Tropical Cyclones in 2018 > Report on Tropical Storm Son-Tinh (1809): 16 – 24 July 2018

3.2 Tropical Storm Son-Tinh (1809): 16 – 24 July 2018

Son-Tinh was the second tropical cyclone affecting Hong Kong in 2018. It necessitated the issuance of the tropical cyclone warning signals on two separate occasions owing to its irregular track. The last time this had happened was in 2010 when severe tropical storm Lionrock affected Hong Kong.

Son-Tinh formed as a tropical depression over the western North Pacific about 650 km northeast of Manila on the morning of 16 July and moved quickly westwards across the Luzon Strait on that day. It continued to move at a fast pace after entering the northern part of the South China Sea on 17 July. Son-Tinh intensified into a tropical storm before noon, reaching its peak intensity with an estimated sustained wind of 85 km/h near the centre on the early morning of 18 July. After moving across Hainan Island and Beibu Wan, Son-Tinh degenerated into an area of low pressure over the northern part of Vietnam on 19 July and its remnant continued to track westward further inland on that day. The low pressure area associated with the remnant of Son-Tinh made a sharp turn to the east over the Indo-China and moved towards Beibu Wan on 20 July. It re-intensified into a tropical depression over Beibu Wan on 22 July and took a northeasterly track, sweeping across the northwestern part of Hainan Island. Son-Tinh then turned north and moved across Leizhou Peninsula on 23 July, before dissipating over Guangxi on 24 July.

According to press reports, Son-Tinh greatly disrupted the traffic of Hainan Island during its passage. Son-Tinh and its remnant also brought torrential rain to Vietnam. At least 32 people were killed, 17 were reported missing and more than 5 000 houses collapsed.

In Hong Kong, the No. 1 Standby Signal was issued at 2:40 a.m. on 17 July when Son-Tinh was about 740 km east-southeast of the territory. Local winds were moderate to fresh east to northeasterlies during the day. As Son-Tinh moved rapidly across the northern part of the South China Sea, the No. 3 Strong Wind Signal was issued at 4:20 p.m. in the afternoon when it was about 350 km south of Hong Kong. Locally, winds generally strengthened in the evening, becoming fresh to strong easterlies and occasionally reaching gale force offshore and on high ground. Son-Tinh came closest to Hong Kong at around 6 p.m. on 17 July as it skirted past about 340 km south of Hong Kong. With Son-Tinh making landfall over Hainan Island and weakening the next morning, all tropical cyclone warning signals were cancelled at 9:40 a.m. Nevertheless, under the influence of a ridge of high pressure over the coastal region of southeastern China, strong winds still affected offshore areas and high ground. The Strong Monsoon Signal was issued immediately afterwards and lasted till 9:00 p.m. that night.

With Son-Tinh re-intensifying into a tropical depression after moving from the northern part of Vietnam into Beibu Wan and moving closer to Hong Kong gradually, the No. 1 Standby Signal was issued again at 3:40 p.m. on 23 July when Son-Tinh was about 460 km west-southwest of Hong Kong. Local winds were moderate to fresh southeasterlies in the afternoon, occasionally reaching strong force offshore and on high ground. When Son-Tinh started to track northwestwards and moved away from Hong Kong the next morning, all tropical cyclone warning signals were cancelled at 10:40 a.m.

During the first passage of Son-Tinh, a maximum sea level (above chart datum) of 2.94 m was recorded at Tsim Bei Tsui and a maximum storm surge (above astronomical tide) of 0.64 m was recorded at Shek Pik and Tsim Bei Tsui. The lowest instantaneous mean sea-level pressure of 1000.1 hPa was recorded at the Observatory headquarters at 4:08 p.m. on 17 July when Son-Tinh was about 350 km south of Hong Kong. During the second passage of Son-Tinh, a maximum sea level (above chart datum) of 2.61 m and a maximum storm surge (above astronomical tide) of 0.26 m were recorded at Tsim Bei Tsui. The lowest instantaneous mean sea-level pressure of 1000.3 hPa was recorded at the Observatory headquarters at 4:36 p.m. on 23 July when Son-Tinh was about 460 km west-southwest of Hong Kong.

With plenty of sunshine, the weather of Hong Kong was very hot on 17 July. As Son-Tinh came closer to Hong Kong, its outer rainbands brought occasional heavy squally showers and thunderstorms to the territory in the evening and the next day. With Son-Tinh moving away from Hong Kong, there were sunny intervals on the afternoon of 18 July. More than 30 millimetres of rainfall were generally recorded over the territory on 17 and 18 July. Over 60 millimetres of rainfall were registered over the northern part of the New Territories.

With Son-Tinh edging closer to Hong Kong again, the local weather turned cloudier with occasional heavy showers and a few squally thunderstorms on 23 July. As Son-Tinh moved away, there were a few showers at first and sunny intervals during the day on 24 July. More than 30 millimetres of rainfall were generally recorded over the territory during these two days.

Son-Tinh did not cause any significant damage in Hong Kong during its first passage. A hoarding in Causeway Bay was blown down and two passersby were injured during the second passage of Son-Tinh.