Typhoon Pongsona - Meteorological History

Meteorological History

During late November, an area of convection persisted about 625 kilometers (390 mi) east-southeast of Pohnpei. Satellite imagery indicated broad cyclonic turning in the lower levels of the atmosphere, and a trough was located near the surface. The disturbance developed rainbands and gradually became better organized. By December 2, the system had an elongated low-level circulation, located to the south of the convection. At 0600 UTC that day, the Japan Meteorological Agency (JMA) classified the system as a tropical depression about 735 km (450 mi) east-northeast of Pohnpei. Shortly thereafter, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert, and at 1800 UTC on December 2 the agency classified the system as Tropical Depression 31W. Initially the depression moved to the west-northwest, and early on December 3 the JTWC classified the system as a tropical storm.

Initially, the circulation was exposed from the convection, although it was able to intensify into Tropical Storm Pongsona at 1200 UTC on December 3 while located 375 km (230 mi) northeast of Pohnpei. It turned to the west on December 4, due to a ridge to the north. It slowly intensified, developing an eye feature on December 5. That day, both the JTWC and the JMA upgraded Pongsona to a typhoon about 1150 km (715 mi) southeast of Guam.

While continuing generally to the west, the eye of Pongsona gradually became better organized. A baroclinic cyclone east of Japan weakened the ridge, which caused the typhoon to turn more to the northwest. By late on December 7, Pongsona developed a well-defined 55 km (35 mi) wide eye as it approached Guam. After the typhoon underwent rapid deepening, the JTWC estimated that Pongsona reached peak winds of 240 km/h (150 mph ), making it a supertyphoon. At 0500 UTC on December 8, the eyewall made landfall on Guam, and two hours later the northern portion of the eyewall crossed over nearby Rota. Around that time, the JMA estimated Pongsona attained a peak intensity of 175 km/h (110 mph ) just to the north of Guam. The typhoon turned to the north-northwest through a weakness in the subtropical ridge a short distance west of the Northern Marianas Islands. On December 9, convection began to weaken as Pongsona began interacting with a mid-latitude system to its north. Dry air became entrained in the southwestern portion of the circulation, and the circulation became exposed from the diminishing convection. As a result, both the JTWC and the JMA declared Pongsona as an extratropical cyclone on December 11 about 1400 km (865 mi) northwest of Wake Island.