Continuing to intensify, Typhoon Kelly reached its maximum intensity on October 15, but a weakening trend began thereafter.

Following multiple outbreaks of tropical cyclone activity across the Western Pacific basin in September 1987, a large ridge developed at unusually low latitudes of the basin, forcing the Intertropical Convergence Zone further south and making environmental conditions unfavorable for tropical cyclone formation.

The next day, satellite imagery indicated that a low-pressure area embedded in the monsoon trough developed 350 km (215 mi) south of Yap.

Although shower activity was initially displaced to the north of the low, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert for the system, on the basis of increased circus outflow.

[4] Despite a 100 km (60 mi) displacement between the surface and upper-level circulations, the JTWC upgraded the depression into a tropical storm at 12:00 UTC.

[5][nb 2] While moving north, the low-and upper-level centers of Kelly became better aligned on October 11, which resulted in strengthening.

[1] Meanwhile, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) also began to monitor the storm, with the agency assigning it the local name Oniang.

[1][8][2] Kelly then emerged into the Sea of Japan, and at 06:00 UTC on October 17, the JTWC declared the system an extratropical cyclone.

[16] Offshore, a 12,346 short tons (11,200 t) freighter, the Eleftheria II, was badly damaged when it slammed onto rocks off the coast of Shikoku, where its 24 crewmen were rescued.