Tropical Cyclones in 2017 > Report on Tropical Depression: 23 – 25 September

3.6 Tropical Depression : 23 – 25 September 2017

A tropical depression formed over the central part of the South China Sea on the night of 23 September, making it the sixth tropical cyclone necessitating the issuance of tropical cyclone warning signals by the Hong Kong Observatory in 2017.

The Tropical Depression formed over the central part of the South China Sea about 620 km south-southeast of Hong Kong on the night of 23 September and tracked west-northwestwards towards Hainan Island. It reached its peak intensity the next morning with an estimated maximum sustained wind of 55 km/h near its centre. Taking on a northwestward course, the Tropical Depression then moved across Hainan Island and Beibu Wan before weakening into an area of low pressure over the northern part of Vietnam on the night of 25 September.

The Observatory issued the Standby Signal No. 1 at 11:10 p.m. on 23 September when the Tropical Depression was about 560 km south-southeast of Hong Kong. Local winds were generally moderate to fresh east to southeasterly the next day, occasionally strong offshore and on high ground. The Tropical Depression came closest to the territory around 8 a.m. on 24 September when it was about 470 km south-southwest of Hong Kong. At the Observatory Headquarters, the lowest instantaneous mean sea-level pressure of 1007.7 hPa was recorded at 2:16 p.m. that day. With the Tropical Depression moving away from Hong Kong, all tropical cyclone warning signals were cancelled at 7:20 p.m. in the evening.

The Tropical Depression did not cause any significant damage in Hong Kong during its passage. Its outer rainbands brought squally showers and thunderstorms to the territory on 24 September, with more than 10 millimetres of rainfall recorded over many places. A maximum sea level (above chart datum) of 2.61 m and a maximum storm surge of 0.35 m (above astronomical tide) were recorded at Tsim Bei Tsui.