 On September 16, the Japan Meteorological Agency (JMA) began monitoring an area of convection as a tropical depression about 1,270 km (790 mi) east-northeast of Guam. The depression was located within an area of low to moderate wind shear under an upper-level anticyclone. Flaring convection was noted along the eastern portion of the developing system before the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert later that day. Early on September 17, the JTWC began issuing advisories on Tropical Depression 16W. Located within a weak steering environment, the storm executed a clockwise loop while quickly strengthening. Several hours after the JTWC issued their first advisory, the JMA upgraded the system to a tropical storm and gave it the name Yagi; the name was contributed by Japan, and is the Japanese word for "goat". Late on September 18, the JMA upgraded Yagi to a severe tropical storm while the JTWC classified the storm as a minimal typhoon.
