The fourth tropical depression of the Atlantic Ocean hurricane season formed today, August 18, 2015 as NASA's Terra satellite passed overhead.
On Aug. 18 at 8:45 a.m. EDT the Moderate Resolution Imaging Spectroradiometer or MODIS instrument aboard NASA's Terra satellite captured a visible image of newborn Tropical Depression 4 in the central Atlantic Ocean. The image showed thunderstorms banding around the center of circulation in all quadrants except the northwest.
The National Hurricane Center (NHC) also uses measurements from the Advanced Scatterometer or ASCAT instrument that flies aboard the EUMETSAT METOP satellite. NHC Forecaster Stewart noted that an "ASCAT-B overpass indicated the system has a well-defined circulation, and there was also a large field of 30-knots and greater wind vectors in the eastern semicircle."
At 11 a.m. EDT (1500 UTC), Tropical Depression Four (TD4) had maximum sustained winds are near 35 mph (55 kph). The center of TD4 was located near latitude 10.6 North, longitude 36.5 West. That puts the center of the depression about 1,665 miles (2,675 km) east of the Windward Islands. The depression was moving toward the west near 13 mph (20 kph) and it is expected to turn toward the west-northwest by Wednesday, August 19. The estimated minimum central pressure is 1009 millibars.
The NHC noted that gradual strengthening is forecast during the next 48 hours, and the depression is expected to become a tropical storm later today, August 18. The storm is expected to reach hurricane strength on August 21.