At 1430 IST, the center of Gaston was swirling south-east of Bermuda near latitude 27.9 north and longitude 52 west, the NHC said.
The storm is moving toward the northwest around 24 kilometers per hour, with maximum sustained winds of near 100 kilometers per hour.
Gaston's tropical storm force winds extend outward up to 240 kilometers from the center.
"Strengthening is forecast during the next 48 hours, and Gaston is expected to become a hurricane by tonight," the NHC said.
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Early Thursday Gaston became the third named hurricane of the Atlantic season, but weakened to a tropical storm later in the day.
Although the Atlantic hurricane season officially runs from June 1 to November 30, this year's first hurricane - Alex - formed in January during an unusual weather event.
The Climate Prediction Center of the National Oceanic and Atmospheric Administration (NOAA) initially estimated the Atlantic would see between 10 and 16 storms this year, but recently updated its prediction to 17.
Last year's number of storms was below average, with 11 tropical storms in the Atlantic, six of which became hurricanes, including two major ones.
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