Tropical Depression 04W strengthened to Tropical Storm Choi-wan east of Mindanao on Sunday. At 11:00 p.m. EDT on Sunday the center of Tropical Storm Choi-wan was located at latitude 8.2°N and longitude 130.4°E which put it about 260 miles (420 km) east of Mindanao. Choi-wan was moving toward the northwest at 9 m.p.h. (15 km/h). The maximum sustained wind speed was 45 m.p.h. (75 km/h) and there were wind gusts to 60 m.p.h. (95 km/h). The minimum surface pressure was 997 mb.
The circulation around former Tropical Depression 04W exhibited more organization on Sunday night and the Japan Meteorological Agency named the system as Tropical Storm Choi-wan. Although the circulation around Choi-wan exhibited more organization, the distribution of thunderstorms was asymmetrical. The strongest thunderstorms were occurring in bands in the western side of Tropical Storm Choi-wan. Bands in the eastern half of the tropical storm consisted of showers and lower clouds. Storms west of the center of Choi-wan generated upper level divergence that pumped mass away to the west and south of the tropical storm. Winds to tropical storm force extended out 85 miles (135 km) from the center of circulation.
Tropical Storm Choi-wan will move through an environment favorable for intensification during the next 24 hours. Choi-wan will move over water where the Sea Surface Temperatures are near 30°C. It will move under the southeastern part of an upper level ridge centered northeast of the Philippines. The ridge will produce easterly winds which will blow toward the top of the tropical storm. Those winds are the reason for the asymmetrical distribution of thunderstorms and they will continue to cause vertical wind shear. The wind shear will not be enough to prevent intensification, but the shear will limit the rate of intensification. Tropical Storm Choi-wan will likely intensify during the next 24 hours.
Tropical Storm Choi-wan will move around the western side of a subtropical high pressure system over the Western North Pacific Ocean. The high will steer Choi-wan toward the northwest during the next 36 hours. On its anticipated track Tropical Storm Choi-wan could approach the central Philippines in 36 hours.