On September 5, 2018, the Joint Typhoon Warning Center (JTWC) began monitoring a tropical disturbance near the International Date Line. Steady development ensued over the following days, and the system organized into a tropical depression on September 6, though operationally, the Japan Meteorological Agency (JMA) did not classify the system as a tropical depression until September 7. The depression soon intensified into a tropical storm, upon which it received the name Mangkhut. Throughout the next two days, the system underwent rapid intensification. Tight banding features wrapped around a developing eye feature. Favorable environmental conditions hastened Mangkhut's development, including low wind shear, ample outflow aloft, high sea surface temperatures, and high ocean heat content. Mangkhut achieved typhoon strength on September 9. A well-defined 18 km (11 mi) eye became evident on satellite imagery as the typhoon approached the Northern Mariana Islands and Guam. The JTWC analyzed Mangkhut as a Category 2-equivalent typhoon with one-minute sustained winds of 165 km/h (105 mph) as it tracked near Rota, around 12:00 UTC on September 10. The JMA assessed the storm's ten-minute sustained winds to be 155 km/h (100 mph) at this time.