Tropical Cyclones in 2021 > Report on Super Typhoon Rai (2122): 12 to 21 December 2021

3.7 Super Typhoon Rai (2122): 12 – 21 December 2021

Rai was the eighth tropical cyclone affecting Hong Kong in 2021. It is rare to have tropical cyclones directly threatening Hong Kong in winter. However, as Rai gradually edged closer to the coast of Guangdong, the Observatory issued the Standby Signal No. 1 on 20 December. This is the latest tropical cyclone warning signal in a year since 1946, breaking the record of Irma in early December 1974.

Rai formed as a tropical depression over the western North Pacific about 2 680 km east-southeast of Manila on the night of 12 December. It moved west-northwestwards and intensified gradually. Rai started to intensify rapidly on 15 December and developed into a super typhoon on the next day. After sweeping across the Philippines, Rai weakened into a severe typhoon and moved westwards across the southern part of the South China Sea on 17 December. Rai re-intensified into a super typhoon the next day and became the first super typhoon occurring over the South China Sea in December since 1961. It reached its peak intensity in the small hours on 19 December with an estimated maximum sustained wind of 205 km/h near its centre. Turning to move northeastwards gradually on the following two days, Rai edged closer to the coast of Guangdong and weakened rapidly. It finally degenerated into an area of low pressure over the seas south of Hong Kong shortly after noon on 21 December.

The Standby Signal No. 1 was issued at 11:20 am on 20 December when Rai was about 630 km south-southwest of Hong Kong. Under the combined effect of Rai and the northeast monsoon, local winds were fresh northeasterlies, occasionally reaching strong force offshore and on high ground on that day. Rai picked up speed to move northeastwards towards the coast of Guangdong and weakened rapidly the next day. It came closest to the territory around 11:00 a.m. on 21 December with its centre about 140 km south-southeast of Hong Kong. As Rai degenerated into an area of low pressure over the seas south of Hong Kong shortly after noon, all tropical cyclone warning signals were cancelled at 12:20 p.m. on that day.

Under the influence of Rai, a maximum sea level (above chart datum) of 2.93 m was recorded at Tsim Bei Tsui. A maximum storm surge of 0.28 m (above astronomical tide) was recorded at Tai Miu Wan. At the Observatory Headquarters, the lowest instantaneous mean sea-level pressure of 1011.9 hPa was recorded at 5:31 a.m. on 21 December.

Affected by the northeast monsoon and the rain-bearing cloud band associated with Rai, it was rainy and cool in Hong Kong on 20 – 21 December. More than 25 millimeters of rainfall were recorded over Lantau Island and the northwestern part of the New Territories on these two days.

Rai did not cause significant damage in Hong Kong. According to press reports, Rai left at least 409 deaths and over 100 000 people were evacuated in the Philippines during its passage.