A distinct center of circulation formed within an area of thunderstorms south-southeast of Okinawa on Thursday night and the Japan Meteorological Agency designated the system as Tropical Storm Prapiroon. At 8:00 p.m. EDT on Thursday the center of Tropical Storm Prapiroon was located at latitude 20.0°N and longitude 130.2°E which put it about 500 miles (800 km) south-southeast of Okinawa. Prapiroon was moving toward the west at 7 m.p.h. (11 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 circulation of Tropical Storm Prapiroon was still organizing. More thunderstorms were forming close to the center of circulation. There were more thunderstorms west of the center than there were east of the center. Several bands of showers and thunderstorms in the outer part of the circulation were revolving around the core of Tropical Storm Prapiroon. The storms in the core were generating upper level divergence which was pumping mass away from the tropical storm.
Tropical Storm Prapiroon formed beneath an upper level ridge that developed between an upper level low northwest of the Philippines and a much larger upper low east of the Marianas. The winds in the upper ridge will be weak and there will be little vertical wind shear. Prapiroon will move over water where the Sea Surface Temperature is near 29°C. Tropical Storm Prapiroon will move through an environment very favorable for intensification. Prapiroon will strengthen and it could intensify rapidly once the inner core organizes. Tropical Storm Prapiroon could strengthen into a typhoon in 24 to 48 hours.
Tropical Storm Prapiroon was moving around the southwestern part of a subtropical ridge which was steering the tropical storm toward the west. Prapiroon will turn toward the north in 12 to 24 hours when it reaches the western end of the ridge. On its anticipated track Tropical Storm Prapiroon could approach Okinawa in about 36 hours. Prapiroon could be a typhoon when it nears Okinawa.