TROPICAL CYCLONES IN 2016

3.4 Tropical Storm Dianmu (1608): 17 – 20 August 2016

Dianmu was the fourth tropical cyclone necessitating the issuance of tropical cyclone warning signal by the Hong Kong Observatory in 2016.

A tropical depression formed over the northern part of the South China Sea about 220 km southwest of Hong Kong on the morning of 17 August. It moved slowly and intensified gradually that day. The tropical depression intensified into a tropical storm and was named Dianmu the next morning. Moving generally westwards, Dianmu made landfall over Leizhou Peninsula on the afternoon of 18 August and entered Beibu Wan that night. It re-intensified as it moved across Beibu Wan, reaching its peak intensity with an estimated sustained wind of 85 km/h on the morning of 19 August. After making landfall over the northern part of Vietnam in the afternoon, Dianmu moved inland and weakened gradually. It finally degenerated into an area of low pressure over the northern part of Myanmar on the morning of 20 August.

The Standby Signal No. 1 was issued at 11:30 a.m. on 17 August when Dianmu was about 220 km southwest of Hong Kong. It was also closest to the territory at the time. Local winds were generally moderate to fresh from the east. At the Observatory Headquarters, the lowest instantaneous mean sea-level pressure of 991.5 hPa was recorded at 4:59 p.m. that day. With Dianmu moving slowly and developing gradually, local winds strengthened during the night, and the Strong Wind Signal No. 3 was issued at 10:15 p.m. when Dianmu was about 240 km southwest of Hong Kong. Local winds were generally fresh to strong from the southeast overnight, occasionally reaching gale force on high ground. As Dianmu moved further away from Hong Kong the next morning, local winds moderated gradually. The Strong Wind Signal No. 3 was replaced by the Standby Signal No. 1 at 11:15 a.m., before all tropical cyclone warning signals were cancelled at 1:15 p.m. that afternoon.

Under the influence of Dianmu, a maximum sea level (above chart datum) of 3.15 m was recorded at Tsim Bei Tsui, while a maximum storm surge of 0.37 m (above astronomical tide) was recorded at Tai Po Kau.

Under the influence of the outer rainbands of Dianmu, local weather was cloudy with squally showers on 17 and 18 August. More than 50 millimetres of rainfall were generally recorded over the territory during these two days, with rainfall amount exceeding 100 millimetres over the eastern part of Kowloon, the eastern part of Hong Kong Island and Shatin.

Dianmu did not cause any significant damage in Hong Kong. According to press reports, there were flooding in many places in Hainan Island due to rainstorms brought by Dianmu. Around 40 000 people were evacuated and transportation services were affected. In Vietnam, a least 16 persons were killed, two were missing and another 15 were injured during the passage of Dianmu.

Information on the maximum wind, period of strong winds, daily rainfall and maximum sea level reached in Hong Kong during the passage of Dianmu is given in Tables 3.4.1 - 3.4.4 respectively. Figures 3.4.1 - 3.4.4 show respectively the track of Dianmu, the rainfall distribution for Hong Kong, a satellite imagery and a related radar imagery of Dianmu.