Early on September 11, the Joint Typhoon Warning Center (JTWC) began monitoring an area of convection roughly 1,100 km (700 mi) east of Guam [REF].
Early on September 15, the JTWC reported that Typhoon Choi-wan had intensified further and had become a Category 4-equivalent Super Typhoon whilst passing over the island of Alamagan [REF].