Rapid weakening occurred due to land interaction, and on September 20, Typhoon Abby transitioned into an extratropical cyclone.

During late August and early September 1986, the Western Pacific monsoon trough became displaced to the east.

By the evening of September 9, a pronounced area of disturbed weather developed southwest of the Truk Atoll.

Aided by low wind shear, the disturbance developed slowly, even though the Hurricane Hunters initially failed to find a well-defined center.

The disturbance drifted towards the northwest,[3] and early on September 12, the Japan Meteorological Agency (JMA) started monitoring the system.

[nb 1][5] Based on ship reports, the Joint Typhoon Warning Center (JTWC) classified the system as a tropical depression at 06:00 UTC on September 13.

[5] Despite reports from Hurricane Hunter aircraft indicating that low-and upper-level circulations were not properly aligned,[3] both the JTWC and JMA estimated that Abby became a typhoon early on September 16.

[5] Twelve hours later, the JTWC stopped monitoring the system[3] while located 410 km (255 mi) southeast of Shanghai.

[10] Even though the storm weakened slightly prior to landfall,[3] some areas of the country received 40 in (1,000 mm) of rain,[11] after already having suffered severe damage from Typhoon Wayne a week earlier.