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.