In an eerie coincidence, Hurricane Isaac has spun up to Category 1 strength over warm Gulf waters and is predicted to hit southeastern Louisiana—and possibly New Orleans directly—seven years to the day after Hurricane Katrina slammed the Crescent City. The tropical cyclone will likely be no stronger than a Category 2 on the Saffir-Simpson scale when it hits land later tonight or early tomorrow, a full step below Hurricane Katrina, which hit land as a Category 3 storm.

But even a Category 2—with sustained winds above 150 kilometers-per-hour—or, for that matter, a tropical storm can boast pressures and winds strong enough to produce a monster storm surge of Gulf waters, particularly if the timing of landfall coincides with high tide. As it stands, Isaac's current 97.6 kilopascals of pressure and sustained winds of more than 100 km/h are enough to produce a surge of as much as 4.5 meters onto southeastern Louisiana. The National Weather Service is suggesting that residents keep an axe on hand if they choose to hunker down in place and ride out the storm, just in case they have to chop through their roof—and that "life threatening" flooding may occur in areas outside of "hurricane protection levees." Isaac appears to be on course to take the eastern approach to the Big Easy, raising the risk that "sections of west Jefferson, east St. Charles and lower Lafourche hurricane protection levees could be overtopped."

The good news is that the rest of the Crescent City's new levees should hold. High walls were constructed and old levees reinforced and made taller in the wake of the Hurricane Katrina disaster that killed some 1,400 people. Gates have been built across the canals and navigation channels that make shipping easier but also provide a quick route for surges to penetrate far inland. It was likely a combination of surges pushing up two such canals that created the monster surge that overtopped flood defenses and drowned New Orleans, though some have argued that shoddy construction of levees in the first place played a key role.

Isaac may also bring some much needed rain—as much as 50 centimeters in parts of Louisiana—to the parched Midwest and Southeast of the U.S. The deluge could help failing crops, although it may prove to be too little too late.

The bad news is that New Orleans may never be fully protected. That would require restoring coastal wetlands south of the city, to absorb storm surges. Dredging, canal building and the general industrial nature of the Louisiana coast make such a restoration improbable if not impossible. The ring of new defenses also leaves these mortally wounded wetlands, interspersed with settlements, relatively unprotected and outside the defensive perimeter. Even a weak Hurricane Isaac, if the tropical cyclone continues on a bad luck trajectory, could prove once again how hard it is to protect the Big Easy.

Image: Courtesy of NASA GOES Project