TROPICAL CYCLONES IN 2013

3.6 Super Typhoon Usagi (1319): 17 - 23 September 2013

Usagi was the sixth tropical cyclone necessitating the issuance of tropical cyclone warning signals by the Hong Kong Observatory in 2013. It is also the second tropical cyclone necessitating the issuance of No. 8 Gale or Storm Signal in the year. Usagi was the most intense tropical cyclone affecting Hong Kong in 2013.

Usagi formed as a tropical depression over the western North Pacific about 1 240 km east-northeast of Manila on 17 September. Moving generally westwards, it intensified into a tropical storm that afternoon and further strengthened into a typhoon the next day. It took on a northwesterly track and intensified further into a super typhoon over the Pacific to the east of Luzon on 19 September, reaching its peak intensity with estimated sustained winds of 205 km/h near its centre the next day. Usagi moved west-northwestwards at about 17 km/h across the Luzon Strait on 21 September. At 5 a.m. sustained wind of 137 km/h was reported at the island station of Batan over the Luzon Strait, with the mean sea-level pressure falling to 939.6 hPa three hours later. Usagi entered the South China Sea and weakened into a severe typhoon during the evening. It moved across the northeastern part of the South China Sea at about 20 km/h on 22 September, made landfall near Shanwei about 160 km east-northeast of Hong Kong that evening, and moved across the coastal areas of Guangdong during the night. Usagi weakened rapidly into a tropical depression the next morning and dissipated over the inland areas of Guangxi in the afternoon.

According to press reports, 29 people were killed, one person was reported missing and over 9.22 million people were affected in Guangdong during the passage of Usagi. The direct economic loss exceeded 17.7 billion RMB. Backflow of seawater occurred in Shantou with serious flooding in many places, and large trees were toppled in the city. In Shanwei, there were interruptions to traffic, telecommunication links, electricity and water supply, with over 65 percent of the area without electricity. Over 152 000 hectares of vegetation were damaged, 14 794 houses collapsed and another 32 879 houses were severely damaged in eastern Guangdong. In Macao, there was minor backflow of seawater inside the harbour.

The Hong Kong Observatory issued the Standby Signal No. 1 at 10:40 a.m. on 21 September when Usagi was about 760 km east-southeast of the territory. Local winds were moderate to fresh north to northwesterlies, occasionally strong on high ground that day. As Usagi edged steadily towards the coast of Guangdong, the Strong Wind Signal No. 3 was issued at 11:40 p.m. when Usagi was about 530 km to the east-southeast. Winds occasionally reached gale force on high ground in the morning on 22 September, but most of the territory remained relatively sheltered. Winds strengthened during the afternoon and became strong northwesterlies offshore, reaching gale force on high ground. The No. 8 NW Gale or Storm Signal was issued at 6:40 p.m. when Usagi was about 160 km to the east-northeast of Hong Kong. West to northwesterly gales affected the Victoria Harbour and the offshore waters of Hong Kong that night, reaching storm to hurricane force on high ground at times. Usagi was closest to Hong Kong around midnight, passing about 80 km to the north of the Hong Kong Observatory. Winds turned to southwesterly in the small hours on 23 September and the No. 8 SW Gale or Storm Signal was issued at 12:25 a.m. Gale force winds persisted over the waters to the south of Hong Kong and on high ground before dawn. The gales gradually subsided in the morning and the Strong Wind Signal No. 3 was issued at 9:20 a.m. All signals were cancelled at 10:25 a.m. as Usagi weakened significantly inland and local winds subsided further.

During the passage of Usagi, a maximum hourly mean wind of 87 km/h was recorded at Waglan Island, while a maximum gust of 128 km/h was recorded at Cheung Chau. A maximum sea level of 3.38 m (above chart datum) was recorded at Tsim Bei Tsui, while a maximum storm surge of 0.99 m was recorded at Tai Po Kau. The lowest instantaneous mean sea-level pressures recorded at some selected stations are as follows:-

Station	Lowest instantaneous mean sea-level pressure (hPa)	Date/Month	Time
Hong Kong Observatory Headquarters	985.7	22/9	10:06 p.m.
Cheung Chau	985.6	22/9	9:19 p.m.
Hong Kong International Airport	987.5	22/9	10:54 p.m.
King’s Park	985.1	22/9	10:16 p.m.
Lau Fau Shan	984.8	22/9	11:22 p.m.
Peng Chau	985.0	22/9	10:07 p.m.

The weather in Hong Kong on 21 September was very hot and dry with the maximum temperature exceeding 34 degrees over most parts of the territory. The weather turned cloudy the next day with squally showers, becoming heavy at night. Heavy squally showers continued to affect Hong Kong in the morning on 23 September. Rain eased off during the day as Usagi weakened significantly inland and moved away from the territory. More than 100 millimetres of rainfall were recorded over most parts of the territory during the three-day period, with rainfall in parts of the New Territories and Lantau Island exceeding 150 millimetres.

In Hong Kong, 17 people were injured during the passage of Usagi, 900 trees were blown down and many incidents of fallen objects were reported. A van and two private vehicles were damaged by a fallen tree in Tuen Mun. In Tai Po, a satellite receiver dish and water pipe on the rooftop of a building were damaged by loose objects. Three vehicles nearby were also damaged during the incident. A maintenance workbench outside a building and wall plaster outside another building fell off in Tuen Mun and Tai Kok Tsui respectively, damaging seven lorries in the former incident. Storm surge induced by Usagi caused minor flooding in some low lying areas in Hong Kong, including Lau Fau Shan, Tuen Mun, Tai Po and Sham Shui Po. A private vehicle was trapped by flood waters in Lau Fau Shan, and two people inside the vehicle had to be rescued by firemen. Flood waters in Tuen Mun interrupted traffic for a period of time. At the Hong Kong International Airport, 215 flights were cancelled and 472 flights were delayed. Four flights bound for Macao were diverted to other places due to the passage of Usagi.

Information on the maximum wind, period of strong and gale force winds, daily rainfall and maximum sea level reached in Hong Kong during the passage of Usagi is given in Tables 3.6.1 - 3.6.4 respectively. Figures 3.6.1 - 3.6.2 show respectively the track of Usagi and the rainfall distribution for Hong Kong. Figure 3.6.3 shows the winds recorded at various stations in Hong Kong. Charts in figures 3.6.4 shows the time traces of wind speed and mean sea-level pressure recroded at Waglan Island. Figures 3.6.5 and 3.6.6 show respectively the satellite imageries and the radar imagery of Usagi.