Typhoon Koinu lingered over the Western North Pacific Ocean near Hong Kong on Saturday. At 11:00 p.m. EDT on Saturday the center of Typhoon Koinu was located at latitude 21.4°N and longitude 114.4°E which put it about 65 miles (105 km) south-southeast of Hong Kong. Koinu was moving toward the west-northwest at 4 m.p.h. (6 km/h). The maximum sustained wind speed was 100 m.p.h. (160 km/h) and there were wind gusts to 120 m.p.h. (195 km/h). The minimum surface pressure was 968 mb.

Typhoon Koinu weakened gradually on Saturday as it lingered near Hong Kong. Since Koinu moved very slowly, the strong winds in the lower atmosphere mixed cooler water to the surface of the ocean. Koinu was unable to extract as much energy from the cooler water and the typhoon weakened. Thunderstorms in Typhoon Koinu did not rise quite as high into the atmosphere.

Even though Typhoon Koinu weakened on Saturday, it still exhibited a well organized circulation. A very small eye was present at the center of Koinu’s circulation. The inner end of a rainband wrapped around the existing eye and eyewall, and concentric eyewalls could be developing. Bands of showers and thunderstorms revolved around the small core of Typhoon Koinu. Storms near the core generated upper level divergence that pumped mass away from the typhoon.

The size of the circulation around Typhoon Koinu decreased as Koinu gradually weakened. Winds to typhoon force extended out 25 miles (40 km) from the center of Koinu’s circulation. Winds to tropical storm force extended out 90 miles (145 km) from the center of circulation. The Hurricane Intensity Index (HII) for Typhoon Koinu was 16.5. The Hurricane Size Index (HSI) was 8.6. and the Hurricane Wind Intensity Size Index (HWISI) was 25.1.

Typhoon Koinu will move through an environment somewhat unfavorable for intensification during the next 24 hours. Koinu will move over water where the Sea Surface Temperatures are near 27°C. It will move under axis of an upper level ridge over China. The upper level winds are weak near the axis of the ridge and there will be little vertical wind shear. However, strong winds in the lower atmosphere will continue to mix cooler water to the surface of the ocean. In addition, the circulation around the northern side of Typhoon Koinu could draw in some drier air from China. Typhoon Koinu is likely to continue to weaken gradually during the next 24 hours due to the mix of cooler water to the surface of the ocean.

Typhoon Koinu will move will move around the southern part of a high pressure system over eastern Asia. The high pressure system will steer Koinu slowly toward the west during the next 24 hours. On its anticipated track the center Typhoon Koinu will move south of Hong Kong on Sunday.

Elsewhere over the Western North Pacific Ocean, Tropical Depression 15W strengthened to Tropical Storm Bolaven east-southeast of Guam. At 11:00 p.m. EDT on Saturday the center of Tropical Storm Bolaven was located at latitude 9.6°N and longitude 152.9°E which put it about 630 miles (1020 km) east-southeast of Guam. Bolaven was moving toward the west-northwest at 5 m.p.h. (8 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 1000 mb. Typhoon Watches were in effect for Guam, Rota, Tinian and Saipan.