[nb 1] Subsequent interaction with a mid-latitude trough induced an extratropical transition as the cyclone slowed and turned north.

The system had lost most of its tropical characteristics by September 1, with the majority of convection sheared northeast over South Korea and Japan.

A further 14 people died in mainland China where coastal flooding from the typhoon's slow movement coincided with the spring tide, resulting in the greatest storm surge since 1949 along the Yangtze estuary and Hangzhou Bay.

On August 23, 1981, following several days of tranquil weather, a tropical upper tropospheric trough (TUTT) began building westward from the International Date Line, near Wake Island.

Early on August 27, a reconnaissance plane measured a central pressure of 994 mbar (994 hPa; 29.4 inHg) and flight-level winds of 85 km/h (53 mph), indicating the system had become a tropical storm.

Early on August 31, Agnes achieved its peak strength with winds of 175 km/h (109 mph) and an estimated central pressure of 947 mbar (hPa; 27.96 inHg).

[1] The JMA estimated the system to have been slightly weaker, with ten-minute sustained winds of 150 km/h (93 mph) and a pressure of 950 mbar (hPa; 28.05 inHg).

Subsequent slowing of the cyclone's forward speed as it turned northward resulted in an extended period of heavy rain for much of South Korea.

The JTWC estimated Agnes to have become predominantly extratropical by the end of September 1, coinciding with the system weakening below typhoon intensity.

Accelerating over the Sea of Japan, the remnant storm deepened slightly before striking the northern Japanese island of Hokkaido on September 4 with an estimated pressure of 976 mbar (hPa; 28.82 inHg).

[2] On August 30, Typhoon Agnes moved through the southern Ryukyu Islands, bringing strong winds and heavy rain to the region.

[10] Significant water rise occurred along Yangtze estuary and Hangzhou Bay in mainland China as Agnes remained just east of the area for 36 hours and the coincidence of the spring tide.

Model simulations of the event estimated that a 1.5 m (4.9 ft) surge affected the Waigaoqiao Free Trade Zone in Shanghai.

United States President Jimmy Carter, wrapping up an inspection of the Chinese Navy, was scheduled to take a cruise out of the Huangpu River; however, the typhoon forced the vessel to remain at port.

Regarded as the heaviest on record in the 20th century, at least 710 mm (28 in) fell over a two-day span in the country, resulting in tremendous flash flooding and landslides.

[14] The disaster relief agency Hope Bridge provided 7.5 billion won ($10.2 million) in aid to areas affected by the storm.