Report on Typhoon Cempaka (2107)

Cempaka was the fourth tropical cyclone affecting Hong Kong in 2021.

Cempaka formed as a tropical depression over the northern part of the South China Sea about 180 km south-southwest of Hong Kong on the night of 18 July. It moved generally northwestwards to west-northwestwards slowly towards the coast of western Guangdong and intensified rapidly. Cempaka intensified into a typhoon in the small hours on 20 July and reached its peak intensity with an estimated maximum sustained wind of 120 km/h near its centre. It started to weaken at night and made landfall near Yangjiang. Cempaka moved across western Guangdong and inland Guangxi, and weakened into a tropical depression progressively on 21 July. It turned to move south-southwestwards the next day and entered Beibu Wan on 23 July. Cempaka finally degenerated into an area of low pressure over Beibu Wan on 24 July.

After the formation of Cempaka on the night of 18 July, the Hong Kong Observatory issued the Standby Signal, No.1 at 9:40 p.m. Cempaka was also closest to the territory at that time. Local winds were generally moderate to fresh easterlies that night and the next morning, occasionally reaching strong force offshore and on high ground. With Cempaka edging closer to the coast of western Guangdong and intensifying, the Strong Wind Signal, No.3 was issued at 4:10 p.m. on 19 July when Cempaka was about 180 km southwest of Hong Kong. Locally, there were strong winds offshore and occasional gales on high ground that night and on the morning of 20 July. As Cempaka moved away from Hong Kong gradually and weakened, local winds moderated and the Strong Wind Signal, No.3 was replaced by the Standby Signal, No.1 at 1:20 p.m. on 20 July. All tropical cyclone warning signals were cancelled at 7:40 p.m. that day.

Under the influence of Cempaka, a maximum sea level (above chart datum) of 2.60 m and a maximum storm surge of 0.41 m (above astronomical tide) were recorded at Tsim Bei Tsui and Tai Po Kau respectively. At the Observatory Headquarters, the lowest instantaneous mean sea-level pressure of 1001.2 hPa was recorded at 4:09 p.m. on 19 July.  

The outer rainbands associated with Cempak brought heavy squally showers and thunderstorms to Hong Kong on 18 - 20 July. The Observatory issued the Amber Rainstorm Warning for four times during this period. The Special Announcement on Flooding in Northern New Territories was also issued on the afternoon of 20 July. More than 200 millimetres of rainfall were recorded over most part of Hong Kong during these three days, with rainfall exceeding 350 millimetres over the eastern part of the New Territories.

A number of reports of fallen trees and flooding were reported in Hong Kong during the passage of Cempaka. A tree toppled at Ngau Chi Wan under intense gusts, damaging the wire fences. A man was washed away by flash floods while hiking in Ma On Shan on 20 July and confirmed dead later.