Weather Nerd

No major flooding in New Orleans area

As expected, New Orleans appears to have avoided any major flooding. The water level has subsided on the Industrial Canal after the dramatic live video of overtopping earlier this afternoon. Moreover, a quick perusal of the Times-Picayune‘s New Orleans Hurricane Center reveals one report after another of no major flooding: in the West Bank, in St. Bernard, in East Jefferson, in the Upper 9th Ward, in Slidell, and so on. So, good news all around. New Orleans truly did dodge a bullet this time.

Things probably will not be so rosy in Houma and environs, where Gustav made a direct hit. But even there, the damage won’t be nearly as bad as it have been. We should all be very grateful the Loop Current did not perform as advertised, this time.

Dr. Jeff Masters does an excellent job explaining how lucky we are that Gustav wasn’t worse:

We got very lucky with Gustav — it could have been another Katrina. Both Gustav and Katrina had similar diameters (not radii) of tropical storm force winds at landfall: 440 miles. However, Katrina affected the coast with a region of hurricane force winds 170 miles across — 45% larger than the 115 miles of coast affected by Gustav. Both storms passed over some very high heat content waters in the Gulf of Mexico: Katrina, over a Loop Current eddy, and Gustav, over the Loop Current itself. Why didn’t Gustav explode into a Cat 5 monster storm when it crossed the Loop Current yesterday? Well, when a hurricane has a well-formed circular eyewall that is aligned vertically from the surface to the upper atmosphere, it acts as a very efficient heat engine that can take heat out of the ocean and convert it to the kinetic energy of its winds. When Katrina hit its Loop Current eddy, the hurricane was under low wind shear and had an ideal structure like this for taking advantage of the heat energy offered to it. Gustav, on the other hand, had just crossed Cuba when it hit the Loop Current. Gustav was under about 15 knots of wind shear, which it had been able to hold off, thanks to its tight, well-formed eyewall. However, passage over Cuba disrupted the eyewall structure just enough to allow the upper-level winds shearing it to penetrate into the heart of the hurricane. These winds ripped up the eyewall and tilted it, so that the surface eye was no longer underneath the upper-atmosphere eye. A tilted eyewall structure is not able to act as an efficient heat engine until it can get itself lined up more vertically, so Gustav was unable to take advantage of the warm Loop Current waters it was traversing. It’s like when your car engine is not firing on all cylinders and you hit the gas pedal — nothing happens. Once Gustav finally did align its eyewall vertically and armored itself against the effects of the wind shear, it had passed beyond the Loop Current and was over cooler waters of much lower heat content. Thus, Gustav was not able to intensify much before landfall. The computer models that predicted a Category 4 hurricane at landfall could easily have been correct, had the shear been a few knots less when Gustav crossed Cuba.

So yeah: New Orleans dodged a bullet. But that’s not to say there won’t still be a need for aid to the affected areas, of course. For information on how you can help, visit Gustav Bloggers or the Hurricane Gustav Wiki. They’ve got lots of links to relief agencies and such.

Incidentally, Dr. Masters’s above-quoted discussion is part of a full update on the tropics, titled “Gustav plows inland; Hanna now a hurricane; Ike and Josephine are on the way.” Read the whole thing.

P.S. Oh, and I almost forgot. This is a great picture. Again — lots more at NOLA.com.