Invest 99-L Is Being Monitored For Tropical Development in the Atlantic; Rain to Spread Through the Caribbean Islands

August 22,2016
We continue to monitor Invest 99-L for the possibility of development into a tropical depression or storm as it heads west-northwest toward the Lesser Antilles early this week.
Invest 99-L is one of three systems we are tracking in the Atlantic, including Fiona in the central Atlantic and newly formed Tropical Depression Seven in the eastern Atlantic.
(MORE: Fiona | T.D. Seven)
Of that trio, Invest 99-L will potentially be the most impactful in the near-future as it spreads locally heavy rain into the Leeward Islands and eventually Puerto Rico.
(MORE: Hurricane Central)

Atlantic Basin Infrared Satellite Image

Invest 99-L: Caribbean Threat?

Invest 99-L is currently a tropical wave – an area of low pressure without a closed, counterclockwise surface circulation – located about 500 miles east of the Lesser Antilles.
(MORE: What is an Invest?)

Invest 99-L Infrared Satellite Image

So far, 99-L has had to battle against dry air, similar to Fiona ahead of it. In the last day or so, some thunderstorm clusters have been bubbling up near the disturbance.
(MORE: Why Tropical Waves Are Important During the Hurricane Season)

Satellite Image: Where the Dry and Moist Air Is

For a tropical cyclone to form, there needs to be persistent convection (thunderstorm activity) near a surface low-pressure circulation.
Sometimes the first system – Fiona, in this case – becomes a sacrificial lamb, helping to prime the atmosphere behind it for the next system.
The National Hurricane Center says this system has a medium chance of development into a tropical depression or tropical storm over the next five days.

For now, a tight consensus of our guidance suggests the disturbance should continue in a general west or west-northwest trajectory the next several days. It should reach the Lesser Antilles late Tuesday or early Wednesday and then spread through the northeastern Caribbean Islands into Thursday.
(MORE: Three Things to Know About Spaghetti Model Forecasts)

Forecast Model Tracks: Invest 99-L

Regardless of whether it is 99-L, a tropical depression or tropical storm, this system could bring heavy rain to the Lesser Antilles beginning late Tuesday. After that, it may lead to an uptick in locally heavy rain across Puerto Rico and the U.S. Virgin Islands.
Beyond that, it's possible this system may threaten the Bahamas, then eventually some portion of the U.S., but it's simply far too soon to determine.
The uncertainty is, in part, due to the fact we don't yet have a closed circulation with which numerical guidance can use as a starting point. It's hard to know where you're going if you don't know where you are.
(MORE: Most Intense U.S. Landfalls Have Happened in a 17-Day Period)
Secondly, any low center that does form may spend time interacting with land areas of the Caribbean, which may further inhibit development.
There's a reason some of the world's best hurricane forecasters at the National Hurricane Center only issue forecasts and outlooks five days out.
To put it bluntly...

Numerical forecast models don't yet have the sufficient skill to determine the eventual track and intensity of a tropical cyclone that hasn't even formed yet while it's still thousands of miles and over a week away.

Keep that important point in mind if you see any social media posts in the days ahead suggesting any landfall more than seven days out. We wrote an example of this overhype two years ago regarding what eventually became Hurricane Cristobal.
(MORE: Where Every U.S. Hurricane Has Hit Since 1985)
We're now headed into, climatologically, the most active time of the Atlantic hurricane season.
(MORE: When Atlantic Season Peak Arrives)

Typical tropical storm and hurricane origin areas and tracks in August. This is **not** a forecast for this current system, merely a representation of August climatology.

This time of year, tropical waves, also known as African easterly waves, line up over Africa south of the Sahara Desert, and emerge into the eastern Atlantic Ocean.
A fraction of these tropical waves serve as the seed for an Atlantic basin tropical storm or hurricane each season.
Regardless of whether Invest 99-L becomes a threat, now is a good time to make sure you have a plan before a hurricane hits.
Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7.