Weather Nerd

Clustering at last?

This may change overnight, but at the moment, if you compare the 2:00 PM computer models with the 8:00 PM computer models, it looks like we’re finally starting to see some “clustering” around a rough track — namely, toward western or central Louisiana.

gustav-cluster-fri.png

The zig-zagging tracks on the west side of the guidance envelope, in eastern Texas and just offshore thereof, tell us that a few models call for a stalling, meandering track near the end of the forecast period. So there is still some disagreement. But overall, roughly speaking, it looks — to this admittedly untrained eye — like the models are starting to come into somewhat better agreement.

The Houston Chronicle‘s Eric Berger notices the same thing:

Since this afternoon the models have come into somewhat better agreement, with a handful swinging east from Texas and the GFDL coming west from New Orleans. In effect they met in the middle. … Whether the models remain in agreement is unclear. Given the history of Gustav, the models will probably scatter again by the morning. Most of Texas and all of Louisiana and Mississippi remain at risk for a landfall.

It seems the National Hurricane Center doesn’t want to get too excited about the current apparent agreement, which, as Berger says, could be short-lived. The NHC’s 11pm EDT discussion makes no mention of “clustering” beyond 48 hours. It states in part:

THERE HAS . . . BEEN LITTLE CHANGE IN THE GUIDANCE…WITH SOME CONTINUING DISAGREEMENT ON HOW MUCH RIDGING WILL EXTEND WESTWARD TO THE NORTH OF GUSTAV AND HOW GUSTAV WILL INTERACT WITH AN UPPER-LEVEL TROUGH OVER THE GULF OF MEXICO. THE GUIDANCE IS TIGHTLY CLUSTERED THROUGH 48 HR AND THEN SHOWS SOME SPREAD. THE GFDL REMAINS ON THE EASTERN SIDE OF THE GUIDANCE ENVELOPE AND CALLS FOR LANDFALL IN SOUTHEASTERN LOUISIANA…WHILE THE UKMET CALLS FOR A WESTWARD TURN AND LANDFALL ON THE MIDDLE TO UPPER TEXAS COAST. THE REMAINDER OF THE GUIDANCE LIES BETWEEN THESE EXTREMES. THE NEW FORECAST TRACK IS AN UPDATE OF THE PREVIOUS FORECAST FOR THE FIRST 72 HR…THEN IS SLOWER AND A LITTLE TO THE LEFT OF THE PREVIOUS FORECAST THEREAFTER. IT LIES IN THE EASTERN SIDE OF THE GUIDANCE ENVELOPE.

Significantly, although the eastern outlier GFDL does indeed bring Gustav ashore in southeastern Louisiana, as NHC says, even that track has edged to the left, and would likely no longer constitute a “worst-case scenario” for New Orleans — though it might still be close enough to cause some flooding problems. But none of the models now bring the storm ashore at, east of, or (worst of all) immediately west of, New Orleans.

That’s good news, and hopefully it stays that way. Whether it’s enough to avoid the necessity of a mandatory evacuation order for New Orleans — which is apparently set for Sunday, not Saturday as I had earlier assumed — remains to be seen. It presumably won’t be enough to halt tomorrow’s phase of the evacuation efforts, as the city will begin “helping citizens without cars leave for shelters in northern Louisiana” by loading them on city-chartered buses and trains.

Speaking of the buses, this is a little concerning:

The private contractor the state hired to provide buses for hurricane evacuations has not come through with enough vehicles in a timely manner, causing the state to look elsewhere to meet the state’s timeline for moving people out of New Orleans and other areas prior to the arrival of Hurricane Gustav, Gov. Bobby Jindal said Friday.

The state contracted for 700 buses with drivers to be made available in an emergency but has “run into challenges” with the primary bus contractor, the governor said during a news conference in Baton Rouge.

“The contractor is not necessarily doing what they promised to do, ” Jindal said. …

The state is making arrangements with another contractor to fill bus orders, while the state is also requesting supplemental buses from the Regional Transit Authority, LA Swift coaches and from Texas public transportation services.

Well, it sounds like they’ve got it under control, at least. We can hope.

Anyway, back to the computer models. I called the apparent clustering on western and central Louisiana “good news” because, from a big-picture perspective, a direct hit on New Orleans is worse than just about any other scenario. But of course, when it comes to a landfalling hurricane, good news for one location is generally bad news for another — in this case the western two-thirds of Louisiana’s coastline, which look increasingly like this storm’s likeliest target. The Times-Picayune reports on the storm surge threat to the central part of the state’s coastline:

The biggest threat to Louisiana residents from Hurricane Gustav is shaping up to be flooding caused by storm surge, says Louisiana State University coastal geologist Robert Twilley.

The Houma area could see surge as deep as 15 feet, if Gustav keeps to its predicted path and intensity, Twilley said.

“If it stays on this southwest Louisiana track, Gustav will be a like a hybrid between Katrina and Rita,” Twilley said. “And right in the middle, you’ve got the huge Atchafalaya basin.”

Twilley is heading up a university-led effort to track the potential threat to the state’s coastal communities from storm surge rushing onshore in advance of the Category 3 storm. The results are being used by the state’s Emergency Operations Center to help guide evacuation and future rescue decisionmaking.

The worst threat of surge is normally on the eastern side of an approaching hurricane, as that’s where the storm’s forward speed is added to its normal wind speed, increasing the forces lifting water up from the ocean’s surface and pushing it ashore.

But Gustav is forecast to take a northwestern diagonal path into a part of the state with huge amounts of water already stored in its most vital coastal wetlands.

Unlike the New Orleans metropolitan area, where mostly elevated interstate highways lead north, east and west to safety, communities closest to the coastline have few elevated roads leading away from danger. The remaining roads often have only two or four lanes and flood easily in unusually high tides or heavy rains.

“How do you get people out of that central area?” he said. “I just hope people don’t wait too long.”

Early storm surge computer model runs on Friday indicated water would be funneled north in a variety of natural and man-made channels, including the Houma Navigation Canal, exacerbating potential flooding.

I’ll have more in the morning. In the mean time, keep visiting the links at right, particularly the New Orleans Hurricane Center for the latest from Louisiana, and Houston Hurricane Central for the latest from Texas.

P.S. Lots of information for Louisiana residents, from the governor, can be found here.

Oh, and Alan Sullivan has posted pretty pictures of a “hurricane sky” in South Florida.

P.P.S. I apologize if you leave a comment and it doesn’t appear for several hours. Unfortunately, on this blog, I am required to individually approve each comment, and sometimes I’m away for my computer for a while. I will get everybody’s comments online as quickly as possible, but alas, there will sometimes be significant delays. No censorship is intended, though. Your comments are very much appreciated!