Isaac Struggling, but Will Still be a Dangerous Storm at Landfall

   Posted by Levi at 4:00pm on August 27, 2012

Tropical Storm Isaac continues to struggle a bit this morning, and despite deepening 6mb in the last 24 hours, the maximum winds have not increased because the core is not tightening into an eyewall. The reason for this is dry air entrainment on the eastern side, which was expected to limit the system, but it has had a bit more potent effect than expected. Isaac has a large circulation and it can take a while to rid itself of this dry air, thus strengthening may stay only gradual today. A new source for the dry air affecting Isaac appears to be the upper-level low to his southwest, which is moving away from the storm and providing good ventilation aloft, but again, the long inflow fetch from Isaac is tapping into some of this dry air. Isaac should be able to become a hurricane as soon as the core tightens, as the current central pressure of 988mb would support Cat 1 winds if an eyewall develops. Good news for the north gulf coast is that Isaac now should not make landfall as a major hurricane, though with the environment aloft as good as it is and the favorable shape of the coastline near the Mississippie Delta, the storm should still be watched for rapid intensification just before landfall. It should also be noted that Isaac will be slowing down considerably near the coast, and if the models are underestimating this slowing, the storm could have more time over water to strengthen just before landfall. The new intensity forecast is adjusted downward, and makes Isaac an upper-end category 1 hurricane at landfall.

The track forecast is finally stable today, after requiring westward shifts for the past several days in a row. With Isaac 36-48 hours from landfall, the track up until the coast is getting better narrowed down. The current track takes Isaac into the Louisiana/Mississippi border. There is still some uncertainty in the track just before and after landfall due to how Isaac will be interacting with a trough to its northeast and a ridge to its northwest, both features trying to pull the storm in different directions. The American models continue to take Isaac WNW through Louisiana into eastern Texas, while the European models take Isaac more north up the Mississippi River into the center of the country. The forecast track is in better agreement with the latter scenario right now. This will be significant because Isaac should be slowing down considerably near landfall, which will allow copious amounts of rainfall to fall along the central gulf coast and areas inland along the track. The forecast track lies just to the right of the multi-model consensus.

We shall see what happens!


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