By: Bob Henson and Jeff Masters From Weather Underground

A tropical wave located a few hundred miles south-southwest of the Cabo Verde Islands on Saturday morning was poorly organized, with only a limited amount of heavy thunderstorm activity and spin. This wave was under low to moderate wind shear of 5 – 15 knots and was over warm ocean waters near 29°C (84°F), but was too close to the equator (near 9°N) to be able to leverage the Earth’s spin and acquire enough spin of its own to develop into a tropical depression. However, the tropical wave may move far enough from the equator to be able to develop by Monday or Tuesday, when it will be approaching the Lesser Antilles Islands. Working against development, at least in the next five days, will be the fast forward speed of the system. The storm will be driven west at 20 – 25 mph by the trade winds associated with an unusually strong Bermuda-Azores High; tropical waves moving at 20 mph or faster usually have trouble achieving the vertical alignment needed to intensify. 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 or Wednesday morning. Once the storm enters the eastern Caribbean, long-range model runs suggest that the system might be very close to the coast of South America, which would interfere with development.

Figure 1. Satellite image of the tropical wave (left) located southwest of the Cabo Verde Islands at 1445Z (10:45 am EDT) Saturday, September 24, 2016. Image credit: NASA/MSFC Earth Science Office.

There was increased model support for development of this tropical wave in the Saturday morning runs of the models compared to their Friday runs. Our top three models for predicting hurricane genesis—the GFS, UKMET and European models—all predicted in their 00Z Saturday runs that this tropical wave would develop into a tropical depression or tropical storm between Monday and Thursday. About 80% of the 20 forecasts from the members of the 00Z Saturday GFS ensemble showed development into a tropical storm, with 40% predicting a hurricane. The European model ensemble was less aggressive developing the storm, probably because of a predicted track too close to the coast of South America—about 30% of its 50 ensemble members predicted a tropical storm in the Caribbean. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 10% and 50%, respectively. The Hurricane Hunters are scheduled to make their first flight into the storm on Tuesday afternoon.

Figure 2. Forecasts out to ten days from the 00Z Saturday European model ensemble (top) and GFS model ensemble (bottom) had a number of their 70 members predicting a hurricane for late in the week in the Caribbean (light blue dots). The operational versions of the models, run at higher resolution (red lines), also showed the storm becoming a hurricane by ten days into the future.

For more on this story go to: https://www.wunderground.com/blog/JeffMasters/potential-trouble-in-tropical-atlantic-karl-brushes-bermuda-megi-eye