Satellite images on Wednesday showed Hurricane Joaquin strengthening to a Category 4 storm as it moved closer to the Bahamas, where residents braced for a potential hit over the weekend.

The U.S. National Hurricane Center (NHC) said Joaquin, which had maximum sustained winds of 130 miles per hour (209 kph), was about 170 miles (275 km) northeast of the southeastern Bahamas.

The NHC said the hurricane was expected to gradually strengthen during the next 48 hours, and added that it expected the storm to affect portions of the central and northwestern Bahamas on Thursday and Friday before moving near or over the Turks and Caicos Islands.

Tropical storm warnings and hurricane watches were in effect for portions of the central Bahamas.