Report on Super Typhoon Yagi(2411)

Super Typhoon Yagi (2411)
1 - 8 September 2024

Yagi was the third tropical cyclone affecting Hong Kong in 2024. The Observatory issued the first No. 8 Gale or Storm Signal in 2024 during the passage of Yagi.

Yagi formed as a tropical depression over the seas east of the central part of the Philippines about 560 km east-southeast of Manila on the morning of 1 September. It moved northwestwards towards Luzon and intensified into a tropical storm that night. Yagi moved across Luzon the next day and entered the northern part of the South China Sea on 3 September. Yagi then tracked generally westwards across the northern part of the South China Sea. Under the favourable conditions of very warm sea water and weak vertical wind shear, Yagi rapidly intensified into a super typhoon from a severe tropical storm on 4 September. Yagi attained its peak intensity with an estimated maximum sustained wind of 230 km/h near its centre in the small hours on 6 September, making it the second strongest tropical cyclone in the South China Sea since the Observatory’s records began in 1950, on par with Super Typhoon Saola in 2023. Yagi turned to move west-northwestwards towards Hainan Island that morning and made landfall near Wenchang, Hainan Island that evening. Yagi continued to maintain super typhoon intensity while moving across Beibu Wan the next day, and made landfall near the northern part of Vietnam in the afternoon. It then moved into inland areas of the northern part of Vietnam and weakened rapidly. Yagi finally degenerated into an area of low pressure on the night of 8 September.

According to press reports, Yagi left 21 deaths, 26 missing and 22 injuries in the Philippines during its passage. More than 3 million people were affected, over 7 000 houses were damaged and economic loss exceeded PHP 2.6 billion. In Guangdong, Guangxi and Hainan, more than 2.7 million people were affected and over 450 000 people were evacuated. Yagi left 4 deaths and 95 injuries in Hainan. More than 32 000 houses were damaged, the number of fallen trees amounted to over 160 000, and economic loss exceeded RMB 78.6 billion. Yagi and its remnant brought torrential rain to Vietnam, Lao PDR, Thailand and Myanmar, triggering landslides and flooding in many places. In Vietnam, there were at least 323 deaths, 22 missing and 1 978 injuries. More than 3.6 million people were affected, over 280 000 houses were damaged, and economic loss exceeded VND 81 trillion. In Lao PDR, there were at least 7 deaths and more than 180 000 people affected. Economic loss exceeded USD 7.9 million. In Thailand, there were at least 52 deaths and 28 injuries. In Myanmar, there were at least 360 deaths, 100 missing and 48 injuries. More than 1.1 million people were affected and over 140 000 houses were damaged.

The Standby Signal No. 1 was issued at 5:40 p.m. on 3 September, when Yagi was about 790 km south-southeast of Hong Kong. Local winds were light to moderate northeasterlies that night and during the day the next day. With Yagi edging closer to the coast of Guangdong, the No. 3 Strong Wind Signal was issued at 6:40 p.m. on 4 September, when Yagi was about 590 km south of Hong Kong. Winds over Hong Kong strengthened significantly the next day. Local winds were generally strong east to northeasterlies during the day, occasionally reaching gale force offshore and on high ground.

As the gale force winds associated with Yagi were expected to affect the territory, the No. 8 Northeast Gale or Storm Signal was issued at 6:20 p.m. on 5 September when Yagi was about 340 km south of Hong Kong. Local winds strengthened further, reaching generally strong to gale force, with winds on high ground even reaching storm force that night.

Yagi came closest to Hong Kong at around 8 p.m. on 5 September, when it skirted past about 330 km south of the territory. With Yagi departing from the territory, local winds moderated gradually the next morning. The No. 8 Northeast Gale or Storm Signal was replaced by the No. 3 Strong Wind Signal at 12:40 p.m. on 6 September. As Yagi moved further away from Hong Kong, all tropical cyclone warning signals were cancelled at 4:20 a.m. on 7 September. However, strong easterly winds still affected the offshore and high ground of the territory at first. The Strong Monsoon Signal was issued immediately afterwards at 4:21 a.m. and cancelled at 12:45 p.m. that day.

Under the influence of Yagi, maximum hourly mean winds of 94, 80 and 78 km/h and gusts of 149, 103 and 106 km/h were recorded at Ngong Ping, Waglan Island and Cheung Chau Beach respectively. A maximum sea level (above chart datum) of 2.86 m was recorded at Tsim Bei Tsui and a maximum storm surge (above astronomical tide) of 0.67 m was recorded at Tai Po Kau. The lowest instantaneous mean sea-level pressures recorded at some selected stations are as follows:

Under the influence of an anticyclone aloft, the weather of Hong Kong was still very hot with sunny intervals during the day on 4 – 5 September. With the approach of Yagi, the weather of Hong Kong deteriorated later on that day. The outer rainbands of Yagi brought heavy showers, violent gusts and thunderstorms to Hong Kong on the night of 5 September and the next two days. Violent gusts of around 139 kilometres per hour were once recorded at Central Pier on the night of 5 September, just below the gusts recorded at the station (launched in 2005) during the passages of Super Typhoon Mangkhut in 2018 and Super Typhoon Saola in 2023. More than 100 millimetres of rainfall were recorded over most parts of Hong Kong on 5 – 7 September and rainfall even exceeded 200 millimetres over the northeastern part of the New Territories.

In Hong Kong, there were at least 581 reports of fallen trees, one report of flooding and one report of landslide during the passage of Yagi. A total of nine people were injured during the passage of Yagi. Two flights were diverted at the Hong Kong International Airport.