Report on Two Tropical Depressions (5 to 8 July)
Two Tropical Depressions
5 to 8 July 2021
On 6 July, the Standby Signal No.1 was issued for two tropical depressions which were respectively located near the southern part of Taiwan and over the central part of the South China Sea. This is a relatively rare event and the last time with a tropical cyclone warning signal issued for two tropical cyclones at the same time was due to severe tropical storm Mac and tropical storm Nancy in September 1979.
The area of low pressure over the western North Pacific around 740 km east-northeast of Manila intensified into a tropical depression in the small hours on 5 July. The tropical depression then moved northwestwards across the Luzon Strait. It reached its peak intensity at night with an estimated sustained wind of 55 km/h near its centre. The tropical depression skirted past the southwestern coastal waters of Taiwan and weakened rapidly on the morning of 6 July. It dissipated near the Taiwan Strait shortly after noon.
Another area of low pressure over the central part of the South China Sea around 230 km south-southeast of Xisha intensified into a tropical depression on the night of 5 July. It tracked generally northwest to north-northwestwards towards Hainan Island on 6 July. The tropical depression reached its peak intensity on the night of 6 July with an estimated sustained wind of 55 km/h near its centre. After moving across the southwestern part of Hainan Island, the tropical depression entered Beibu Wan on 7 July. It made landfall over the northern part of Vietnam on 8 July and then degenerated into an area of low pressure.
In Hong Kong, the Standby Signal No.1 was issued at 4:15 a.m. on 6 July when the first tropical depression near the southern part of Taiwan was about 660 km east of Hong Kong and the second tropical depression over the central part of the South China Sea was about 820 km south-southwest of Hong Kong. Local winds were generally moderate easterlies in the morning. The first tropical depression was closest to Hong Kong before noon with its centre about 610 km east of Hong Kong. It dissipated near Taiwan Strait shortly after noon. Meanwhile, as the second tropical depression over the central part of the South China Sea continued to edge closer to Hong Kong, the Standby Signal No.1 remained in force. Locally, there were occasional strong winds offshore and on high ground on the afternoon of 6 July and the next day. The second tropical depression came closest to Hong Kong around 2 a.m. on 7 July as it skirted past about 570 km south-southwest of the territory. With the second tropical depression making landfall over Hainan Island and moving away from Hong Kong, its threat to Hong Kong diminished and all tropical cyclone warning signals were cancelled at 2:10 p.m. on 7 July.
Under the influence of the two tropical depressions, a maximum sea level (above chart datum) of 2.49 m was recorded at Tsim Bei Tsui. A maximum storm surge of 0.22 m (above astronomical tide) was recorded at Tai Miu Wan and Tai Po Kau. At the Observatory Headquarters, the lowest instantaneous mean sea-level pressure of 1004.7 hPa was recorded at 4:15 a.m. on 6 July.
Affected by the outer rainbands of these two tropical depressions, there were occasional squally showers and thunderstorms in Hong Kong on 6 July and the early part of 7 July. More than 20 millimetres of rainfall were recorded in many places of Hong Kong. With the subtropical ridge over southeastern China extending westwards, showers in Hong Kong abated gradually during the day on 7 July and local weather became very hot with sunny intervals.
The two tropical depressions did not cause significant damage in Hong Kong.