Super typhoon Choi wan 2009

Typhoon Choi-wan formed as weak tropical disturbance early on September 11, 2009, about 1100 kilometers (700 mi) to the east of Guam.[131] During that day the disturbance rapidly developed and was designated as a tropical depression early the next day by both the JMA and the JTWC before intensifying further and being named as Tropical Storm Choi-wan on September 12.[132][133][134][135] During September 13, Choi-wan's rapid intensification slowed down barely intensifying into a severe tropical storm, before early the next day it was upgraded to a typhoon and rapidly intensified during the day to become a Category 4-equivalent typhoon.[136][137][138] Choi-wan then intensified further during September 15, as it moved through the Northern Marina Islands with the Japan Meteorological Agency reporting peak wind speeds of 185 km/h (115 mph), whilst the Joint Typhoon Warning Center reported 1-minute peak wind speeds of 260 km/h (160 mph) which made it the first Category 5-equivalent typhoon of the 2009 Pacific typhoon season and the strongest storm thus far in 2009.[139][140][141][142]

With the help of excellent poleward outflow and high ocean heat content, Choi-wan remained at its peak intensity until early on September 17 when deep convection started to erode in the northwestern quadrant as the tropical upper tropospheric trough cell to the northwest was no longer providing good outflow.[143] Choi-wan was then downgraded to a typhoon by the JTWC as it started to undergo an eyewall replacement cycle, with the JTWC reporting a secondary peak intensity of 150 km/h, (90 mph), during the next day.[138][144] During September 19, Choi-wan rapidly weakened, as it moved into an unfavorable environment with higher amounts of vertical wind shear, causing the storm's deep convection to erode. As a result of this and dry latitude air wrapping into the low level circulation center, the JTWC decided to downgrade Choi-wan to an extratropical system and released their final advisory early the next day, before the JMA followed suit later that day.[145][146]

Despite the intensity of Choi-wan when it passed over the Northern Marina Islands, there was no casualties reported.[147] It was determined afterwards by the US Navy that the whole island of Alamagan was uninhabitable, with all but one of the structures, a facility for laboratory and research, completely destroyed and most of the islands' trees downed.[148] As a result it was determined that all of the residents of Alamagan and Agrihan needed to be completely evacuated to Saipan.[149]

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