It takes Scott Kiser only a split second to name the one city in the U.S., and probably the world, that would sustain the most catastrophic damage from a category-5 hurricane. "New Orleans," says Kiser, a tropical-cyclone program manager for the National Weather Service. "Because the city is below sea level-with the Mississippi River on one side and Lake Pontchartrain on the other-it is a hydrologic nightmare." The worst problem, he explains, would be a storm surge, a phenomenon in which high winds stack up huge waves along a hurricane´s leading edge.