Mawar was the fifth tropical cyclone necessitating the issuance of tropical cyclone warning signal by the Hong Kong Observatory in 2017.
Mawar formed as a tropical depression over the northern part of the South China Sea about 270 km east-southeast of Dongsha on the afternoon of 31 August. It drifted north-northwestwards slowly and intensified into a tropical storm on the night of 1 September. Tracking slowly to the northwest towards the coast of eastern Guangdong over the next two days, Mawar intensified into a severe tropical storm on the morning of 2 September and reached its peak intensity with an estimated sustained wind of 90 km/h near its centre. It then weakened into a tropical storm on 3 September, making landfall near Shanwei that night and degenerating into an area of low pressure over inland Guangdong the next day.
According to press reports, torrential rain and squalls brought by Mawar caused severe flooding in the Chaozhou-Shantou region and the Pearl River Delta, seriously disrupting transportation services. Electricity supply to around 110 000 households was interrupted in Guangdong and flooding was reported in many places in Macao.
In Hong Kong, the Standby Signal No. 1 was issued at 2:20 a.m. on 2 September when Mawar was about 400 km east-southeast of the territory. Local winds were light on 2 September and the lowest instantaneous mean sea-level pressure of 1002.8 hPa was recorded at the Observatory headquarters at 3:21 p.m. on 2 September when Mawar was about 310 km east-southeast of Hong Kong. With Mawar moving gradually closer to the coast of Guangdong, local winds became moderate west to northwesterlies during the day on 3 September. Although Mawar made landfall near Shanwei and weakened gradually on the night of 3 September, it continued to edge closer to Hong Kong. The Strong Wind Signal No. 3 was issued at 10:40 p.m. that night when Mawar was about 190 km east-northeast of Hong Kong. Winds strengthened gradually and became fresh to strong southwesterlies around dawn. Mawar came closest to the territory around 8 a.m. on 4 September, passing at a distance of about 120 km to the north-northeast of Hong Kong. As Mawar started to move away from Hong Kong and continued to weaken, local winds subsided and the Strong Wind Signal No. 3 was replaced by the Standby Signal No. 1 at 10:20 a.m. on 4 September. With Mawar degenerating into an area of low pressure over inland Guangdong in the afternoon, all tropical cyclone warning signals were cancelled at 2:10 p.m.
A maximum sea level (above chart datum) of 2.89 m and a maximum storm surge (above astronomical tide) of 0.44 m were recorded at Tsim Bei Tsui during the passage of Mawar.
Under the influence of a continental airstream, the weather in Hong Kong was hazy with sunny intervals and evening squally thunderstorms on 2 September. The rainbands of Mawar brought heavy squally showers and thunderstorms to the territory on 3 and 4 September. More than 50 millimetres of rainfall were generally recorded in Hong Kong during the 3-day period of 2 – 4 September, with rainfall over Lantau Island, Cheung Chau, Kwai Chung and Tuen Mun exceeding 100 millimetres.
In Hong Kong, several private cars were damaged by fallen trees in Sai Wan and Tai Wai. Fallen trees also led to a disruption of light rail services. Village houses in Shui Hau Tsuen of Lantau Island were flooded during the heavy rain on the evening of 3 September.