Mangkhut was the fifth tropical cyclone affecting Hong Kong in 2018. After Hato in 2017, the Hurricane Signal No. 10 was issued again during the passage of Mangkhut and lasted for ten hours. It was the second longest duration of Signal No. 10 since World War II, just after the 11 hours of York in 1999. Mangkhut is characterized by its extensive circulation, ferocious winds and fast movement as well as special wind structure. It brought damaging winds and record-breaking storm surges to Hong Kong, causing widespread and serious impacts.
Tropical depression Mangkhut formed over the western North Pacific about 2 330 km east of Guam on 7 September. Moving westwards quickly, it intensified gradually in the following few days. Mangkhut developed into a super typhoon on 11 September. It turned to the northwest on 14 September, reaching its peak intensity before making landfall over Luzon with an estimated maximum sustained wind of 250 km/h near the centre. Mangkhut weakened after crossing the northern part of Luzon and continued to track northwestwards quickly across the northern part of the South China Sea towards the coast of Guangdong. Mangkhut weakened into a severe typhoon on the morning of 16 September and made landfall in the vicinity of Taishan of Guangdong before dusk. It then moved into western part of Guangdong and weakened further. Mangkhut degenerated into an area of low pressure over Guangxi the next night.
According to press reports, Mangkhut brought torrential rain and squalls to Luzon. There were at least 82 deaths, 138 injuries and two missing. Around 15 000 houses collapsed. Mangkhut brought damaging winds and severe storm surge to the coast of Pearl River estuary, leading to damages of many buildings and coastal structures, as well as serious inundation of low lying areas. In Macao, 40 people were injured and more than 5 500 people were evacuated. There were a number of reports of building damages. The height of the inundation in Inner Harbour once reached 1.9 metres or higher above ground. At least six people were killed and more than 3.3 million were affected in Guangdong, Guangxi, Hainan, Guizhou and Yunnan.
As Mangkhut was a fast-moving storm and posed a serious threat to Hong Kong, the Hong Kong Observatory issued the Standby Signal No. 1 well in advance at 10:20 p.m. on 14 September when Mangkhut was about 1 110 km east-southeast of the territory, the farthest distance on record. Local winds were light to moderate northeasterlies during the day of 15 September. As Mangkhut edged closer to the coast of Guangdong quickly, the No. 3 Strong Wind Signal was issued at 4:20 p.m. on 15 September when Mangkhut was about 650 km southeast of Hong Kong. Local winds strengthened at night, becoming fresh to strong northerlies. With Mangkhut maintaining its course towards the Pearl River Estuary, the Observatory issued the No. 8 Northeast Gale or Storm Signal at 1:10 a.m. on 16 September when Mangkhut was about 410 km southeast of the territory. Local winds continued to strengthen afterwards, with gale to storm force northerlies offshore and on high ground. As winds were expected to increase further when Mangkhut came closer to Hong Kong, the Increasing Gale or Storm Signal No. 9 was issued at 7:40 a.m. when Mangkhut was about 200 km south-southeast of the territory. With local winds picking up rapidly afterwards, the Observatory issued the Hurricane Signal No. 10 at 9:40 a.m. when Mangkhut was about 160 km south-southeast of the territory. At the time when tropical cyclone signals number 8, 9 and 10 were issued, the storm was the farthest away from Hong Kong for the corresponding signal since 1961. The destructive storm to hurricane force winds affected Hong Kong for a long period of time during the day of 16 September. Mangkhut came closest to the Hong Kong Observatory Headquarters around 1 p.m. with its centre was located about 100 km to the south-southwest. With Mangkhut shirting past to the southwest of Hong Kong, local winds veered from the northeast to the southeast gradually. Mangkhut made landfall over the vicinity of Taishan of Guangdong before dusk. With Mangkhut moving away from Hong Kong and weakening gradually, hurricane force winds no longer affected the territory. The No. 8 Southeast Gale or Storm Signal was issued at 7:40 p.m. to replace the No. 10 Signal. With local winds subsiding continuously, the No. 3 Strong Wind Signal was issued at 5:20 a.m. on 17 September, followed by the No. 1 Standby Signal at 2:40 p.m. Mangkhut moved further inland and weakened, and all tropical cyclone warning signals were cancelled at 7:10 p.m. that night.
While Mangkhut weakened after moving across the northern part of Luzon with weaker convection over the eyewall, the spiral rainband between 100 and 200 kilometres from its centre remained intense and the structure was intact. Analysis of microwave satellite images (Figure 2.2.6), Doppler weather radar images (Figure 2.2.7), surface observations and flight reconnaissance data revealed that the winds associated with the spiral rainband outside the eyewall were stronger than those near the eyewall. When Mangkhut skirted past to the south of Hong Kong, the intense spiral rainband swept across Hong Kong during the day. Moreover, Mangkhut moved rapidly over the northern part of the South China Sea (speed reaching 35 km/h). With Hong Kong staying in the right semicircle of the storm (also known as the dangerous semicircle) for a long time, the superposition of wind speed and moving speed of the storm brought ferocious winds to Hong Kong most of the time during the day. In view of the special wind structure of Mangkhut, the wind strength experienced by Hong Kong was the strongest among the Pearl River Delta areas even though Mangkhut tracked closer to Macao, Zhuhai and Taishan.
Table 2.2.1 showed the maximum wind recorded at various stations in Hong Kong during the passage of Mangkhut. Under the influence of Mangkhut, the wind strength over Hong Kong was generally stronger than that of the tropical cyclones necessitating the issuance of No. 10 signals in the recent three decades, including York in 1999, Vicente in 2012 and Hato in 2017 (Table 2.2.2). The maximum 60-minute mean wind speeds recorded at Waglan Island and Cheung Chau were 161 km/h and 157 km/h respectively. Both are the second highest records at the corresponding stations, just lower than the record high of Ellen in 1983. Gusts over 150 km/h were registered in most parts of the territory on that day and a maximum gust of 256 km/h was recorded at Tate’s Cairn, ranking after Wanda in 1962 and Ruby in 1964. A maximum 10-minute mean wind of 124 km/h was registered at North Point anemometer located inside the Victoria Harbour (Figure 2.2.9), the first time sustained hurricane force winds were recorded at the station since the start of its operation in 1998. Besides, the automatic weather station under testing at Clear Water Bay even recorded a maximum 10-minute average wind speed of 191 km/h (Note), which is believed to be the highest record near the surface since the Observatory's commencement of automatic weather station installation in Hong Kong in the 1980s.
The lowest instantaneous mean sea-level pressures recorded at some selected stations are as follows:
Mangkhut's track is a typical one causing severe storm surge in Hong Kong. When it skirted past to the south-southwest of Hong Kong, the associated ferocious east to southeasterly winds pushed water towards the shore and piled up against the coast. In addition, Mangkhut’s extensive circulation drove a more extensive area of the ocean which in turn raised the water level further. The severe storm surge induced by Mangkhut raised the water level in Hong Kong generally by more than two metres, resulting in an unusually high water level in many places in Hong Kong. Five of the six tide stations of the Observatory (including Quarry Bay, Tai Po Kau, Tsim Bei Tsui, Tai Miu Wan and Shek Pik) registered record breaking storm surges. Among them, the water level increases recorded at the tide station at Quarry Bay and Tai Po Kau were 2.35 metres and 3.40 metres respectively. As the tide station at Waglan Island was severely damaged by Mangkhut, the highest sea level was not recorded. The sea level (the sum of astronomical tide and storm surge) of Quarry Bay rose to a maximum of 3.88 metres (above Chart Datum, same below), exceeding the 3.57 metres registered during the passage of Hato in 2017, and only lower than the record high of 3.96 metres set by Wanda in 1962. A maximum water level of 4.71 metres was recorded at Tai Po Kau, also only lower than the record high of 5.03 metres set by Wanda. For the maximum sea level recorded at various tide stations in Hong Kong on 16 September 2018, please refer to Figure 2.2.11.
The subsiding air ahead of Mangkhut's circulation brought mainly fine and very hot weather to Hong Kong on 14 and 15 September. Temperature at the Hong Kong Observatory soared to 35.1 degrees on 15 September, the second highest record for September. Under the influence of the circulation of Mangkhut, the weather in Hong Kong deteriorated rapidly with heavy rain and squalls on 16 September. More than 150 millimetres of rainfall were recorded over most parts of the territory on that day. Red Rainstorm Warning and Special Announcement on Flooding in the Northern New Territories were issued by the Observatory. Under the influence of the rain bands associated with Mangkhut, there were still occasional squally showers on 17 September.
In Hong Kong, at least 458 people were injured during the passage of Mangkhut. There were more than 60 000 reports of fallen trees, the highest number on record. Many incidents of blowing down and falling objects as well as building damages were reported. A tower crane of a construction site in Tai Kok Tsui was blown down. The wall of a building and a rooftop home also collapsed. The roof of a refuse collection centre in Sau Ming Ping was also blown away. At least 500 reports of smashed windows or glass curtain walls were received. Among them, glass curtain walls of several commercial buildings in Hung Hom, Wan Chai, Central and Mong Kok were damaged. Windows were broken in several apartment buildings in Tseung Kwan O. An air conditioning unit was crashed into an apartment in Lai King, injuring a person inside. Electricity supply to over 40,000 households in Hong Kong was interrupted (Figure 2.2.13), including Sai Kung, Cheung Chau, Kat O, Tung Ping Chau, individual buildings in Tseung Kwan O and Heng Fa Chuen, and rural areas in the western and northern New Territories. Power outage to some 13,500 households lasted for more than 24 hours, and the electricity supply to some remote areas and individual buildings were not fully restored even after four days. Supply of fresh water in some places was also affected due to power outages.
The destructions caused by the heavy rain, storm surge and high waves induced by Mangkhut are more serious than those of Hato in 2017. Severe inundation triggered by storm surge and huge waves were observed in a number of coastal areas, including Tai O, Shek Pik, Mui Wo, Cheung Chau, Heng Fa Chuen, Siu Sai Wan, South Horizons, Lei Yue Mun, Tseung Kwan O, Sha Tin, Tai Po, Sai Kung, Yuen Long, Lau Fau Shan, Sha Tau Kok, Shek O and Peng Chau. Many coastal structures suffered from different levels of damages, including sewage treatment works, public beaches, waterfront promenades and sports ground. Flooding was serious in Tai O, Lei Yue Mun and Tsang Tai Uk in Shatin and many residents were evacuated. Sea water flowed into the estates and underground car parks in Hung Fa Chuen and Tseung Kwan O south, submerging a number of private vehicles inside. The cycle tracks and subways near Shing Mun River in Shatin, coastal area of Tolo Harbour, Lam Tsuen River in Tai Po were inundated. A number of villages houses in Nam Wai in Sai Kung, Lau Fau Shan, Sam Mun Tsai San Tsuen in Tai Po and San Tsuen in Sha Tau Kok were seriously flooded. Hundreds of vessels of various sizes were stranded, sunk or seriously damaged by the powerful waves. Farmland, fish rafts and fish ponds in all districts suffered different levels of damage.
Sea, land and air transportation services were paralyzed on the day Mangkhut battered Hong Kong. Owing to fallen trees and flooding, parts of the major roads were still closed and public transports could not be fully resumed the next day. Most of the public bus services were suspended and there were limited services of East Rail Line and Light Rail of MTR. Ferry services resumption was affected due to the damage of facilities at a number of ferry terminals. 889 flights were cancelled at the Hong Kong International Airport.
For comparison between Mangkhut and the tropical cyclones necessitating the issuance of the Hurricane Signal No. 10, please refer to the Observatory’s Blog – "A Wake up Call from Mangkhut" (http://url.hko.hk/3oIYlUG9).
Note: The automatic weather station at Clear Water Bay is located on a complex terrain with the anemometer at an elevation over 70 metres above sea level. The corresponding wind speed near sea level is estimated to be lower than 185 km/h.