TWC: Invest 99-L May Develop Into a Tropical Depression or Tropical Storm; Southeast U.S. Should Watch Closely

We continue to monitor Invest 99-L for the possibility of development into a tropical depression or storm as it moves into the Lesser Antilles, then heads in the general direction of the Bahamas late in the week. It’s possible this system could affect Florida and other parts of the Southeast U.S. late in the weekend into next week, however uncertainty remains high.

Invest 99-L is one of two systems we are tracking in the Atlantic, the other being Gaston in the eastern Atlantic.

(MORE: Hurricane Central)

Below is a look at what to expect from Invest 99-L in the Caribbean first, and then what the long-term future holds for any potential Bahamas or U.S. impacts.

Invest 99-L: Caribbean Impact

A Hurricane Hunters reconnaissance mission examined Invest 99-L early on Tuesday and found that it had an elongated and poorly defined circulation center.

Invest 99-L Infrared Satellite Image

The highest cloud tops, corresponding to the most vigorous convection, are shown in the orange and red colors. The approximate location of 99-L is circled.

Convection has been pulsing up at times in association with 99-L, despite a battle against some dry air near the eastern Caribbean Sea. However, the National Hurricane Center said the shower and thunderstorm activity was disorganized as of Tuesday afternoon.

(MORE: Why Tropical Waves Are Important During the Hurricane Season)

Satellite Image: Where the Dry and Moist Air Is

This satellite image shows the amount of water vapor in the atmosphere’s middle levels. Dry air shows up as dark orange and red. The location of the current system is shown by a white circle.

For a tropical cyclone to form, there needs to be persistent convection (thunderstorm activity) near a well-defined area of low pressure at the earth’s surface.

The National Hurricane Center says this system has a medium chance of development into a tropical depression or tropical storm over the next 2 to 5 days. Another Hurricane Hunters reconnaissance mission will investigate 99-L on Wednesday, if necessary.

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

The lines indicate possible tracks of Invest 99-L from various numerical forecast models.

Regardless of whether it is 99-L, a tropical depression or tropical storm, this system will 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, the Virgin Islands and Hispañiola.

The National Weather Service has issued a flash flood watch for Puerto Rico and the U.S. Virgin Islands. Rainfall totals of 2-4 inches (locally higher) are expected through Wednesday night in those areas.

Caribbean Rainfall Forecast Through Friday

Bahamas, Florida Threat?

Beyond that, this system, in whatever form it’s in, should arrive in the southeast or central Bahamas by Friday or Saturday.

(MORE: Most Intense U.S. Landfalls Have Happened in a 17-Day Period)

Once it reaches this area, the environment may feature low-enough wind shear and sufficient moisture to allow a tropical depression or storm to finally form, if it hadn’t done so already.

By this time, upper-level high pressure should be established over the southern Appalachians eastward to the coast of Virginia and North Carolina.

Potential steering flow in the upper-atmosphere late in the week/weekend. Upper high could steer the system west once it’s near the Bahamas.

This steering flow could, then, push our system (T.D. Eight or “Hermine”) toward Florida later in the weekend or early next week.

(MORE: Where Every U.S. Hurricane Has Hit Since 1985)

An alternate, but lower-probability scenario is for “it” to merely stall out near the central Bahamas through much of next week before eventually being picked up by the jet stream and carried out to the open Atlantic.

It is simply too early to make a definitive call.

For now, if you have interests along the Southeast coast, including Florida and the Bahamas, check back with us at weather.com for any important forecast changes in the days ahead.

(FORECAST: Turks and Caicos | Nassau, Bahamas | Miami | Daytona Beach | Orlando)

We’re now headed into, climatologically, the most active time of the Atlantic hurricane season.

(MORE: When Atlantic Season Peak Arrives)

August’s Typical Origins and Tracks

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.