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Typhoon Choi-wan was a powerful typhoon that became the first Category 5 equivalent-super typhoon to form during the 2009 Pacific typhoon season. Forming on September 11, 2009, about 1100 km (700 mi) to the east of Guam, the initial disturbance rapidly organized into a tropical depression. By September 12, the depression intensified into a tropical storm, at which time it was given the name Choi-wan. The following day, rapid intensification took place through September 14. Choi-wan attained its peak intensity on September 15, as it moved through the Northern Mariana Islands with the Japan Meteorological Agency reporting peak windspeeds of 195 km/h (120 mph 10-minute sustained). Additionally, the Joint Typhoon Warning Center reported the storm to have attained winds of 260 km/h (160 mph 1-minute sustained). The typhoon remained very powerful until September 17 when the storm's outflow weakened. The typhoon underwent an eyewall replacement cycle, leading to intensity fluctuations. By September 19, Choi-wan rapidly weakened as strong wind shear caused convection to diminish. The following day, the system transitioned into an extratropical cyclone and dissipated several hours later over open waters. Despite the intensity of Choi-wan when it passed through the Northern Mariana Islands, no casualties were reported. However, following the storm, the United States Navy deemed that the island of Alamagan was uninhabitable, with all but one of the structures completely destroyed and most of the islands' trees downed. In response to this, all residents on the island were evacuated to nearby Saipan. ==Meteorological history== 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. A broad, low-level circulation center had formed within the system and became increasingly organized, showing signs of developing convective banding.〔 A Tropical Cyclone Formation Alert was released later that day as convection was rapidly consolidating around the low, which was embedded within the eastern end of the monsoonal trough. It was then declared as a tropical depression by the Japan Meteorological Agency (JMA) and the JTWC with the latter designating it as Tropical Depression 15W The tropical depression was tracking westward along the southern edge of the subtropical ridge situated east of the international dateline.〔 During September 12, the depression continued to develop quickly with both the JMA and the JTWC reporting that the depression had intensified into a Tropical Storm later that day, with the JMA assigning the international name of Choi-wan and the international designation of 0915. During September 13, Choi-wan gradually developed whilst moving west with the JMA reporting that Choi-wan had become a severe tropical storm late that day, before reporting along with the JTWC early the next day that Choi-wan had intensified further and had become a weak Typhoon with multiple spiral bands that had wrapped into the fully consolidated center to form a banding eye. During September 14, Choi-wan intensified further whilst moving along the southwestern edge of a deep level subtropical ridge of the northeast, intensifying into a Category 4-equivalent typhoon on the Saffir-Simpson Hurricane Scale with 1-minute winds of 220 km/h (140 mph).〔 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. Later that day the JMA reported that Typhoon Choi-wan had reached its 10-minute peak wind speeds of 195 km/h (120 mph) and a pressure of 915 hPa (mbar). Typhoon Choi-wan then intensified further and reached its peak 1-minute wind speeds of 260 km/h (160 mph) which made it the first Category 5-equivalent tropical cyclone on the Saffir-Simpson Hurricane Scale anywhere in the world in 2009.〔 With the help of excellent poleward outflow and high ocean heat content, Typhoon 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 (TUTT) cell to the northwest was no longer providing good outflow. 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.〔 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. The JMA then reported that the extratropical low had dissipated completely, early on September 21.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Typhoon Choi-wan (2009)」の詳細全文を読む スポンサード リンク
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