Tropical Storm Sepat formed near Honshu on Thursday. More thunderstorms formed near the center of a low pressure system near the southeast coast of Honshu on Thursday and the Japan Meteorological Agency designated the system as Tropical Storm Sepat. At 8:00 a.m. EDT on Thursday the center of Tropical Storm Sepat was located at latitude 32.6°N and longitude 134.7°E which put it about 100 miles (160 km) southwest of Susami, Japan. Sepat was moving toward the northeast at 32 m.p.h. (50 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 998 mb.
The distribution of thunderstorms around Tropical Storm Sepat was asymmetrical. The strongest thunderstorms were in several bands south and east of the center of circulation. Bands in the other parts of the circulation consisted primarily of showers and lower clouds. Storms southeast of the center of Sepat were generating upper level divergence which was pumping mass away to the northeast of the tropical storm. Winds to tropical storm force extended out about 150 miles (240 km) to the east of the center of circulation.
Tropical Storm Sepat will move through an environment marginally favorable for intensification during the next 12 to 24 hours. Sepat will move over water where the Sea Surface Temperature is near 24°C. An upper level trough west of Japan will produce strong southwesterly winds which will blow toward the top of the circulation. Those winds will cause moderate vertical wind shear, which is the primary reason for the asymmetrical distribution of thunderstorms. Moderate vertical wind shear and cooler water will cause the structure of Tropical Storm Sepat to change to that of an extratropical cyclone. Sepat could strengthen during the extratropical transition because upper level divergence will cause the surface pressure to decrease.
The upper level trough will steer Tropical Storm Sepat rapidly toward the northeast. On its anticipated track the center of Tropical Storm Sepat will pass near the coast of Honshu. Sepat will bring gusty winds and it could drop locally heavy rain.