The storm reached its peak intensity on October 17 with ten-minute sustained winds of 185 km/h (115 mph) according to the Japan Meteorological Agency (JMA).

Rapid weakening ensued due to interaction with the mountainous terrain of Luzon and the disheveled core of Koppu emerged over the South China Sea on October 19.

Prior to Koppu's landfall, PAGASA raised Public Storm Warning Signals for numerous provinces; nearly 24,000 people evacuated accordingly.

[1] One day later, the Japan Meteorological Agency (JMA) upgraded the low-pressure area to a tropical depression, yet the low-level circulation center was exposed owing to strong vertical wind shear.

[5] The JTWC upgraded the system to a tropical depression early on October 13, although fragmented convective bands were wrapping tighter into a broad LLCC which remained exposed.

[6] At noon, tracking westward along the southern periphery of the deep-layered subtropical ridge, the system intensified into a tropical storm and was named Koppu by the JMA.

[9][10][11] Under a favorable environment of low vertical wind shear and radial outflow, Koppu deepened rapidly and intensified into a typhoon in the afternoon of October 15, when a tightly curved convective band was wrapping into an eye revealed by a microwave imagery.

[17] Frictional effects from land interaction with Luzon started to erode the typhoon rapidly after landfall, despite excellent dual outflow channels.

[18] Before noon or in the afternoon on October 18, Koppu emerged into the South China Sea as a disorganized system which had begun to encounter with stronger easterly vertical wind shear.

[19] Tracking northward very slowly along the western periphery of an extension of the deep-layered subtropical ridge to the east-northeast, the proximity to Luzon was inhibiting the system from consolidating, leading the JMA to downgrade Koppu to a severe tropical storm when main convection had been sheared to the west early on October 19.

[24] PAGASA began issuing Public Storm Warning Signals (PSWS) for the Philippines starting on October 15, at which time Koppu was situated 755 km (469 mi) east of Baler, Aurora.

[37] A state of calamity was declared for the provinces of Aurora, Cagayan, Isabela, Nueva Ecija, Nueva Vizcaya, Tarlac, Pangasinan, Quirino, three cities namely Dagupan, Ilagan and Tuguegarao, and to nine towns specifically Arayat, Baler, Cabatuan, Calumpit, Camiling, General Nakar, Infanta, Ramos and Sugpon owing to widespread flooding.