From Wunderblog, Sept. 23:
Karl Approaches Bermuda; Trouble in the Caribbean Next Week?
After nearly a week as a lackluster system, Tropical Storm Karl is finally gaining strength as it heads toward a close encounter with Bermuda. As of the 11 am EDT advisory, Karl was located about 250 miles south of Bermuda, moving north at 12 mph. Karl’s top sustained winds were holding at 60 mph, its peak intensity thus far. Karl is continuing its multi-day struggle with vertical wind shear that’s tended to push its showers and thunderstorms (convection) east of its center. On Thursday night, the storm managed to consolidate a healthy core of convection around its center, but Karl remains somewhat asymmetric, with a comma-shaped structure and a large band of convection well to its east.....
.... An African tropical wave that could be trouble
A tropical wave located a few hundred miles west of the coast of Africa and about 350 miles southeast of the Cabo Verde Islands on Friday morning was poorly organized, with only a limited amount of heavy thunderstorm activity and spin. This wave is currently too close to the equator (near 8°N) to be able to leverage the Earth’s spin and acquire enough spin of its own to develop into a tropical depression, and is not likely to develop through this weekend as it heads rapidly west at 20 - 25 mph. However, the tropical wave may move far enough from the equator to be able to develop by early next week, when it reaches a point about halfway between the Lesser Antilles Islands and the coast of Africa. There was increased model support for development of this tropical wave in the Friday morning runs of the models compared to their Thursday morning runs. Our top three models for predicting hurricane genesis—the GFS, UKMET and European models—all predicted in their 00Z Friday runs that this tropical wave would develop into a tropical depression or tropical storm between Monday and Thursday next week. About 60% of the 20 forecasts from the members of the 00Z Friday GFS ensemble showed development, and about 30% of the 50 members of the European model ensemble did so. Troublingly, a considerable number of the ensemble model runs showed this storm becoming a hurricane in the Caribbean. Working against development, at least in the next five days, will be the fast forward speed of the system—tropical waves moving at 20 mph or faster usually have trouble getting organized. However, the storm does not have as much dry air to contend with compared to other storms we have seen this year, and it would not be a surprise to see this system be close to tropical depression or tropical storm status when it begins moving into the Lesser Antilles Islands on Tuesday night. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 0% and 20%, respectively....MORE