Tropical Storm Bopha - Meteorological History

Meteorological History

On November 23, a large area of convection persisted 650 km (400 mi) south of Pohnpei, near the equator at a latitude of 0.6ºN. The system had a poorly defined, elongated atmospheric circulation and was located in an area of moderate wind shear and restricted outflow, due to a subtropical ridge to the north. As a result, the Joint Typhoon Warning Center (JTWC) assessed a low chance for tropical cyclogenesis. The center slowly consolidated, with a well-defined mid-level circulation. Late on November 25, the JTWC issued a tropical cyclone formation alert after it organized further, noting that the system had developed an anticyclone which was providing outflow. Around the same time, the Japan Meteorological Agency (JMA) classified the system as a tropical depression, about 410 km (255 mi) south-southwest of Pohnpei. At 2100 UTC on November 25, the JTWC also upgraded the system to Tropical Depression 26W.

Upon developing, a depression had deep convection located in rainbands around the increasingly consolidating center; however, the center initially was difficult to locate. With minimal wind shear and overall favorable conditions, the system was expected to gradually intensify as it moved generally westward, steered by the ridge to the north. On November 26, the movement became nearly stationary as the center organized further. Later that day, the JMA upgraded the depression to Tropical Storm Bopha, and early on November 27 the JTWC followed suit after a strong burst of thunderstorms over the center. Its westward motion increased on November 28, later becoming west-southwestward. The convection gradually became better organized and wrapped into the center, indicative of a strengthening storm. However, early on November 29 the appearance degraded, with the thunderstorms limited to the southern periphery due to reduced outflow to the south. The circulation became exposed, and the JTWC noted that Bopha was unable to intensify significantly due to its low latitude and correspondingly low Coriolis effect. Late on November 29, convection increased, aided by warm waters and low wind shear. At 0000 UTC on November 30, the JMA upgraded Bopha to a severe tropical storm. A few hours later, the JTWC upgraded the storm to a typhoon, and the JMA followed suit at 1800 UTC that day. At that time, Bopha was located about 980 km (610 mi) east-southeast of Palau.

As the system continued to intensify, organized bands of thunderstorms began to develop rapidly around the system, mostly on the western half of the storm. They later merged with Bopha, which led to the system's increase in size. Several hot towers also began to rise up near the low level circulation center, with one of the hot towers reaching 17 km (11 mi) into the atmosphere. At around the same time, on November 30, a ragged, eye-like feature appeared in microwave imagery, which later transitioned into a pinhole eye by December 1. The stadium effect was also observed in the thick convection surrounding the eyewall, involving thin strips of clouds from the eyewall curve outward from the surface with height. At around the same time the system's eye structure improved, it began to explosively intensify, becoming a category 4 storm in 18 hours, while being less than five degrees from the equator.

On December 1, the system, as a category 4 typhoon, maintained a well defined, small eye, which later dissipated, but reformed by December 2. As Bopha strengthened further into a super typhoon on December 2, the well-defined eye underwent a small eyewall replacement cycle by late December 2, and by 1000 UTC on December 2, the system developed double eyewalls, which were only seen in microwave imagery. AT 1230 UTC on December 2, Bopha neared Palau, with a closest distance of 50 km (31 mi). At that time, the system's outflow improved, and convection near the eye thickened. Early on December 3, due to the eyewall replacement cycle, the system weakened slightly into a category 3 typhoon; however, a few hours later, as the eyewall replacement cycle ended, Bopha reintensified into a category 4 typhoon, while the eye became well-defined again. Late on December 3, as the system continued to strengthen, the system rapidly intensified into a category 5 super typhoon, deepening to an unofficial pressure of 918 mbar (hPa) (27.11 inHg) as stated by the JTWC at peak intensity, as the eye started to became well defined at 27 km (17 mi) across. The system's convection also became more compact and organized. At that time, an anticyclone located northeast of Bopha continued to provide ample radial outflow and low vertical wind shear over the system, causing favorable conditions, leading to its re-intensification.

At 2100 UTC on December 3, Bopha made landfall over Baganga, Mindanao, as a category 5 super typhoon, and the eye dissipated just an hour after landfall. Bopha began to weaken, with convection on the eastern side of the storm decreasing as it moved through the Sulu Sea. It made landfall on the island of Palawan on December 5, before briefly weakening to a tropical storm on December 6. The JMA continued tracking the system as a typhoon. Overnight, the convection around the storm center weakened, but made another burst on December 7, with an eye-like feature shown only in microwave imagery. After a few hours, the system reintensified into a category 1 typhoon and developed a 27 km (17 mi), well defined eye.

Late on December 7, Bopha's central dense overcast started to increase slightly, and the typhoon rapidly intensified from a Category 1 to a Category 4 typhoon in only six hours. Upon reintesifying, Bopha developed a well-defined eye that had deep convection near the center of the storm. On December 8, the system started to weaken due to increasing moderate wind shear. Late on December 8 into December 9, Bopha weakened rapidly from a typhoon to a tropical storm, with its convection being blown to the northeast by wind shear, exposing the low level circulation center. The remaining thin convection in the center consisted of few convective thunderstorms associated with stratocumulus clouds.