The word on New Orleans is this:

We lose the equivalent of about a football field every day in Louisiana. Most of the "land" south of the city is just canals, lakes, and wetlands. There are some spots where communities exist, but mostly it's just wet. Lake Borgne is connected directly to both Lake Pontchartrain near the Rigolets and the Gulf of Mexico. The theory on the doomsday scenario is a storm moving up from the ESE heading WNW and passing south of the City. This piles the water up to the coast, pushes it through Lake Borgne and into Lake Pontchartrain. The north winds from the eye to our south pushes water to the Southshore of Lake Pontchartrain. Water then breaches over and through the levees. Much of our city is 5' below sea level and lower. Most estimates are that a Cat 5 puts between 20-25' (3 stories ish) into a great part of the Southshore (area roughly south of Lake Pontchartrain and roughly north of the Mississippi River). This is New Orleans, Kenner, Metairie and smaller communities. My property is exactly sea level. My house is roughly 3' raised. That means I'd have to swim up to one of my pecan trees or maybe on top of the either of the doctor's 2-story houses that sit across the street and next door to me.

Steve