Tropical Storm Koinu formed over the Western North Pacific Ocean southeast of Taiwan on Friday night. At 5:00 a.m. EDT on Saturday the center of Tropical Storm Koinu was located at latitude 16.0°N and longitude 130.9°E which put it about 850 miles (1730 km) southeast of Taipei, Taiwan. Koinu was moving toward the west-northwest at 11 m.p.h. (17 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 997 mb.
A low pressure system over the Western North Pacific Ocean southeast of Taiwan strengthened on Friday night and the Japan Meteorological Agency designated the system as Tropical Storm Koinu. The distribution of thunderstorms around Tropical Storm Koinu was asymmetrical. Many of the thunderstorms were occurring in bands in the western half of Koinu’s circulation. Bands in the eastern side of Tropical Storm Koinu consisted primarily of showers and lower clouds. Storms near the center of Koinu’s circulation generated upper level divergence that pumped mass away to the west of the tropical storm. Winds to tropical storm force extended out 170 miles (275 km) from the center of circulation.
Tropical Storm Koinu will move through an environment favorable for intensification during the next 24 hours. Koinu will move over water where the Sea Surface Temperatures are near 30°C. It will move under the southern part of an upper level ridge over the Western North Pacific Ocean. The ridge will produce easterly winds that will blow toward the top of Koinu’s circulation. Those winds will cause moderate vertical wind shear. The wind shear will inhibit intensification, but the shear will not be strong enough to completely stop intensification. Tropical Storm Koinu is likely to intensify gradually during the next 24 hours.
Tropical Storm Koinu will move around the southwestern part of a high pressure system over the Western North Pacific Ocean. The high pressure system will steer Koinu toward the northwest during the next 24 hours. On its anticipated track Tropical Storm Koinu will move closer to Taiwan. Koinu could approach southern Taiwan in four days. It could be a typhoon when it approaches Taiwan.