Khanun was the seventh tropical cyclone to affect Hong Kong in 2017 and for the fifth time in the year, the No. 8 Gale or Storm Signal had to be issued by the Observatory, equalling the record in 1964 and 1999 in terms of the number of No. 8 Signals issued in a year.
Khanun formed as a tropical depression over the western North Pacific about 650 km east-northeast of Manila on the morning of 12 October. It moved west-northwestwards and intensified into a tropical storm that night. Khanun moved across the northern part of Luzon the next day, drifting west-southwestwards slowly and re-organizing after entering the South China Sea. It turned northwestwards on 14 October and kept intensifying, evolving from a severe tropical storm into a typhoon by nighttime. Turning west-northwestwards the next day towards the south China coast, Khanun intensified further into a severe typhoon, reaching peak intensity with an estimated maximum sustained wind of 155 km/h near its centre. It then moved generally westwards and started to weaken rapidly under the influence of the northeast monsoon. Khanun became a tropical storm by the time it crossed Leizhou Peninsula in the early morning on 16 October, and degenerated into an area of low pressure over Beibu Wan during the day.
According to press reports, at least seven people were injured in Macao during the passage of Khanun. Transportation services were seriously disrupted. Under the combined influence of Khanun and the northeast monsoon, over 970 000 people were affected in Guangdong, Hainan, Zhejiang, Guangxi and Fujian. There was also widespread heavy rain in Taiwan, with roads damaged and electricity supply to 14 000 households disrupted.
In Hong Kong, the No. 1 Standby Signal was issued at 10:40 a.m. on 14 October when Khanun was about 700 km southeast of the territory. Under the combined effect of Khanun and the northeast monsoon, fresh northerlies continued to affect Hong Kong, occasionally reaching strong force offshore and on high ground during the day. With Khanun edging closer to the south China coast, the No. 3 Strong Wind Signal was issued at 7:10 p.m. that night when Khanun was about 570 km southeast of Hong Kong. Local winds became fresh to strong northerly during the night and occasionally reached gale force offshore and on high ground. As Khanun continued to move closer to the coast of Guangdong and further intensified, the No. 8 Northeast Gale or Storm Signal was issued at 8:40 a.m. on 15 October when Khanun was about 260 km south-southeast of Hong Kong. Strong to gale force northerly winds generally affected the territory in the morning and occasionally reached storm force on high ground. Khanun came closest to Hong Kong around 3 p.m. that day with its centre passing about 210 km south-southwest of Hong Kong. Local winds started to turn northeasterly in the afternoon. With Khanun weakening and moving away from Hong Kong, local winds moderated gradually. The No. 3 Strong Wind Signal and No. 1 Standby Signal were issued at 7:20 p.m. and 10:40 p.m. respectively, before all tropical cyclone warning signals were cancelled at 2:20 a.m. on 16 October.
Under the influence of Khanun, maximum hourly mean winds of 104, 85 and 65 km/h and maximum gusts of 151, 106 and 99 km/h were recorded at Tate's Cairn, Waglan Island and Cheung Chau respectively. A maximum sea level (above chart datum) of 2.96 m was recorded at Tsim Bei Tsui, and a maximum storm surge (above astronomical tide) of 1.05 m was recorded at Tai Miu Wan. The lowest instantaneous mean sea-level pressures recorded at some selected stations are as follows:
Locally, it was mainly cloudy with one or two rain patches at night on 14 October. Under the influence of the outer rainbands of Khanun, there were squally showers on 15 and 16 October. More than 40 millimetres of rainfall were recorded over most parts of the territory during the 3-day period, and rainfall even exceeded 70 millimetres in the northern part of the New Territories and the western part of Lantau Island.
In Hong Kong, at least 22 people were injured during the passage of Khanun and there were more than 580 reports of fallen trees. One person was hit on the head by falling branches in Tsuen Wan. Two private cars were damaged by fallen galvanized iron sheets in Sham Shui Po. There were 12 people in distress while engaging in water sports activities under stormy weather and required the assistance of rescuers, and 22 campers stranded on Sharp Island off Sai Kung were taken to safety by marine police. Temporary traffic arrangements were implemented in Lantau Link as a result of the windy condition, leading to serious congestion on the roads to and from the airport. More than 600 flights were cancelled or delayed at the Hong Kong International Airport.