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.
Brendan you may have spoke to soon: Plaquemines Parish levee overtopped, subdivision threatened… http://www.nola.com/huSt. Bernard Parish firefighters are rolling to a levee overtopping in Braithwaite in Plaquemines Parish. Plaquemines Parish workers have been trying to bolster the levee along the Clearwater Canal that is being overtopped by floodwaters that threaten the Braithwaite Park subdivision.
St. Bernard Parish is sending all available firefighters to the area to help with any rescue efforts that take place, as well as shore the levee with sandbags.Plaquemines Parish workers have been furiously working on the levee since mid afternoon..
“We don’t think our efforts are going to be successful so we need to get everyone out now,” Parish President Billy Nungesser said. rricane/index.ssf/2008/09/plaquemines_parish_levee_threa.html
Brendan-
I’m getting this via WWL and the T-P, but there is an overtopped levee on the eastbank of Plaquemines Parish between Braithwaite and Scarsdale (upriver from NAS New Orleans and across the river from the Coast Guard reservation), and water is beginning to come in in lower Jefferson (mainly Lafitte).
tmi3rd
Maybe the models need to take into account the possibility of eyewall tilt. Once such a tilt can be empirically confirmed, adjustments can be made.
Two entities with the same diameter but with different radii?
You want to explain that to all us Math majors out here, bippy?
Well, first of all, two different, non-circular objects (such as two hurricane whose wind fields aren’t perfectly symmetrical) could both have, say, a maximum T.S.-force wind radius of 200 miles east of the center, but one might only have T.S. winds out 150 miles on the west side, and the other might only have ‘em 100 miles out on the west. So Storm A would have a diameter of 350, while Storm B would have a diameter of 300. But they’d both have a maximum wind radius of 200.
However, I don’t think Dr. Masters was actually saying they have “the same diameter but with different radii.” I think he was simply correcting the media’s improper use of the word “radii” to refer to the diameter. That is to say, his point was that 440 miles was not wind radius of Katrina and of Gustav (which would imply a diameter of 880 miles, assuming perfect symmetry), but rather, it was the diameter of both storms. He’s correcting terminology, not suggesting that the storms have different radii but the same diamaters. That’s how I read it, anyway.