. FEMA estimates about 2.5 million Florida homes are in flood-hazard zones and many of those homes are in the densely populated Miami-Dade and Broward counties where more than 4.5 million people live. That includes the cities of Miami and Fort Lauderdale.
One of the best ways to understand the threat to low-lying areas is to see them from above. Veteran helicopter pilot Paul Barth took CBS News' Jeff Glor on a tour of the Miami coastline that's under theBarth witnessed the devastation firsthand the last time a .
"This is a low area here, not very much but two or three feet above sea level, so if we have a strong storm surge there's not much to protect it from the coastline moving in," Barth said. "There's no barrier island on this side, this mouth of Key Biscayne there, so it's wide open to the Atlantic Ocean."
Just north of Miami, the Broward County emergency operations center is already running full tilt with members from every county department on hand.
Broward County Mayor Barbara Sharief says her biggest worry right now is storm-surge flooding and with residents complying with evacuation orders.
"So our concern is that we're going to have to try and rescue people or we may have fatalities so right now this preparation process is to minimize that," Sharief said.
Mayor Sharief says Broward County is ready for whatever Irma brings their way.
She warns people who decide to ride out the storm that once winds reach a sustained level of 45 mph, the county cannot send emergency personnel in for them. Winds from Irma will likely be three times that speed.