August 14, 2007

Dean holds steady

Filed under: Tropical — Erin @ 4:10 pm

Tropical Storm Dean has held its intensity in the face of a small pocket of 20kt shear. Although its convection dropped off significantly during the diurnal minimum, this is cyclical, especially for a weak system. Dean used the day to do something more important than blowing up blobs of clouds, start developing the classic tropical cyclone structure. It has feeder bands forming, and as soon as it gets out of the pocket of shear, I’m expecting it to take off. The timing on this is important: If it breaks into the minimal shear area to its west at night (night for it, a few hours ahead of people in the Americas), the effect will be more pronounced in a short time. This is because of diurnal max. Dean is skipping along a tad south of due west at 20 mph.

I think it’s now fair to start discussing where Dean will end up and how strong he will be. I highly doubt that he will recurve, and I am not sold on a U.S. East Coast landfall either (including Florida’s east side). In the short term, I think it’s fair to say that Dean will be a weather-maker for the islands off Venezuela. I also have a bad feeling that he will get into the Caribbean. The water there is even hotter than in the Gulf. A major hurricane seems highly likely if this happens. Many people in the U.S. see a powerful hurricane hit Hispaniola, and see the mountains tear it up and weaken it (a good example is Georges of 1998), so that it has a hard time restrengthening before its final landfall, and they cheer. But the island nations can’t afford a hit any more than New Orleans can. The 12:00 run of the HWRF showed a Hurricane Allen-like system barreling headlong into the Dominican Republic at peak intensity. Needless to say, this would be horrific. The one good thing about Dean is that, since it’s a long-track Cape Verde storm, there’s plenty of time to watch it, and prepare for landfall.

Beyond the Caribbean, as I have stated, I have a really bad suspicion that this is ultimately a Gulf problem. I’m not going to change this guess until and unless the real, observed weather conditions change as Dean continues on its path and gets closer to the point where it must “decide” whether to curve out of the Caribbean or not. Fluctuations of models, which have had it from Central America to an Atlantic fish storm, don’t qualify. Real, large-scale changes in weather do.

The NHC, which has a tendency to underforecast the intensity of hurricanes, has Dean as a 95kt Category Two system in five days. Again, how quickly Dean gets its act together will determine when (NOT “if”) it becomes a hurricane and thus when it may hit Cat-2 or higher intensity. It has followed the NHC’s predicted intensity forecast thus far, and tropical storms usually take several advisories to get fully “with it” and reach a level of stability that permits them to develop eyes and become hurricanes. I see no reason to deviate significantly from the NHC forecast, with the one caveat that Dean might rapidly intensify after it becomes a hurricane.

Note: If anyone tries to tell you that Dean, or any other new TS, has an eye at any intensity under about 55 kt, they don’t know what they are looking at. Former hurricanes that made landfall, of course, are a different matter, and the eye structure could persist long after it lost hurricane-force winds. But newly-formed storms don’t start to develop eyes until they approach hurricane intensity.

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