Potential Tropical Cyclone Nine Nearing Windward Islands; Tropical Storm Watches, Warnings Issued

Jon Erdman
Published: August 17, 2017

A tropical depression or tropical storm is expected to form later Thursday before striking the Windward Islands Friday, and may pose a threat early next week in parts of Central America's Caribbean coast.

The National Hurricane Center (NHC) initiated advisories on Potential Tropical Cyclone Nine late Thursday morning, the disturbance formerly known as Invest 91L, about 350 miles east of the Windward Islands.

(MORE: What is a Potential Tropical Cyclone?)

Tropical storm warnings were issued for parts of the Windward Islands, including Martinique, St. Lucia, Barbados and St. Vincent and the Grenadines. Warnings are issued when tropical storm conditions are expected within 36 hours. A tropical storm watch was posted for Dominica, where those conditions are possible within that timeframe.

(MORE: Hurricane Central)

Watches and Warnings

The NHC said in a Thursday morning discussion that data from NOAA buoys indicates the low-pressure circulation has become better defined.

A hurricane hunter aircraft departing from St. Croix will investigate the system Thursday afternoon to determine if it is a tropical depression or tropical storm. The next Atlantic tropical storm will be called "Harvey."

Infrared Satellite: Potential Tropical Cyclone Nine

Regardless of what it's called, it will arrive in the Windward Islands Friday, bringing 2 to 4 inches of rain, which could trigger dangerous flash flooding and landslides in mountainous terrain.

After that, the system will then track westward through the rest of the Caribbean Sea, and may eventually pose a threat to parts of Central America and/or Mexico's Yucatan Peninsula early next week. 

Forecast Path

It's not clear how strong this possible tropical cyclone will become through its Caribbean trek, due to potential interaction with dry air. Interests throughout the Caribbean should monitor the progress of this system through early next week.

Two Other Disturbances to Monitor

A few hundred miles to the east, a second disturbance, designated Invest 92L by the NHC, is festering.

Despite showing some decent spin, this second disturbance appears to be in a more formidable battle against dry air.

A fresh surge of such dry air, known as the Saharan Air Layer (SAL), is pushing toward 92L right now, adding to the dry air it seems to be fighting with to its north and west. This dry air may mix in with the disturbance, preventing thunderstorms from persisting and clustering.

Water Vapor Satellite Imagery

Furthermore, if 92L tracks farther north, as expected, wind shear – the change in wind direction with height – is stronger to the north of the Leeward Islands, according to an analysis from the University of Wisconsin.

This hostile wind shear that typically rips apart tropical disturbances and weaker tropical cyclones may inhibit 92L from developing significantly, if it gains enough latitude north of the Leeward Islands this weekend.

Satellite and Wind Shear

A hurricane hunter mission has been tentatively scheduled for Saturday afternoon to determine if Invest 92L has organized and strengthened enough to be classified as a tropical depression or tropical storm.

It's worth noting even if 92L doesn't develop into a tropical cyclone the next few days, this disturbance will likely continue migrating through the Bahamas, then into the Gulf of Mexico next week and will be monitored for any potential later development.

Finally, a tropical wave emerged off the west African coast Wednesday, well east of the other disturbances, kicking off its journey across the eastern Atlantic Ocean.

This latest wave may develop by the weekend but won't near the longitude of the Lesser Antilles until early next week.

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What form that system takes – tropical wave, depression, storm or hurricane – and where it exactly tracks remain unknown at this time, though the majority of forecast guidance suggests this system will track north of the Leeward Islands.

(MORE: Why Tropical Waves are Important During Hurricane Season)

The red dots represent where named storms have developed in the Atlantic during mid-August.

We are in the climatological peak of the hurricane season, so each tropical wave or area of low pressure in the Atlantic Basin must be watched closely for development. As the image to the right shows, named storms can form in about every part of the Atlantic Basin this time of year.

As an example, the disturbance that formed into Gert to the northeast of the Bahamas over the weekend was tracked for nearly 10 days before it finally developed. That tropical wave also originated over Africa.

That said, not all tropical waves or low-pressure systems that emerge from Africa become tropical depressions or tropical storms, but they are all monitored closely, particularly when atmospheric conditions are ripe for them to spin up.

(MORE: Where Every U.S. Landfalling Hurricane Began Its Journey)

Now is a good time to make sure you have a plan in case of a hurricane strike. The Federal Alliance for Safe Homes has an excellent website to help you make your plan.

Check back with weather.com for updates in the days ahead on these latest systems and the rest of hurricane season.

(MORE: NOAA Predicts Active Rest of the Hurricane Season)

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