In the evening of September 21, a low pressure area developed to the east-southeast of Palau. The low gradually drifted west and became more well organized prompting the JTWC to issue a Tropical Cyclone Formation Alert (TCFA) on it. Later, the LLCC became well organized with flaring deep convection developing around it, so the JMA started monitoring the low as a tropical depression. Later on September 23, the JTWC also initiated advisories on the system designating it with "20W". Early on September 24, the JMA further upgraded 20W to a tropical storm and named it "Nesat". Nesat continued to drift west with expanding deep convection around the entire system and consolidating convection around the LLCC. The mid-level warm anomaly near the system continued to intensify and convective banding near the LLCC became more and more tighter. As a result, the JMA upgraded Nesat to a severe tropical storm on September 25. Late on the same day, the JMA further upgraded Nesat to a typhoon.