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MIAMI (CBSMiami) – The seventh tropical depression of the season has strengthened into Tropical Storm Gaston.

At 11:00 p.m. Monday, the center of the system was about 450 miles west-southwest of the southernmost Cabo Verde Islands.

Gaston's maximum sustained winds were up to 40 mph with higher gusts. Tropical-storm-force winds extend outward up to 60 miles from the center.

Gaston is moving toward the west-northwest near 18 mph. This general motion, with a slight decrease in forward speed, is expected during the next couple of days.

Additional strengthening is expected, and Gaston is forecast to become a hurricane Tuesday night or Wednesday.

There are no coastal watches or warnings in effect.

Meanwhile, Tropical Depression Fiona is hanging on. There's also a third tropical wave moving in the middle of the Atlantic Ocean. It has a high chance of becoming a tropical disturbance within the next few days.

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