Risk modeler Eqecat says Isaac's pace and size “poses a significant threat to the Gulf Coast.”

“Should it occur during the time of high tide, a surge of 6-12 feet is possible in southeastern Louisiana, Mississippi and Alabama,” the modeler adds.

Because Isaac is headed for New Orleans, the storm has evoked memories of Hurricane Katrina—which made landfall on Aug. 29, 2005—but Katrina was a Category 3 hurricane that brought surges of 15-25 feet.

And since Katrina, the Army Corps of Engineers constructed levees around New Orleans to protect it against a 100-year storm. Eqecat says the defense system is “expected to hold back surge from Isaac in the New Orleans region.”

Tom Larsen, senior vice president at Eqecat, says location also plays a role. A landfall slightly to the west of New Orleans means the storm's stronger right side would push surge into the Mississippi River. Furthermore, engineers have said the Crescent City could be more at risk of river flooding and some areas are expected to receive 20-30 hours of rain.

Plenty of flooding from storm surge and heavy rain may occur in other areas. The NHC says Isaac should move northward into Arkansas and into the Ohio Valley as a tropical depression after making landfall.

“There is a potential flood risk for a lot of people,” adds Knabb.

Catastrophe modeler Risk Management Solutions (RMS) reports “some areas in Southern Louisiana, Southern Mississippi and Southern Alabama could receive up to 20 inches of rain, rivaling amounts seen in 2008 Hurricanes Ike and Gustav.”

Analytics provider CoreLogic says nearly 270,000 residential properties valued at about $36 billion could be at risk for storm surge in seven metro areas from New Orleans to the Florida Panhandle. But CoreLogic's estimation is based on Isaac making landfall as a Category 2 hurricane.

Flooding—including flooding caused by storm surge—is not covered by standard homeowners' and renters' insurance policies but it is available from the National Flood Insurance Program and a few private insurers.

According to data compiled by the Insurance Information Institute, Florida is by far the leader in residents with flood insurance, with more than 2 million policies in 2011. Louisiana has nearly 494,000 policies, with Mississippi and Alabama with about 76,610 and 57,340 policies, respectively.

Isaac's damage will likely not be limited to flooding. With winds approaching 85 mph as it nears the coast, localized damage to roof and non-engineered buildings can be expected, according to the KC Wind Damage Scale from Karen Clark & Co.

Additionally, winds of that speed will cause significant tree damage and localized, long-lasting power outages.

“Note that large storms, even if they don't reach major hurricane intensity, have the potential to gain large amounts of kinetic energy and inertia and can therefore have impacts that may be typically associated with stronger storms,” says Margaret Joseph, catastrophe analyst at RMS.

Larsen says Isaac will probably be more about localized failures rather than widespread damage. Also, some tornadoes could be spawned by the storm.