Monday, July 3, 2017

Invest 94L Likely to Develop Into a Tropical Depression Later This Week; Possible Threat to Lesser Antilles This Weekend

Jon Erdman
Published: July 3,2017

Invest 94L, a tropical disturbance between Africa and the Lesser Antilles, is now likely to become the next Atlantic tropical depression or tropical storm later this week, continuing what has been a somewhat fast start to the 2017 Atlantic hurricane season.
(MORE: What is an Invest? | Hurricane Central)
For now, this area of disturbed weather is a festering cluster of convection about 700 miles southwest of the Cabo Verde Islands, not yet halfway between the west African coast and the Caribbean's Lesser Antilles.

Current Infrared Satellite Image: Invest 94L
This broad area of low pressure originated from a tropical wave, one of dozens of such disturbances that track across the tropical Atlantic Ocean from Africa each hurricane season. Think of it as a seed of a potential tropical cyclone later on.
(MORE: Why We Watch the Cabo Verde Islands in Hurricane Season)
Invest 94L will eventually sweep northwestward and then west-northwestward the next several days around the south side of the Bermuda-Azores high-pressure cell typically in place in summer.
The National Hurricane Center has assigned a "high" chance this tropical disturbance will eventually develop into a tropical depression or tropical storm sometime later this week.
The next Atlantic tropical storm will be named "Don."

Possible Tropical Development Area

Obstacles to Development

Invest 94L may have a couple of barriers to overcome.
First, dry air from the northeast Atlantic Ocean has arrived to the north and west of the disturbance.
You can see this nicely in a satellite loop from UW-CIMSS, showing the yellow-, orange- and red-shaded areas pouring southwestward into the strip of the tropics between Africa and the Lesser Antilles.
It's not clear whether 94L will be able to insulate itself from this dry air, or if it will ingest some of it which would limit its development.

Water Vapor Satellite Image
(MORE: A Typical July in the Atlantic Hurricane Season)
Secondly, wind shear, the change in wind speed and/or direction with height that can rip apart a fledgling tropical disturbance by blowing thunderstorms away from developing low pressure, increases closer to the Lesser Antilles.
This wind shear isn't terribly strong for this early in the season, but it may be enough to mess with this disturbance later in the week.

Current Satellite and Wind Shear
One factor working in favor of this next tropical wanna-be is warm ocean water.
Michael Lowry, tropical scientist at UCAR and FEMA, noted sea-surface temperatures between Africa and the Lesser Antilles were the warmest for any June in six years. This can add instability to maintain thunderstorms near a developing tropical cyclone.

Lesser Antilles Threat

While it's not a certainty a tropical depression or tropical storm will form, numerical forecast guidance has been consistent suggesting the system will reach the longitude of the Lesser Antilles this coming weekend.
Origin points of all July named storms in the Atlantic Basin since 1950.
(MORE: Strange Start to the Atlantic Hurricane Season)
Whether "it" passes over or north of the islands, and whether it is in the form of a tropical depression, tropical storm or just a tropical wave, is still too early to call.
Those in the Lesser Antilles should monitor the progress of this system closely.
While the majority of July tropical storms in the Atlantic Basin have formed in the western Atlantic Ocean and Gulf of Mexico, a number of storms have also formed between the Lesser Antilles and Africa.
Just a couple weeks ago, Tropical Storm Bret formed east of the Windward Islands, at an unusually low latitude for June.
Interestingly, since 1950, there have been only six systems that became tropical depressions east of the Lesser Antilles in the first half of July which later became hurricanes. Bertha in 2008 was the last system to do so.

US Concern?

Invest 94L is still over 3,000 miles away from the southeast U.S. coast.
Given there is still significant uncertainty regarding the Lesser Antilles, it is far too soon to determine if this system will eventually pose a threat to the mainland U.S.
Assuming it survives as a tropical cyclone of some sort beyond the Lesser Antilles next week, potential U.S. impact would depend on the configuration of winds aloft steering the storm, either curving it out to sea away from the coast or pushing it closer to some parts of the coast.
(MORE: Why Long-Range Model Forecasts For the Tropics Can't Often Be Trusted)
This is a good time to make sure you have a plan in case a hurricane threatens. The Federal Alliance for Safe Homes has an excellent site that can help you make a hurricane plan.
Check back with us at for the latest on this possible tropical development.
MORE: Atlantic Basin Retired Hurricanes and Tropical Storm Names

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