Saffir–Simpson hurricane wind scale
On September 17, The JTWC upgraded Usagi to a tropical storm, as the system continued to consolidate and wrap tighter when slowly tracking westward along the extreme southern periphery of the subtropical ridge. Early on September 18, JMA upgraded Usagi to a severe tropical storm; at noon, both JMA and The JTWC upgraded Usagi to a typhoon, as deep convection had completely wrapped around a developing eye with radial outflow and low vertical wind shear. On September 19, Usagi began explosive intensification and formed a round eye; as the result, The JTWC upgraded Usagi to a category 5 equivalent-super typhoon on the Saffir–Simpson hurricane wind scale at noon. However, in a post season reanalysis, it was concluded that Usagi peaked with winds at 155 mph, making it a strong category four on the Saffir-Simpson hurricane wind scale. At 18Z, Usagi reached its peak intensity with 10-minute maximum sustained winds at 205 km/h (125 mph) and the atmospheric pressure at On September 20, Usagi began an eyewall replacement cycle with a inner eyewall and a outer eyewall separated by a clearly defined moat, as well as the system began to weaken late on the same day due to land interaction between Taiwan and Luzon.