Computer Models Nailed Gustav's Track
Kudos to the Geophysical Fluid Dynamic Laboratory (GFDL) weather model. It provided an amazingly accurate forecast of Hurricane Gustav's track more than FIVE days before landfall (recall I showed this last Thursday, and compared it to Katrina). The above image shows the similarity between Gustav's five-day model simulation and a satellite image of the real Gustav just prior to coming ashore. The model was about 12 hours too fast, but gradually got the timing right in subsequent simulations.
Congratulations are also in order to the National Hurricane Center who pegged southeast Louisiana in the middle of its track guidance 5-6 days before landfall. Watch this remarkable animation of the Hurricane Center's evolving track forecast from the time it began issuing advisories on Gustav. Notice how consistently accurate the forecast was for the landfall location.
The intensity forecast by some of the models (like the GFDL), while by no means terrible, wasn't as stellar as the track forecast. You'll notice in the GFDL simulation above, Gustav is much more symmetric than the real Gustav. The model forecast a pressure of about 935 mb and maximum winds of 140 mph (Category 4) compared to 954 mb and maximum winds of 115 mph for the real thing. Some of the models did not capture the effect of the wind shear which disrupted Gustav's structure and prevented it from strengthening over the warm Gulf of Mexico. Chief meteorologist Jeff Masters at wunderground provides an excellent technical explanation of how the structure was impacted:
...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.
The forecasting experience with Gustav reinforces what we already know: track forecasts are pretty good (but by no means perfect) while intensity forecasts are much more difficult. I think we'll see that again with Hurricane Hanna...which we'll be updating you on in just a couple hours...
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