Typhoon Ma-on (international designation: 1106, JTWC designation: 08W, PAGASA Name: Ineng) was a powerful typhoon that affected southern Japan. It was the sixth named storm and second typhoon of the 2011 Pacific typhoon season.
The origins of the typhoon were from an area of convection that persisted near Wake Island on July 9. The system slowly organized as it developed a low-level circulation. With low wind shear and generally favorable environmental conditions, tropical cyclone forecast models anticipated the development of a tropical cyclone from the system. Early on July 11, the Joint Typhoon Warning Center (JTWC) issued a tropical cyclone formation alert, and a few hours later the Japan Meteorological Agency (JMA) reported the formation of a tropical depression about halfway between Wake Island and the Northern Marianas Islands. The JTWC followed suit by initiating advisories on Tropical Depression 08W.
Upon its formation, the depression tracked westward due to a ridge to its north. The circulation was initially broad and ill-defined, while its convection was disorganized due to dry air. It was able to intensify due to generally favorable conditions, and the JMA upgraded the depression to Tropical Storm Ma-on at 0600 UTC on July 12. Gradually the thunderstorms became concentrated around the center, despite restricted outflow to the north and west. Ma-on intensified at a slower than climatological rate, although an eye feature became evident by early on July 13. At 0000 UTC that day, the JMA upgraded Ma-on to a severe tropical storm, and 18 hours later they upgraded it to a typhoon. By that time, it was located about 970 km (575 mi) southeast of Iwo Jima. A ragged eye became apparent on satellite imagery, and after developing an anticyclone aloft, its outflow became much better defined.
By July 15, Typhoon Ma-on had a well-defined eye with the strongest convection in its southern periphery. It continued intensifying, and the JTWC estimated 1-minute sustained winds of 135 mph (220 km/h). Late on July 15, Ma-on weakened slightly due to stronger wind shear, which caused its eyewall to break apart in the northwest quadrant. It re-intensified the next day after an eyewall replacement cycle commenced. At 0600 UTC on July 16, the JMA estimated peak 10-minute sustained winds of 175 km/h (110 mph) while the typhoon was located about 1185 km (735 mi) southeast of Okinawa. Around that time, Ma-on began a motion to the northwest due to a weakening of the subtropical ridge, and it briefly entered the area warned by the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA); the agency gave it the local name Ineng.
Typhoon Ma-on on July 15
Late on July 17, Ma-on underwent another eyewall replacement cycle and weakened, despite developing improved outflow and convection in the northern quadrant. The typhoon’s large size prevented re-intensification – gale force winds extended 370 km (200 mi) east of the center. In addition, the intrusion of dry air diminished thunderstorms in the western periphery. By July 18, Ma-on reached the western extent of the ridge and began a motion to the north toward Japan. The next day, it turned to the northeast as it paralleled the Japan coastline just offshore. According to the JTWC, Ma-on briefly weakened to a tropical storm before re-intensifying into a typhoon, making landfall along the Kii Peninsula in Honshu. The JMA maintained the system as a typhoon until 0000 UTC on July 20, when Ma-on moved ashore. By the time it moved across Japan, the storm was moving due eastward, and after emerging from the country it turned to the southeast. Increased wind shear displaced the convection to the east, although slight re-intensification was expected. However, the JTWC downgraded Ma-on to a tropical depression on July 21 after the storm lost much of its convection. The circulation became ill-defined, and the JTWC discontinued advisories on July 22, noting the system was in the process of dissipation. However, the JMA maintained Ma-on as a severe tropical storm until July 23, by which time the storm had turned to the northeast. The agency discontinued advisories on July 24.