Report on Tropical Storm Lionrock (2117)

Tropical Storm Lionrock (2117)
7 - 10 October 2021

Lionrock was the sixth tropical cyclone affecting Hong Kong in 2021. While Lionrock skirted past at about 490 km of Hong Kong, under the combined effect of Lionrock’s extensive circulation and the northeast monsoon, the Observatory issued the first No. 8 Gale or Storm Signal in the year. Lionrock is also the farthest tropical cyclone necessitating the issuance of No. 8 Gale or Storm Signal in Hong Kong since 1961.

A monsoon depression developed into a tropical depression over the central part of the South China Sea about 290 km east-southeast of Sanya on the afternoon of 7 October. It generally tracked northwestwards towards Hainan Island. The tropical depression was named Lionrock on the morning of 8 October and intensified into a tropical storm. Lionrock reached its peak intensity in the afternoon with an estimated maximum sustained wind of 75 km/h near its centre. When Lionrock moved across Hainan Island on 9 October, it took on an erratic track and made an anti-clockwise loop. Lionrock turned to move west-northwestwards in that afternoon and entered Beibu Wan at night. It degenerated into an area of low pressure over the coast of northern Vietnam on the night of 10 October.

According to press reports, Lionrock left six injuries in Macao. Besides, it also brought torrential rain to Zhuhai and triggered extensive flooding.

The Strong Monsoon Signal was issued at 11:40 p.m. on 6 October. Under the combined effect of the northeast monsoon and the outer circulation of the monsoon depression, local winds were generally fresh easterlies, reaching strong force offshore on 7 October. With the monsoon depression developing into a tropical depression and edging closer to Hainan Island, the No. 3 Strong Wind Signal was issued to replace the Strong Monsoon Signal at 4:40 a.m. on 8 October when Lionrock was about 630 km south-southwest of Hong Kong. Under the combined effect of Lionrock and the northeast monsoon, local winds were generally strong with occasional gales offshore and on high ground on 8 October. Local winds further strengthened on the early morning of 9 October and the No. 8 Southeast Gale or Storm Signal was issued at 6:40 a.m. when Lionrock was about 550 km southwest of Hong Kong. Under the influence of the outer rainbands of Lionrock, local winds were strong to gale force east to southeasterlies, with occasional storm force winds offshore and even hurricane force occasionally on high ground over the southwestern part of Hong Kong on 9 October. Lionrock came closest to Hong Kong at around 2 p.m. on 9 October with its centre passing about 490 km southwest of Hong Kong. With Lionrock entering Beibu Wan and departing from Hong Kong, local wind moderated and the No. 3 Strong Wind Signal was issued to replace the No. 8 Southeast Gale or Storm Signal at 4:40 a.m. on 10 October. As local winds further subsided, the No. 1 Standby Signal was issued to replace the No. 3 Strong Wind Signal at 12:40 p.m. on 10 October and all tropical cyclone warning signals were cancelled at 2:20 p.m. on that day.

Under the influence of Lionrock, maximum hourly mean winds of 103, 76 and 74 km/h and maximum gusts of 154, 123 and 101 km/h were recorded at Ngong Ping, Cheung Chau and Waglan Island respectively. A maximum sea level (above chart datum) of 3.12 m and a maximum storm surge (above astronomical tide) of 0.60 m were recorded at Tsim Bei Tsui. The lowest instantaneous mean sea-level pressures recorded at some selected stations are as follows:

The outer rainbands of Lionrock brought squally heavy showers and thunderstorms to Hong Kong on 7 – 9 October. The rain was particularly heavy and persistent on 8 October with more than 200 millimetres of rainfall generally recorded over Hong Kong, necessitating the issuance of the second Black Rainstorm Warning this year. Landslip Warning and Special Announcement on Flooding in the northern New Territories were also in force on that day. The rainfall recorded at the Observatory on 8 October even reached 329.7 millimetres, more than two times of October’s monthly total normal figure of 120.3 millimetres and the highest daily rainfall on record for the month of October. When Lionrock moved away from Hong Kong, showers eased off during the day on 10 October. More than 400 millimetres of rainfall were generally recorded over the territory during the four-day period of 7 – 10 October, with rainfall exceeding 700 millimetres over parts of Hong Kong Island.

Lionrock caused two deaths and at least 14 others injured in Hong Kong during its passage. There were over 1 100 reports of fallen trees, 6 reports of flooding and 3 reports of landslip. About 300 hectares of farmland in the New Territories were affected. 15 flights were diverted at the Hong Kong International Airport. A catamaran was overturned by huge waves over the waters near Po Toi Island on 7 October when the Strong Monsoon Signal was in force. Two people on board fell into the sea and one of them was drowned. About 10 containers fell into the sea from a freighter passing through the waters off the Ninepin Island. The scaffolding of a residential building in Happy Valley collapsed on the morning of 8 October, resulting in one death and one injury. The heavy downpour on that day also caused serious flooding to some roads. A number of vehicles were trapped by flood waters at Princess Margaret Road, Tai Hang Road and Shun Lee Tsuen Road. On 9 October, the scaffolding of a construction site in Tsuen Wan collapsed under strong wind. The fallen trees in Kowloon Tong and Deep Water Bay damaged three private cars and a police vehicle.