93L in Eastern Atlantic Growing More Organized

By: Dr. Jeff Masters , 2:43PM,GMT on July 28,2014

An area of disturbed weather located near 10°N, 33°W at 8 am EDT Monday, about 500 miles southwest of the Cape Verde Islands, was designated Invest 93L by NHC early Monday morning. This disturbance is a more serious threat than Tropical Depression Two of last week, and has the potential to develop into a strong tropical storm before reaching the Lesser Antilles Islands on Friday or Saturday. Visible satellite loops on Monday morning showed that the disturbance had only a modest amount of spin, but infrared satellite images showed that the system's heavy thunderstorm activity had increased significantly since Sunday. Wind shear was light, 5 - 10 knots, which should aid development. Ocean temperatures were decent for development, about 27.5°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. Analysis of the Saharan Air Layer (SAL) made at 8 am EDT Monday July 28, 2014 using data from the Meteosat-9 satellite. Dry, dusty air was present from the coast of Africa westwards across the tropical Atlantic, but was located well to the north of tropical disturbance 93L. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.


Figure 2. MODIS true-color image of 93L from approximately 9:30 am EDT July 28, 2014, when the storm was about 500 miles southwest of the Cape Verde Islands. The disturbance was embedded in the Intertropical Convergence Zone (ITCZ), the band of heavy thunderstorms that circles the globe in the tropics. Image credit: NASA.

Forecast for 93L
The 12 UTC Monday forecast from the SHIPS model predicted that wind shear will remain low to moderate, 5 - 15 knots, for the remainder of the week, aiding development. Sea Surface Temperatures (SSTs) will cool a bit to 27°C on Tuesday and Wednesday, which will tend to slow development. Two of our three reliable models for predicting tropical cyclone genesis, the GFS and UKMET models, predicted in their 00Z Monday 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 8 am EDT Monday Tropical Weather Outlook, NHC put the 5-day odds of development at 70%, up from their 40% forecast from Sunday.

All of the models predict that the disturbance will continue due west or west-northwest 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 evening. The GFS is slower, predicting a Saturday morning arrival in the islands. Once the disturbance crosses west of about 55°W longitude on Thursday, ocean temperatures will warm to about 28°C, which should aid development. Dry air to the north will likely interfere with development late in 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.

The long-range fate of 93L remains highly uncertain, and will depend upon exactly how far south the center ends up consolidating when the storm develops. Most of the 20 members of the 06Z Monday 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 next week; four of the members showed 93L hitting the Southeast U.S. coast. Most of the members of the 00Z Monday run of the European ensemble model showed 93L moving into the Eastern Caribbean on a more southerly trajectory without recurving.

Jeff Masters