Tropical Storm Choi-wan formed over the Western North Pacific Ocean east of the Northern Marianas on Wednesday. At 11:00 p.m. EDT on Wednesday the center of Tropical Storm Choi-wan was located at latitude 16.3°N and longitude 149.5°E which put the center about 150 miles (240 km) east of Alamagan. Choi-wan was moving toward the west-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 1000 mb.
A low pressure system over the Western North Pacific Ocean east of the Northern Marianas strengthened on Wednesday and the Japan Meteorological Agency designated the system as Tropical Storm Choi-wan.
The distribution of thunderstorms in Tropical Storm Choi-wan was asymmetrical. Thunderstorms were occurring in bands in the western side of Choi-wan’s circulation. Bands in the eastern side of Tropical Storm Choi-wan consisted primarily of showers and lower clouds. Storms near the center of Choi-wan generated upper level divergence that pumped mass away to the west of the tropical storm. The removal of mass caused the surface pressure to decrease.
The strongest winds in Tropical Storm Choi-wan were occurring in the bands of thunderstorms in the western side of Choi-wan. Winds to tropical storm force extended out 115 miles (185 km) in the western half of Choi-wan’s circulation. The winds in the eastern side of Tropical Storm Choi-wan were blowing at less than tropical storm force.
Tropical Storm Choi-wan will move through an environment that will be somewhat 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 southern part of an upper level ridge that is south of Japan. The upper level ridge will produce easterly winds that will blow toward the top of Choi-wan’s circulation. Those winds will cause some vertical wind shear, but the wind shear will not be enough to prevent intensification. Tropical Storm Choi-wan will intensify during the next 24 hours. Choi-wan could strengthen to a typhoon by the end of the week.
Tropical Storm Choi-wan will move around the western end of a high pressure system that is over the Western North Pacific Ocean. The high pressure system will steer Choi-wan toward the west-northwest during the next 24 hours. On its anticipated track, Tropical Storm Choi-wan will move over Alamagan, Agrihan, Pagan, and Anatahan on Thursday.
Tropical Storm Choi-wan will bring strong winds and heavy rain to Alamagan, Agrihan, Pagan, and Anataham. Heavy rain could cause floods in some locations. Choi-wan could also bring gusty winds and heavy rain to Tinian and Saipan.
A Flash Flood Watch is in effect for Tinian and Saipan.
A Tropical Storm Warning is in effect for Tinian and Saipan.
A Tropical Storm Warning is in effect for Alamagan and Pagan.
A Typhoon Watch is in effect for Alamagan and Pagan.
A Tropical Storm Watch is in effect for Agrihan.