grounds for the Philadelphia Phillies and Toronto Blue Jays.
Storm surge has accounted for half the U.S. deaths from hurricanes, tropical storms and cyclones over the last half-century, according to a hurricane center study.
The surge helped destroy nearly half the structures along a 40-mile (64 kilometer) stretch of the Florida Keys during the 1935 Labor Day Hurricane, which killed over 400 people, including World War I veterans working on a railway project.
Storm surge flooding up to 28 feet (8 meters) above normal tide levels were associated with Hurricane Katrina in 2005, directly or indirectly causing at least 1,500 deaths, according to the hurricane center.
Even tropical storms can cause major coastal flooding. Hurricane Sandy lost its tropical characteristics before making landfall in 2012, but its enormous size drove catastrophic storm surge onto the New Jersey and New York coastlines.
A quarter of Florida's population, 6.4 million people, were warned to evacuate low-lying areas.
In Miami, Patricia Magalhaes and her family decided to stay in their high-rise in the waterfront Brickell neighborhood because electricity is typically restored quickly after storms.
"Even if the streets do flood, I have enough supplies for a week without leaving my apartment," she said.
She learned about storm surge's power while living in New York City during Sandy. She was surprised to see cars floating in the streets, and friends living downtown lacked power for weeks.
"I think Miami is more prepared than New York for something like that. I feel safer. We have a plan," Magalhaes said.
Associated Press writers Patrick Whittle in Portland, Maine; Curt Anderson and Jason Dearen in Miami; and Seth Borenstein in Washington, D.C., contributed to this report.