Koppu was the seventh tropical cyclone that necessitated the issuance of a tropical cyclone warning signal in Hong Kong in 2009. It was also the third time that the No. 8 Gale or Storm Signal had to be issued in the year.
Koppu developed into a tropical depression over the western North Pacific about 490 km northeast of Manila on 12 September and moved west-northwestwards to cross the Luzon Strait that night. It entered the northern part of the South China Sea with a westerly track on the morning of 13 September and intensified into a tropical storm that evening. Koppu intensified further into a severe tropical storm on the morning of 14 September and turned to move northwestwards towards the coast of Guangdong. It became a typhoon that afternoon, and took up a west-northwesterly track at night. Koppu reached its peak intensity with estimated maximum winds of 140 km/h near its centre in the small hours of 15 September. It made landfall over the coast of western Guangdong near Taishan in the morning and weakened into a severe tropical storm. It became a tropical storm that afternoon. Koppu weakened into a tropical depression on the small hours of 16 September and then dissipated over Guangxi. According to press reports, Koppu brought severe storm surge and rainstorm to Guangdong, where at least nine people were killed, another nine people missing, over 1.4 million people were affected and over 1,200 houses collapsed. The direct economic losses amounted to RMB$1.75 billion. In Macau, back-flow of sea water brought severe flooding to the coastal and low lying areas there. A Panamanian cargo vessel ran aground near Galon Island in Zhuhai, spilling some 50 tonnes of oil.
In Hong Kong, the Standby Signal No. 1 was issued at 8:35 p.m. on 13 September when Koppu was about 520 km to the southeast. Winds in Hong Kong were moderate easterly that night. Local winds freshened from the northeast on the morning of 14 September, occasionally strong offshore and on high ground. The Strong Wind Signal No. 3 was issued at 11:15 a.m. when Koppu was about 280 km to the south-southeast. As Koppu continued to move closer to Hong Kong, the northeasterlies became generally strong in the afternoon, with occasional gales force offshore and on high ground. The No. 8 Northeast Gale or Storm Signal was issued at 5:55 p.m. when Koppu was about 180 km to the south-southeast. Gales from the east affected the territory at night, reaching storm force offshore and on high ground. Winds gradually changed its direction to the southeast on the small hours of 15 September and the No. 8 Southeast Gale or Storm Signal was issued at 12:35 a.m. At the Hong Kong Observatory Headquarters, the lowest instantaneous mean sea-level pressure of 996.0 hPa was recorded between 12:42 a.m. and 1:16 a.m. when Koppu was at its closest to Hong Kong, passing about 130 km to the south-southwest. Gales from the southeast persisted until around dawn and gradually subsided in the morning. The Strong Wind Signal No. 3 was issued at 10:15 a.m. to replace the No. 8 Gale or Storm Signal. Winds subsided further thereafter and the No. 3 Signal was replaced by the Standby Signal No. 1 at 1:35 p.m. All signals were cancelled at 3:40 p.m.
The weather in Hong Kong was sunny on 13 September but squally thunderstorms affected Hong Kong in the evening. It was cloudy with squally showers on 14 September. Heavy squally showers affected Hong Kong on 15 September and more than 100 millimetres of rainfall were recorded in many parts of Hong Kong. The Amber Rainstorm Warning was issued at 3:55 a.m. and 5:55 p.m. respectively. The combined effect of the storm surges of Koppu and high tides resulted in a maximum sea level of 3.43 metres at Tai Po Kau, just below the highest sea level of 3.47 metres recorded during the passage of Typhoon Utor in July 2001.
In Hong Kong, at least 74 people were injured, four of them seriously during the passage of Koppu. There were 48 reports of fallen trees, of which 11 of them in Hong Kong Island, 19 in Kowloon and 18 in the New Territories. There were also five reports of loose scaffoldings and one report of a fallen external wall. Eight reports of flooding were received, with Tai O being worst hit. In Tai O, the combined effect of storm surge and heavy rain brought about by Koppu and high tides resulted in flood waters reaching 1.5 metres deep there, bringing damage to the goods and equipment in the shops there and around ten people have to be evacuated. There were also many incidents of vehicles being affected by flooding. In Ho Man Tin and Wong Tai Sin, four drivers had to be rescued by firemen when their taxis were affected by flood waters. In Yuen Long, a large tree was blown down and damaged a warehouse nearby. In addition, a scaffolding was also blown off by strong winds in Yuen Long, damaging four vehicles nearby. Fallen objects were reported in Yau Ma Tei and Kwai Chung, injuring one person. In Tsim Sha Tsui, strong winds damaged the door of a commercial building and a person was injured by pieces of broken glass. At the Hong Kong International Airport, eight flights were diverted.