National Hurricane Center: Chances rise that Atlantic could see two tropical systems this week

082116 8 AM WEATHER MAPThe week ahead could see two tropical systems form in the Atlantic, and some forecasters are cautioning that the coastal U.S. should keep a close eye on one.

National Hurricane Center meteorologists said Sunday that the first wave likely to become a depression is one just off the western coast of Africa. It will likely follow a similar track to that of fizzling Tropical Storm Fiona.

The next tropical storm will be named Gaston.

The chances that the wave closest to the United States  – Invest 99-L – could become a tropical depression declined as the day wore on, but forecasters cautioned that outlook could change when it nears Haiti, the Dominican Republic and the southeastern Bahamas later this week.

“The most impressive thing about 99-L when viewing satellite loops is its very large size and excellent spin,” wrote Weather Underground co-founder and hurricane specialist Jeff Masters in a blog this morning. “The storm could be a long-range threat to the U.S. East Coast from Florida northwards in about seven to 10 days.”

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Invest 99-L

Invest 99-L

Invest 99-L was about 1,100 miles east of the Lesser Antilles and moving to the west-northwest at 15-20 mph as of 2 p.m.

Forecasters put its chances of becoming a tropical system by Tuesday at 10 percent and by Friday at 40 percent. Dry air was knocking down those chances for development from 50 percent in earlier forecasts.

Other conditions, however, including low wind shear and sea surface temperatures near 82 degrees are favorable for more organization.

“Environmental conditions could become more conducive for development late this week when the system is expected to be near Hispaniola and the southeastern Bahamas,” forecasters said in their 2 p.m. update.


The negative phase of the MJO, in red, dominates the Atlantic basin. Courtesy Mark Sudduth,

Mark Sudduth, a geographer and founder of, said part of 99-L’s problem is the Madden Julian Oscillation, which is no where near the Atlantic right now. The traveling line of thunderstorms and unsettled air, circles the globe, and helps embryonic tropical waves strengthen.

“We will see what happens when the energy makes its way into the southwest Atlantic later next week. Until then, 99-L will not become Gaston,” Sudduth said.

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Forecasters were far more certain about a second low-pressure area which moved off the coast of Senegal in western Africa on Saturday and was a few hundred miles southeast of the Cabo Verde Islands as of 2 p.m.. They listed its chances of becoming a tropical system at 70 percent by Tuesday and 90 percent by Friday.

“Environmental conditions are conducive for development, and a tropical depression is likely to form in the next day or two while the system moves westward and then northwestward over the eastern tropical Atlantic Ocean,” the 2 p.m. update said.

Tropical Storm Fiona, which poses no risk to Florida, continued its northwest trek in the Atlantic on Sunday. It weakened during the morning hours, with its maximum wind strength dropping from 45 mph at 5 a.m. to 40 at 11 a.m. — barely tropical-storm strength.

“Some weakening is forecast during the next 24 hours, and Fiona could weaken to a tropical depression later today or tonight,” the 11 a.m. update said.

Saharan Air Layer

Saharan Air Layer