Friday, July 7, 2017

Invest 94L May Become a Tropical Depression; Could Pass Near Lesser Antilles This Weekend

Jon Erdman
Published: July 5,2017

Invest 94L, a tropical disturbance between Africa and the Lesser Antilles, may become the next Atlantic tropical depression, possibly adding to what's been an odd start to the 2017 Atlantic hurricane season.
(MORE: What is an Invest? | Hurricane Central)
This cluster of convection we've been watching since last weekend is now about 900 miles west of the Cabo Verde Islands, nearly halfway between the west African coast and the Caribbean's Lesser Antilles.

Current Infrared Satellite Image: Invest 94L
Thunderstorms have become a bit more concentrated near this broad area of low pressure, but have not yet been persistent enough to merit an upgrade to either Tropical Depression Four or Tropical Storm Don.
(MORE: Why We Watch the Cabo Verde Islands in Hurricane Season)

Lesser Antilles Weekend Threat?

This system will track west-northwestward the next several days around the south side of the Bermuda-Azores high-pressure cell typically in place in summer.
The setup that will steer the tropical system in the general direction of the Lesser Antilles this weekend.
Numerical forecast guidance has been consistent suggesting the system will reach the longitude of the Lesser Antilles this weekend.
In general, if the system develops into a tropical depression or tropical storm sooner, it should get tugged a bit farther north by the weekend, increasing the odds the center remains north of the Leeward Islands.
This general track north of the Leewards this weekend is agreed upon by the majority of ensemble forecast model tracks.

GFS Ensemble Model Tracks - Invest 94L
However, if 94L remains weak, it may instead take a more southerly track closer to or over parts of the Lesser Antilles. This scenario is suggested by a smaller number of ensemble model tracks.
Those in the Lesser Antilles should continue to monitor the progress of this system closely.
Invest 94L will have a couple barriers to overcome along the way.
First, intrusions of dry air from the northeast Atlantic Ocean are virtually surrounding the disturbance, seen nicely in this 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.
(MORE: A Typical July in the Atlantic Hurricane Season)
Secondly, wind shear is affecting Invest 94L, and that may continue through late week. Wind shear is 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.
This wind shear isn't terribly strong for this early in the season, but it may be enough to keep this disturbance from developing further.

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.

U.S. Concern?

Invest 94L is still roughly 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.
The majority of forecast guidance suggests a general track somewhere between Bermuda and The Bahamas later next week. Whether the system is "Don," a tropical depression or merely a tropical wave is yet to be determined.
Potential U.S. impact later next week 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)

Continuing a Fast Start to the Season

Origin points of all July named storms in the Atlantic Basin since 1950.
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.
However, there are relatively few that have formed as far east as where Invest 94L may develop.
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.
Just a couple weeks ago, Tropical Storm Bret formed east of the Windward Islands, at an unusually low latitude for June.
(MORE: Strange Start to the Atlantic Hurricane Season)
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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