Weather Nerd

Gustav slowly intensifying; New Orleans likely to dodge the worst

According to a recon report at 7:03 PM EDT, Hurricane Gustav’s minimum central pressure has fallen another 4 millibars — from 957 to 953 millibars — since 3:17 PM. That’s in addition to the 5-millibar drop between 9:48 AM and 3:17 PM EDT. Gustav’s intensification isn’t rapid, but it’s steady, and the storm is looking better on the visible satellite loop.

I’ve mentioned before that wind speed increases often take a while to “catch up” with pressure drops, and that appears to be happening here. Dr. Jeff Masters predicts that “the winds should start to increase by late tonight in response to the falling pressure.” It seems unlikely that Gustav will regain Category 4 intensity, but Dr. Masters writes: “Given the recent improvement in Gustav’s organization, I believe that the storm has time to intensify into at least a Category 3 hurricane with 125-130 mph winds by landfall.”

On the other hand, it’s still possible that Gustav’s winds could ramp up to 125-130 mph — and then ramp back down to ~115 mph just before landfall. Precision in the intensity forecast is impossible, even at this late date. Regardless, though, it looks like this will be a Category 2 or 3 at landfall, not a Category 4 or 5.

The million-dollar question remains, landfall where? And what of New Orleans?

With regard to the first question, the official forecast track “line” has Gustav’s center coming ashore just southwest of Houma — where The Weather Channel’s Jim Cantore has been sent — and then proceeding directly over Morgan City and Lafayette.

However, as I mentioned earlier, the National Hurricane Center has hinted at a possible westward shift in the track later tonight. On the other hand, Alan Sullivan, who tends to rely on a more intuitive reading of the satellite map rather than scrutinizing the computer models, still thinks a worst-case scenario for New Orleans — landfall near Grand Isle — is possible.

Even if that happens, it might not be Apocalypse Now for the Big Easy, given that Gustav is unlikely to come ashore as the monster hurricane once anticipated. After I linked to Sullivan’s post stating that Gustav “certainly seems to be heading in [Grand Isle’s] direction,” he added this addendum:

Will the major levees hold? Probably. Gustav may not quite be strong enough to cause another wholesale flood of the city.

Eric Berger agrees that New Orleans is likely to be spared the worst, as does Dr. Masters, who has a detailed analysis of the threat to New Orleans:

Gustav’s storm surge is not likely to breach the New Orleans levees–if they perform as designed
Gustav is a very large storm. Like Katrina, Gustav may carry a larger storm surge to the coast than its wind speeds might suggest. Currently, Gustav’s diameter of tropical storm force winds is 340 miles. By landfall, this number is forecast to increase to 360 miles, which would make Gustav 80% as large as Katrina was at landfall. NHC’s current storm surge forecast calls for a storm surge of 10-14 feet to the right of where the center of Gustav comes ashore. The latest computer generated storm surge map shows that highest surge will be along the levee system along the east side of New Orleans. Storm surge levels of this magnitude are characteristic of a Category 3 hurricane. The levee system of New Orleans is designed to withstand a Category 3 storm surge. If Gustav intensifies more than the NHC forecast is calling for, there is a significant threat of multiple levee failures in the New Orleans levee system resulting in flooding of portions of the city. However, the latest 12Z (8 am EDT) model runs have shifted their landfall points a bit further west, reducing the odds of a Category 4 storm surge in New Orleans. My best guess is that New Orleans will suffer a Category 2 or 3-level storm surge. The levees will hold with that level of storm surge, if they perform as designed.

Barring unexpected changes in the forecast, it will soon be time for the media to ramp down the pre-landfall hype about a possible calamity in New Orleans, lest they contribute to the public’s sense of cynicism about storms “never living up to the hype.” The “hype” surrounding Gustav was fully justified last night, before the unforeseen post-Cuba weakening, but now it is growing less so, at least with regard to the Big Easy.

In other news, Tropical Storm Hanna — which I confess I’ve barely been following at all, as Gustav has been so all-consuming — is increasingly looking like a hurricane threat to the Southeast coast late this week or early next weekend. Hanna to Savannah? Maybe. I’ll have more to say about Hanna once we’re finished with Gustav.