Tropical Cyclones in 2021 > Report on Tropical Storm Koguma (2104): 11 to 13 June 2021

3.1 Tropical Storm Koguma (2104): 11 to 13 June 2021

Koguma was the first tropical cyclone affecting Hong Kong in 2021.

A monsoon depression developed into a tropical depression over the northern part of the South China Sea about 500 km south-southwest of Hong Kong on the afternoon of 11 June. It generally tracked west-northwestward towards Hainan Island. The tropical depression moved across Hainan Island on the morning of 12 June and was named Koguma in the afternoon. Koguma intensified into a tropical storm over Beibu Wan in that evening and reached its peak intensity with an estimated maximum sustained wind of 65 km/h near its centre. It made landfall over the northern part of Vietnam on the morning of 13 June and degenerated into an area of low pressure over inland Vietnam in the afternoon.

In Hong Kong, the Standby Signal No.1 was issued at 4:15 p.m. on 11 June when Koguma was about 500 km south-southwest of the territory. Local winds were generally fresh east to southeasterlies and occasionally reached strong force offshore and on high ground at night and the next morning. Koguma came closest to the territory at around 8 p.m. on 11 June as it skirted past about 480 km south-southwest of Hong Kong. With Koguma making landfall over Hainan Island on the morning of 12 June and moving away from Hong Kong, local winds subsided gradually and all tropical cyclone warning signals were cancelled at 2:10 p.m. on 12 June.

Under the influence of Koguma, a maximum sea level (above chart datum) of 2.63 m and a maximum storm surge of 0.34 m (above astronomical tide) were recorded at Shek Pik and Tai Po Kau respectively. At the Observatory Headquarters, the lowest instantaneous mean sea-level pressure of 1004.0 hPa was recorded at 5:44 p.m. on 11 June.

Under the influence of the outer rainbands of Koguma, there were occasional heavy squally showers and thunderstorms in Hong Kong on 11 and 12 June. Amber Rainstorm Warning was issued on the night of 11 June. More than 40 millimetres of rainfall were generally recorded in Hong Kong during these two days, with rainfall exceeding 70 millimetres over the eastern part of Kowloon.

Koguma did not cause significant damage in Hong Kong. According to press reports, Koguma left at least 1 death and 2 missing as well as at least 130 houses collapsed during its passage to Vietnam.