Here comes Wilma

Tropical Depression 24 is here, but it won't be called that for very long. All indications are that this will be Tropical Storm Wilma on Sunday, and Hurricane Wilma by Tuesday. The areal coverage of the deep convection continues to increase, low-level spiral banding has appeared, and upper-level outflow is now good on the west and north sides of the depression. The upper level anti-cyclone has overhead has grown better defined, and wind shear remains a low five knots.

Global computer models forecast that the shear will continue to remain low the next several days over the western Caribbean, where the depression is expected to remain. The chances of this storm growing to hurricane strength are high, and I expect this will be a major hurricane of a least Category 3 strength five days from now. The last three GFDL model runs have consistently been bringing the storm to major hurricane strength.


Figure 1. Historical tracks of tropical depressions that have formed in the western Caribbean in October.

Steering currents are expected to remain weak, and the computer models are forecasting a slow movement to the west or west-southwest the next three days. After that, most of the models agree on a more northerly track towards Cuba as a trough of low pressure swinging across the U.S. exerts a pull on the system. This is a typical track for October systems forming in the western Caribbean, as we can see from the historical track map shown in Figure 1. If this system does eventually affect the U.S., the most likely target would be the Florida Keys or the southwest coast of Florida. Historically, however, most storms forming in October in the western Caribbean miss the U.S. entirely, affecting just the Cayman Islands, Cuba, and the Bahamas. There is a chance that this trough would be too weak to recurve the system, and that instead it would continue west or drift southwest towards Honduras like Category 5 Hurricane Mitch did in October 1998.

Cape Verdes tropical disturbance
A tropical disturbance about 500 miles west of the Cape Verde Islands is experiencing high wind shear that will prohibit development for the next few days as it tracks west-northwest over the open ocean.

Katrina's winds revisited
In my last blog entry on this subject, we discussed the Florida Sun-Sentinel article commenting on new findings that indicate Katrina was only a Category 3 hurricane at first landfall near the mouth of the Mississippi, and a Category 1 hurricane over New Orleans. The article was rather imprecise in its use of the Category system for ranking hurricanes, and I interpreted the article to mean that Katrina was a Category 1 at landfall in Mississippi. Upon re-reading the article, I think what they were trying to say was that Katrina had Category 1 force winds over New Orleans, not that the storm itself was a Category 1. As several of you have pointed out, it is pretty difficult to have a hurricane with a 927 mb pressure (Katrina's pressure at landfall in Mississippi) with just Category 1 winds. Katrina was a least a strong Category 2, and perhaps a weak Category 3 hurricane at landfall in Mississippi. While Katrina did have unusualy high winds aloft compared to surface winds (which NHC noted on one of their discussions during the storm), this difference was not enough to make Katrina a Category 1 hurricane at landfall in Mississippi. Sorry for sowing the confusion!

My next post will be Sunday morning about 10 am.

Jeff Masters