Early on August 8, Dianmu formed in the Northwestern Pacific Ocean and by 2100 UTC (5 p.m. EDT) Dianmu had maximum sustained winds near 35 knots (40 mph) and was about 180 miles southwest of Okinawa, Japan.
On August 9 at 04:35 UTC (12:35 a.m. EDT), the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua satellite captured an infrared image of Tropical Storm Dianmu. The AIRS infrared image showed very cold cloud-top temperatures and showed banding of strong thunderstorms around the north, east and south of the center of circulation. Those cloud tops were so high that they were colder than -63 Fahrenheit. The center of circulation was also clearly visible in the infrared image and appeared as a small circle.