Category 5

Crikey.
UPDATE: Via Kevin, who reminded me of this:

A direct hit from a powerful hurricane on New Orleans could furnish perhaps the largest natural catastrophe ever experienced on U.S. soil. Some estimates suggest that well over 25,000 non-evacuees could die. Many more would be stranded, and successful evacuees would have nowhere to return to. Damages could run as high as $100 billion. In the wake of such a tragedy, some may even question the wisdom of trying to rebuild the city at all.