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.
Peter and Rose weaken to tropical depressions
Tropical Storm Peter brought heavy rains of one to five inches to the northern Leeward Islands, Virgin Islands, and Puerto Rico on Tuesday, but weakened to a tropical depression on Tuesday night. Relentless 25-to-30-knot southwesterly wind shear, combined with dry air, are predicted to cause Peter to degenerate to a remnant low by Thursday.
In the eastern Atlantic, Rose also weakened to a tropical depression on Tuesday night. High wind shear, dry air, and marginally warm waters should weaken Rose to a remnant low by Saturday.
Remnants of Odette may rise again
The remnants of Tropical Storm Odette, declared post-tropical south of Nova Scotia on Saturday evening, could regroup later this week as a subtropical cyclone as they arc southeastward atop warmer waters in the central Atlantic, about 500 miles west-northwest of the Azores Islands. In that case, the name Odette would be revived.
In its 8 a.m. EDT Wednesday Tropical Weather Outlook, the NHC gave ex-Odette 2-day and 5-day odds of regeneration of 30% and 50%, respectively. By the end of the week, Odette is expected to encounter high wind shear and finally lose any of the tropical characteristics it could briefly reacquire.
Bob Henson contributed to this post.
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