 Towards the end of September , the monsoon trough spawned a tropical disturbance east @-@ northeast of Yap , which became a tropical depression on September 24 . There were initially several circulations , with a tropical upper tropospheric trough to the northeast increasing outflow . After slowing and turning to the northeast , the depression intensified into Tropical Storm Koppu on September 26 . After the storm developed a large eye feature , the JTWC upgraded it to typhoon status on September 27 , although the JMA did not follow suit until the following day while near Chichi @-@ jima . Also that day , Koppu passed 95 km ( 60 mi ) west of Iwo Jima , and the JMA estimated peak 10 minute winds of 130 km / h ( 80 mph ) . The JTWC estimated peak 1 minute winds of 165 km / h ( 105 mph ) , before an approaching trough caused the typhoon to accelerate northeastward . The convection diminished near the center , causing Koppu to become extratropical on September 30 . The remnants continued generally northeastward through the Aleutian Islands , eventually passing south of mainland Alaska on October 7 . 
