Tropical Storm Sanba formed over the Gulf of Tonkin east of Vietnam on Wednesday. At 11:00 a.m. EDT on Wednesday the center of Tropical Storm Sanba was located at latitude 18.9°N and longitude 108.2°E which put it about 180 miles (280 km) southeast of Haiphong, Vietnam. Sanba was moving toward the northwest at 9 m.p.h. (15 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 1003 mb.

A low pressure system over the Gulf of Tonkin east of Vietnam strengthened on Wednesday and the Japan Meteorological Agency designated the system as Tropical Storm Sanba. The distribution of thunderstorms in Tropical Storm Sanba was asymmetrical. Many of the thunderstorms were occurring in bands in the eastern side of Sanba’s circulation. Bands in the western part of Tropical Storm Sanba consisted primarily of showers and lower clouds. Storms near the center of Sanba’s circulation generated upper level divergence that pumped mass away to the west and north of the tropical storm. Winds to tropical storm force extended out 50 miles (80 km) in the eastern side of Tropical Storm Sanba. The winds in the western part of Sanba’s circulation were blowing at less than tropical storm force.

Tropical Storm Sanba will move through an environment somewhat favorable for intensification during the next 24 hours. Sanba will move over water where the Sea Surface Temperatures are near 28°C. It will move under the western part of an upper level ridge centered northwest of the Philippines. The upper level ridge will produce southeasterly winds that will blow toward the top of Sanba’s circulation. Those winds will cause some vertical wind shear but the wind shear will not be strong enough to prevent intensification. Some of the air circulating around the eastern side of Tropical Storm Sanba will flow over Hainan. The additional friction caused by the land will cause the air in that part of Sanba’s circulation to slow down. Tropical Storm Sanba could intensify gradually during the next 24 hours.

Tropical Storm Sanba will move around the western part of a high pressure system over the Western North Pacific Ocean. The high pressure system will steer Sanba toward the north-northwest during the next 24 hours. On its anticipated track the center of Tropical Cyclone Sanba will pass west of Hainan. Sanba will move toward northeastern Vietnam.

Bands in the eastern side of Tropical Storm Sanba are already producing gusty winds and locally heavy rain in parts of Hainan. Bands in the western side of Sanba’s circulation are bringing rain showers to parts of northern Vietnam. Tropical Storm Sanba will continue to produce gusty winds and locally heavy rain in parts of Hainan and northern Vietnam. Heavy rain could cause flash floods in some locations.