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Gustav fails to strengthen significantly over Loop Current

August 31st, 2008 - 2:33 pm

For nearly a week now, forecasters have been worried about what would happen when Hurricane Gustav tracked over the warm, deep waters of the Gulf of Mexico’s Loop Current. It’s easy to see why: it was the Loop Current, after all, that strengthened Katrina and Rita into Category Five super-hurricanes in 2005. It seemed almost a fait accompli that something similar would happen with Gustav, particularly once it strengthened to a 150-mph Category Four monster yesterday over the Caribbean, meaning it would presumably have a more advanced “starting point” for its expected intensification in the Gulf.

It was this fear of rapid, massive intensification over the Loop Current that spurred New Orleans Mayor Ray Nagin to declare yesterday that Gustav would be the “mother of all storms” and the “storm of the century.” And indeed, while Nagin’s comments were slightly hyperbolic, there seemed little reason when he made them to hope that Gustav would fail to significantly intensify this morning and afternoon over the southern Gulf. The best we could hope for, we thought, was a gradual, non-rapid intensification, followed by significant weakening over the northern Gulf.

Instead, we’ve been blessed by a totally unexpected positive development: Gustav has essentially failed to strengthen at all over the Loop Current. Now those dangerous waters are in the storm’s rearview mirror. Gustav is moving toward less energetic waters — still capable of supporting intensification, but probably not rapid intensification.

gustav-pastloop-sm.jpg
Gustav’s track (black for its previous course, red for its forecast track, black dot for its current position), roughly drawn by yours truly on the Tropical Cyclone Heat Potential map.

Admittedly, Gustav has shed a small handful of millibars from its central pressure in recent hours — but not enough to qualify as significant deepening. Meanwhile, the storm’s core has remained woefully disorganized for a major hurricane, as you can see on the satellite loop, and the winds are barely Category 3 strength, if that. Officially, the top sustained winds are 115 mph, but that may be generous; a strong case can be made that Gustav has been a glorified Cat. 2 all morning and afternoon today. (The NHC has, I suspect, been reluctant to downgrade it because they don’t want people to prematurely sound the “all clear,” lest Gustav then re-strengthen unexpectedly.)

Why has the Loop Current failed to strengthen Gustav much? Alan Sullivan explained this morning:

In time-lapse water-vapor imagery, a skilled eye can see the plume of mid-level dry air wrapping around the outside of Gustav in an arc from the western Gulf, across Yucatan, curving north over western Cuba, and entering the east side of the cyclonic whirl. This plume is hindering convection there, but the fierce winds in the storm core are lifting energy from the Loop Current and cycling it aloft through the western eyewall. Yes, it’s counterintuitive, but this is how hurricanes work.

Now, as of 5:00 PM EDT, there are signs Gustav is finally beginning to get its act together — too late for the Loop Current to do its worst, but soon enough, perhaps, for some intensification before landfall (followed, hopefully, by some weakening). The National Hurricane Center’s 5pm discussion states: “VISIBLE SATELLITE IMAGERY DURING THE PAST COUPLE OF HOURS SUGGESTS THAT GUSTAV IS GETTING A LITTLE BETTER ORGANIZED…WITH A HINT OF AN EYE RETURNING.” Sullivan sees the same thing:

[I]n the last two hours I have seen a marked increase in convection around the center of the storm. Although the eye is still cirrus-lidded, there is a complete ring of overshooting cloud tops. Gustav has reformed a real eyewall, for the first time since it left Cuba. I expect [the] pressure drop to sharpen at the 8 PM update. In that case we shall have an answer to the question of intensity: some strengthening this evening; some weakening just before the eye reaches the Delta. I’m guessing 120 mph winds at landfall.

But landfall where? The NHC has shifted its official forecast track at 5:00 PM, and suggests in the discussion that it may go further left:

THE NEW GUIDANCE RUNS…MOST NOTABLY THE HWRF…SHOW A SLIGHT LEFT TURN AFTER 12 HR WITH A LEFTWARD SHIFT IN THE LANDFALL POINT ON THE LOUISIANA COAST. THE NEW FORECAST TRACK IS SHIFTED SLIGHTLY TO THE LEFT OF THE PREVIOUS TRACK…AND SINCE IT LIES NEAR THE EASTERN EDGE OF THE GUIDANCE ENVELOPE SOME ADDITIONAL SMALL SHIFTS MAY BE NECESSARY LATER.

This would be excellent news for New Orleans. But Sullivan, watching not the computer models but the satellites, sees things differently:

With forward speed at 18 mph, Gustav will be ashore much sooner than previously expected. This means less time for the core to decay before it passes just SW of New Orleans — the worst track. And it certainly seems to be heading in that direction. Maybe it will waver left or right. Maybe.

We’ll see who is right.

I’ll have more to say about the track in my next update, probably around 8:00 PM EDT. (Please note, the timestamps at the top of these posts are in PDT. Sorry for any confusion.)

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