In Vietnam, the typhoon severely affected the rice harvest and winter crop cultivation. The Nam Định Province suffered the brunt of the typhoon, where the 134 people died and 289 sustained injuries. There, 400 schools and 230 nurseries were flattened. Throughout the country, the storm destroyed 271,000 ha (669,655 acres) of farmland. Typhoon Agnes originated from an isolated area of convection located near the equator on October 28. Satellite imagery indicated a possible low pressure area near 149° E. Although the disturbance was not attached to the monsoon trough, an upper-level anticyclone became situated aloft, providing the system with strong ventilation. Midday on October 30, the Joint Typhoon Warning Center (JTWC) started tracking the system, with the Japan Meteorological Agency (JMA) upgrading the system to a tropical depression on the same day. Late on October 31, shower activity increased drastically in both coverage and organization as the disturbance tracked northwest. Early the next day, the JTWC issued a Tropical Cyclone Formation Alert. Shortly thereafter, a Hurricane Hunter aircraft investigated the system and found winds of ; these winds were significantly higher than intensity estimates from the Dvorak technique. At 06:00 UTC on November 1, the JTWC issued its first warning on the system, upgrading it to a tropical storm and naming it "Agnes". The JMA also classified Agnes as a tropical storm at this time. Although Agnes initially continued to track northwest, passing through the Caroline Islands, by the evening of November 1, Agnes moved northward enough to feel the effects of a subtropical ridge anchored to its north, causing the storm to turn west-northwest. Despite its asymmetric structure, the JTWC classified Agnes as a typhoon midday on November 2. The storm began to rapidly intensify on November 3, with the JMA upping Agnes to a severe tropical storm at 00:00 UTC, and to a typhoon six hours later. Meanwhile, the JTWC elevated the intensity to , which would make it a Category 3 hurricane on the United States-based Saffir-Simpson Hurricane Wind Scale (SSHWS). That afternoon, however, the typhoon began to level off in strength. Around this time, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) also monitored the storm and assigned it with the local name "Undang".