Disturbance off the Coast of Africa a Potential Threat to the Lesser Antilles

An area of disturbed weather near 11°N, 31°W, about 500 miles southwest of the Cape Verde Islands, has the potential to develop into a tropical depression later this week as it heads west at 10 - 15 mph towards the Lesser Antilles Islands. Visible satellite loops on Sunday morning showed that the disturbance had only a minor amount of spin. Infrared satellite images showed that the system's heavy thunderstorm activity was modest at best, and had not changed significantly since Saturday. Wind shear was moderate, 10 - 15 knots, which should allow slow development. Ocean temperatures were marginal for development, about 27°C. Water vapor satellite loops showed that the atmosphere was reasonably moist in the area, though a large area of dry air lay a few hundred miles to the north.


Figure 1. MODIS true-color image from approximately 11:30 am EDT July 27, 2014, showing a tropical disturbance (marked with an "L") about 500 miles southwest of the Cape Verde Islands. The Intertropical Convergence Zone (ITCZ), the band of heavy thunderstorms that circles the globe in the tropics, is also apparent. Clusters of thunderstorms in the ITCZ may compete for moisture and energy, slowing development of the disturbance. Image credit: NASA.

Forecast for the disturbance
Two of our three reliable models for predicting tropical cyclone genesis, the GFS and UKMET models, predicted in their 00Z Sunday runs that the disturbance would develop into a tropical depression by Thursday. Several of our less reliable models, the NAVGEM and Canadian models, also predicted development. The only reliable model that did not predict development was the European model, which historically has had the highest incidence of failing to predict development when development actually occurs. The fact that two out of three of the reliable genesis models predict development bolsters the odds that development will actually occur. In their 2 pm EDT Sunday Tropical Weather Outlook, NHC put the 5-day odds of development at 40%, up from their 20% forecast from Saturday evening.

All of the models predict that the disturbance will continue due west or just north of due west at 10 - 15 mph for the next five days. The UKMET is the fastest of the models, predicting that the disturbance will arrive in the Lesser Antilles Islands on Friday. The GFS is slower, predicting a Saturday arrival in the islands. Once the disturbance crosses west of about 50°W longitude, ocean temperatures will warm to about 28°C, which should aid development. Dry air to the north will likely interfere with development by the middle of the week, and we will have to see if the moderate levels of wind shear forecast to occur over the tropical Atlantic will be capable of driving this dry air into the core of the system, disrupting formation. The disturbance may also have trouble disentangling itself from the Intertropical Convergence Zone (ITCZ), the band of heavy thunderstorms that circles the globe in the tropics, which lies just to the south of the disturbance. Clusters of thunderstorms in the ITCZ may compete for moisture and energy, slowing development of the disturbance. Most of the 20 members of the 06Z Sunday run of the GFS ensemble model (which runs the GFS model at low resolution 20 times with slightly different initial conditions to show a range of possible outcomes) showed the disturbance missing the U.S. East Coast and recurving out to sea early next week, but it is too early to assess the odds of this.

Jeff Masters