Tropical Storm Vamei - Meteorological History

Meteorological History

On December 19, a small low-level circulation was located along the northwest coastline of Borneo; at the same time a plume of cold air progressed southward through the South China Sea on the southeastern periphery of a ridge over the Far East. The vortex drifted southwestward, reaching open water by December 21. The northerly air surge was deflected after interacting with the circulation, and at the same time a portion of the air surge crossed the equator. The southerly flow turned eastward, then northward, and in combination with the northerly flow it wrapped into the vortex, resulting in rapid development of the low-level circulation, just a short distance north of the equator. By December 25, an area of scattered convection persisted about 370 km (230 mi) east of Singapore within an area of low wind shear, in association with the low-level circulation. Continuing slowly westward, the convection deepened and organized further, and at 1200 UTC on December 26 the disturbance developed into a tropical depression about 230 km (145 mi) east of Singapore, or 156 km (97 mi) north of the equator. This was the first recorded occurrence of a tropical cyclone near the equator.

The depression strengthened further and officially attained tropical storm status at 0000 UTC on December 27, based on the analysis by the Japan Meteorological Agency (JMA), though the Joint Typhoon Warning Center (JTWC) unofficially classified it as a tropical storm six hours prior. Shortly thereafter, an eye with a 39 km (24 mi) diameter became apparent on satellite imagery, along with rainbands extending southward to the opposite side of the equator. At 0600 UTC, the JMA first classified the system as Tropical Storm Vamei, about 65 km (40 mi) northeast of Singapore, and the agency estimated the storm attained peak winds of 85 km/h (50 mph) at the same time. However, the JTWC upgraded Vamei to typhoon status with peak winds of 140 km/h (85 mph), based on a United States Navy ship report from within the eye; a second ship reported wind gusts of 195 km/h (120 mph) in the southern portion of the eyewall. The storm was small and compact, with gales extending about 45 km (30 mi) from its center. At about 0830 UTC on December 27, Vamei made landfall approximately 60 km (35 mi) northeast of Singapore, in the southeastern portion of the Malaysian state of Johor. Initially, the Malaysian Meteorological Department classified the cyclone as a tropical storm, though it was later re-assessed as a typhoon at landfall.

Tropical Storm Vamei weakened quickly as it crossed the extreme southern portion of the Malay Peninsula, and late on December 27 the JMA downgraded it to tropical depression status before the cyclone emerged into the Straits of Malacca. The JTWC initially maintained it as a minimal tropical storm, though the agency downgraded the storm to depression status as the center again approached land. Early on December 28, Vamei moved ashore along northeastern Sumatra, and at 0600 UTC the JMA classified the storm as dissipated. However, convection persisted near the circulation over land, believed to have been caused by the process known as upper-level diffluence. On December 29, what was originally believed to be a separate system reached the southeastern Bay of Bengal. In a post-season re-evaluation, the JTWC classified the system as a continuation of Vamei, based on analysis of satellite imagery that indicated the circulation of Vamei crossed Sumatra without dissipating. Convection re-developed, and late on December 30 the JTWC classified the cyclone as a tropical storm about 390 km (245 mi) west-southwest of the northwestern tip of Sumatra; initially, due to being treated as a separate system, it was classified as Tropical Cyclone 05B. Vamei quickly developed good outflow and organization, though increased wind shear on December 31 rapidly weakened the storm; by late that day, the center was exposed from the deep convection, and Vamei quickly dissipated.