On September 9, an area of convectional cloudiness associated with the monsoon trough formed 370 km (230 mi) to the northwest of Palau.

[1] Satellite imagery showed that a consolidating Low Level Circulation Centre (LLCC) with convection had started to develop and was wrapping toward the center.

On September 11, the LLCC started to show improvement and was under moderate vertical shear with good westward outflow, but the JMA still upgraded the system to a minor tropical depression.

Early of September 13, both JMA and JTWC upgraded the system as a tropical storm and assigned its international name, Koppu.

But the JTWC issued their final advisory early on September 15, as Koppu was moving over land, and was expected to dissipate quickly.