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Tropical systems to unleash torrential downpours in northern Caribbean

By Alex Sosnowski, AccuWeather senior meteorologist
August 10, 2017, 7:44:43 AM EDT

Downpours will push westward across the northern islands of the Caribbean Sea, while a second tropical system, dubbed 99L, may develop and approach the United States next week.

Unsettled weather, similar to a tropical depression or a weak tropical storm, will affect areas from the the Bahamas to Cuba into Thursday. However, this system is not likely to become an organized tropical system.

Static Atlantic Tropics

These islands will experience a pulse of drenching showers and locally gusty thunderstorms over a 12- to 36-hour period.

The showers and thunderstorms may postpone some activities such as a day at the beach or boating adventures. However, on a more serious note, some communities may be hit with torrential rainfall and life-threatening flash flooding.

Flooding rainfall and road closures occurred across the United States Virgin Islands and the British Virgin Islands from late Monday to Monday night. Homes and businesses sustained major damage in some communities. A significant number of people were still without power on Tortola Wednesday morning.

Static Tortola Flooding Monday Aug. 7

Torrential rain unleashed dangerous flash flooding on the island of Tortola, British Virgin Islands, on Monday evening, Aug. 7, 2017. (Photo/Mahde Said)

Late this week, some of this moisture may feed into the general pattern of showers and thunderstorms and enhance rainfall and gusty storms along the southeastern coast of the United States.

99L may develop, become a concern for US

A more organized batch of showers and thunderstorms, referred to as 99L, was located hundreds of miles farther east over the Central Atlantic.

When batches of thunderstorms in the Atlantic Ocean have the potential to become a tropical depression or storm, they are given a number between 90 and 99. The "L" designation refers to a system under investigation in the Atlantic.

"We noticed an uptick in thunderstorms and some spin to the cloud cover on Monday," according to AccuWeather Hurricane Expert Dan Kottlowski.

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"However, 99L will be moving through a zone of dry air and strong wind shear into Friday," Kottlowski said.

Wind shear is the changing of speed and direction of winds at different layers of the atmosphere. Strong wind shear can prevent tropical development or diminish the intensity of tropical storms and hurricanes.

While 99L could become a tropical depression at any time, rapid strengthening seems unlikely in the short term.

It is likely that 99L will take a general path just northeast of the Leeward Islands later this week.

"Should 99L survive the dry air and wind shear, the system may develop and strengthen near or just northeast of the Bahamas this weekend," Kottlowski said.

Static 99L Possible Tracks 2 pm

The next name on the list of Atlantic tropical storms is Gert.

An arm of showers and thunderstorms associated with 99L will extend southward in parts of the Leeward Islands later Thursday and Friday. Cleanup efforts in the British and United States Virgin Islands following flooding from Monday night could be hindered.

In the long range, if the system develops, then it could be drawn northward along the Atlantic coast of the U.S. next week. Interests in the Bahamas and along the U.S. Atlantic coast should monitor the progress of 99L.

Hundreds of miles farther west, Franklin will inundate Mexico with flooding and mudslides over the span of a few days this week. Franklin may become the first hurricane of the 2017 Atlantic season.

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