Hurricane Gustav: Changing Predictions

There has been enough change in the predictions regarding Gustav to make it worth a fresh look. Bottom line: It is weaker than previously predicted, it is still almost exactly on the same track, and it may be speeding up a little bit. Tropical Storm and soon after hurricane force winds may strike the Northern Gulf Coast by 2:00 or so Monday Morning (or even sooner), certainly during the night tonight. The eye may cross the coast about mid day Monday (tomorrow). The center of the predicted path is just far enough west of New Orleans proper to worry a lot about the storm surge.

Details:

1) The Hurricane Prediction Center is now saying that Gustav was ripped apart more than previously realized when it passed over western Cuba, and there are shearing winds working the top of the storm. This has caused the storm to drop to Category Three level. (Why does this happen? See note below.) Here is a recent satellite view. Notice how badly organized Gustav is at the moment:

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2) At this moment, Gustav is passing over the very warm Loop Current, and will gain strength. Expect it to get better organized over the next several hours, and to regain Category Four strength. Later, as the outer hurricane force winds are just about on land, you can expect this storm to weaken again (owing mainly to less warm water) and drop back to a Category Three (or maybe Two?) as it strikes.

3) The track of the storm is a little wobbly and the prediction has been shifted slightly to the east, but in general, the steering winds in the case of this storm, over the next 36 hours or so, are behaving in a way that this track has a lot of confidence. Here is the three-day map:

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Note: How hurricanes start and operate is a bit complex, but there is one rule of thumb that you can use to conceptualize how they work. The hurricane is essentially a giant turbine that runs because of the movement of heat from warm ocean water to the upper layer of the troposphere, which is quite cold. Therefore, there are two things that determine the degree of organization and the total energy of a hurricane: 1) The differential between surface and tropospheric heat, and 2) the lack of interference with the flow. The differential is mainly a matter of varying sea surface temperature, so as the sea surface temperature goes up, the storm gets stronger, as it goes down, it gets weaker. The warmest sea surface temperatures in the Gulf are where Gustav is right now as I write this, and later it will pass over cooler waters.

Meanwhile, Cuba, or any land mass, interferes with the turbine just as a flock of geese or a big piece of plywood or whatever thrown into a jet engine will interfere with that turbine’s operation. On the other end of it, high altitude shearing winds … winds that represent air movement contrary to the ideal direction of air for the turbine, weaken the storm. An analogy would be the old trick of sticking a potato on the end of a car’s exhaust pipe. Or, putting is a slightly different way, the stuff that happens at the top of the hurricane, at high altitude, is the follow through. The shearing winds mess with the follow through.

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