A center of circulation developed within a larger cyclonic rotation associated with the monsoon near Hainan Island and the Japan Meteorological Agency classified the system as Tropical Storm Dianmu.  At 5:00 a.m. EDT on Thursday the center of Tropical Storm Dianmu was located at latitude 20.5°N and longitude 109.7°E which put it about 265 miles (430 km) east of Hanoi, Vietnam.  Dianmu was moving toward the west at 10 m.p.h. (16 km/h).  The maximum sustained wind speed was 40 m.p.h. (65 km/h) and there were wind gusts to 50 m.p.h. (80 km/h).  The minimum surface pressure was 985 mb.

The circulation of Tropical Storm Dianmu consists of a smaller counterclockwise rotation inside a much larger cyclonic flow associated with the monsoonal circulation.  Thunderstorms clustered around the western and southern portion of the smaller counterclockwise rotation and a distinct inner core developed that is separate from the broader center of the monsoonal gyre.  Additional spiral bands formed outside the core.  The thunderstorms are generating upper level divergence, primarily to the south and west of Tropical Storm Diamnu.

Tropical Storm Dianmu is moving through an environment that is favorable for additional strengthening.  It is moving over water where the Sea Surface Temperature is near 31°C.  An upper level ridge centered over China is producing easterly winds which are blowing across the top of Tropical Storm Dianmu.  Those winds are causing moderate vertical wind shear, which is slowing the rate of intensification of the tropical storm.  The proximity of Dianmu to the coast is also slowing the rate of intensification.  Tropical Storm Dianmu has 12 to 18 hours to intensify before it moves over land.

The upper level ridge over China is steering Tropical Storm Dianmu toward the west and that general motion is expected to continue.  On its anticipated track the center of Tropical Storm Dianmu could reach the coast of Vietnam in 12 to 18 hours.  Dianmu could cause some minimal wind damage and storm surge, but heavy rain will be the greatest threat.  Tropical Storm Dianmu could cause flash flooding when it moves inland over Southeast Asia.