98L in Eastern Atlantic close to becoming a tropical depression » Yale Climate Connections

The main story in the Atlantic continues to be a tropical wave designated 98L in the eastern Atlantic, which was close to tropical depression status early Wednesday afternoon. The system is expected to become Tropical Storm Sam later this week, and could threaten the Caribbean by the middle of next week.

Disturbance 98L well-organized

Early Wednesday afternoon, the tropical wave designated 98L by the National Hurricane Center (NHC) was located a few hundred miles west-southwest of the Cabo Verde Islands. Satellite images showed 98L was well-organized, with a concentrated area of heavy thunderstorms and a pronounced spin at mid-levels of the atmosphere. However, 98L had only a broad and elliptical surface circulation, without a well-defined surface circulation needed for it to be classified as a tropical depression. The wave was moving westward at 10-15 mph.

98L has strong model support for development, and conditions appear quite favorable for it to intensify throughout the next five days. Wind shear is predicted to be light to moderate (5-15 knots), sea surface temperatures (SSTs) will be at least 28 degrees Celsius (82°F), and the atmosphere is predicted to be moist, with a mid-level relative humidity of 55-70%. But the system’s development will be slowed by its close proximity to the equator (near 10°N), which will keep 98L from gaining much spin from Earth’s rotation.

98L is predicted to continue tracking west to west-northwest relatively slowly, at 10-15 mph, over the next week, potentially making it a threat to the Lesser Antilles Islands in seven to eight days. The future track will depend heavily on how quickly 98L develops, and how strong it gets. A stronger storm is more likely to track farther to the north and miss the Lesser Antilles; a weaker and slower-to-organize storm will track farther to the west and potentially enter the Caribbean. 98L’s future track will also depend upon the timing and strength of troughs of low pressure passing to the north next week, and possibly on how strong the remains of tropical storms Peter, Odette, and Rose are. Remains of these storms could create a weakness in the ridge of high pressure steering 98L, allowing it to track farther to the north and miss the Lesser Antilles.

The 0Z and 6Z Wednesday runs of the GFS and European model ensembles showed a potential threat to the Leeward Islands by the middle of next week, with the European model showing more of a threat. Compared to model runs on Tuesday, Wednesday’s model runs were slower, showing 98L would likely make its closest approach to the Lesser Antilles Islands on Wednesday, September 29. In its tropical weather outlook issued at 8 a.m. EDT Wednesday, the NHC gave 98L 2-day and 5-day odds of development of 90%. The next name on the Atlantic list of storms is Sam.