A low-pressure area formed south of Guam on September 22. On the next day, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert on the system. Owing to the low-level circulation center (LLCC) rapidly consolidating with convective banding, JTWC upgraded it to a tropical depression late on the same day. Overall, the environment was favorable for further development, with a developing anticyclone over the LLCC, weak vertical wind shear, and good poleward outflow due to a cold-core low north of the system.
Early on September 24, JTWC upgraded the system to a tropical storm; however, according to RSMC Best Track Data, the system intensified into a tropical depression at the same time. At noon, the Japan Meteorological Agency (JMA) upgraded the system to a tropical storm and named it Jangmi, when it was tracking northwestward under the steering influence of the subtropical ridge in the northeast. Several hours later, Jangmi entered the Philippine Area of Responsibility, earning the name Ofel from the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA).
Shortly after JMA upgraded Jangmi to a severe tropical storm early on September 25, JTWC upgraded it to a typhoon for its developing eye; moreover, according to RSMC Best Track Data, Jangmi intensified into a typhoon simultaneously. Tracking toward a weakness in the subtropical ridge induced by a mid-latitude trough to the north-northeast on September 26, Jangmi formed an cirrus-filled and ambiguous eye, as the typhoon gradually intensified owing to strong radial outflow provided by a mesoscale anticyclone aloft, which persisted into a tropical upper tropospheric trough cell to the east.
At 00Z on September 27, JTWC upgraded Jangmi to a super typhoon, with a round 28 nautical miles (52 km; 32 mi) eye with an intense and symmetric core of deep convection and excellent convective banding over all quadrants. Six hours later, the typhoon reached category 5 strength on the Saffir–Simpson hurricane wind scale. At 12Z, Jangmi attained peak intensity by the ten-minute maximum sustained winds reaching 115 knots (215 km/h, 130 mph) and the atmospheric pressure decreasing to 905 hPa (26.7 inHg). In terms of ten-minute maximum sustained winds, it became the most intense tropical cyclone in the Northwest Pacific Ocean since Angela in 1995, tied with Nida in the next year, until Megi surpassed all of them in 2010.
Jangmi weaken gradually due to land interaction with Taiwan early on September 28. After JTWC downgraded Jangmi to a typhoon, it made landfall over Nan'ao in Yilan, Taiwan at 15:40 TST (07:40 UTC) with ten-minute maximum sustained winds of 100 knots (185 km/h, 115 mph) and an atmospheric pressure of 925 hPa (27.3 inHg). Because of the frictional effects of land, the typhoon showed trochoidal motion overland, unexpectedly shifting toward the south-southwest shortly. Jangmi weakened significantly by the mountainous terrain of Taiwan after landfall; therefore, it lost the majority of deep convection. Under the steering influence by the eastern subtropical ridge, the system began to track northward toward a break situated east of Shanghai, China, and it arrived at the Taiwan Strait from Taoyuan at 04:20 TST on September 29 (20:20 UTC on September 28).
Early on September 29, Jangmi weakened into a severe tropical storm in the East China Sea, when JTWC downgraded it to a tropical storm. Despite lacking significant deep convection, the storm still had good radial outflow, especially the poleward outflow tapped into strong mid-latitude westerlies. As JMA downgraded Jangmi to a tropical storm at noon, the system started to track northeastward along the weakened western finger of the subtropical ridge. Jangmi underwent extratropical transition while accelerating east-northeastward and then eastward on September 30, as well as upper-level westerly winds sheared the remaining convection to the east and south of the low-level circulation center with frontal features. Late on the same day, JTWC reported that Jangmi became extratropical with a final warning.
JMA reported that Jangmi became an extratropical cyclone near the Ōsumi Islands early on October 1, and the system kept moving eastward rapidly during that day. Remaining weak and shallow, it began to drift east-southeastward slowly by early on October 2. Since the noon on the same day, the system turned to drift southward very slowly. It ultimately dissipated near Iwo Jima on October 5.
Read more about this topic: Typhoon Jangmi (2008)
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