In the middle of September , the monsoon trough spawned a rapidly organizing disturbance east @-@ northeast of Luzon , with weak wind shear and favorable conditions . On September 16 , the JMA classified it as a tropical depression , and the JTWC initiated advisories the next day . The system moved to the northwest due to the subtropical ridge to the northeast and later to the north . On September 18 , the JMA upgraded the depression to Tropical Storm Choi @-@ wan , the same day that PAGASA classified it as Tropical Storm Roskas . An eastward @-@ moving trough turned the storm to the northeast , bringing the track over Okinawa and Amami Ōshima on September 19 . Choi @-@ wan continued gradually intensifying , becoming a typhoon on September 20 to the southeast of Japan . That day , the JMA estimated peak winds of 130 km / h ( 80 mph ) , and the JTWC estimated peak 1 minute winds of 185 km / h ( 115 mph ) on September 21 , after Choi @-@ wan developed a well @-@ defined eye . The typhoon weakened due to increasing wind shear , deteriorating to severe tropical storm status on September 22 before JMA declared it extratropical on September 23 . The remnants of Choi @-@ wan continued to the northeast , exited the basin on September 24 , and eventually struck southern Alaska on September 25 .