Nesat was the fourth tropical cyclone that necessitated the issuance of a tropical cyclone warning signal by the Hong Kong Observatory in 2011. It also necessitated the issuance of the first No. 8 Gale or Storm Signal in the year.
Nesat formed as a tropical depression over the western North Pacific about 1 840 km east of Manila on 23 September and moved west-northwestwards. It intensified into a tropical storm on 24 September and further into a severe tropical storm on the following day. Nesat intensified into a typhoon over the western North Pacific about 560 km east of Manila on 26 September, reaching its peak intensity with an estimated maximum sustained wind of 145 km/h near its centre. On 27 September, Nesat crossed Luzon and entered the South China Sea in the afternoon. It continued to move west-northwestwards at about 20 km/h across the northern part of the South China Sea on 28 September. Nesat took up a northwesterly track at a speed of about 22 km/h that night, moving closer to the south China coast. It moved generally west-northwestards on 29 September and made landfall over the northeastern part of Hainan Island in the afternoon, entering Beibu Wan that evening. On 30 September, Nesat first weakened into a severe tropical storm and moved across Beibu Wan, then made landfall again over the coast of northern Vietnam in the afternoon and weakened into a tropical storm. It moved further inland that evening and dissipated over northern Vietnam on 1 October.
According to press reports, Nesat triggered flooding in the Philippines where 35 people were killed. In Hainan Island, Guangdong and Guangxi, at least 4 people were killed and one person missing during the passage of Nesat. Over 183 000 hectares of farmland were inundated and over 5 300 houses collapsed or destroyed, with direct economic loss exceeding 7.4 billion RMB. In addition, around 3 000 houses were destroyed and 11 boats sank during the passage of Nesat over Vietnam.
In Hong Kong, the Standby Signal No. 1 was issued at 10:40 p.m. on 27 September when Nesat was about 770 km southeast of Hong Kong. Local winds were moderate east to northeasterlies that night, freshening from the northeast and becoming strong on high ground on 28 September as Nesat moved closer. The Strong Wind Signal No. 3 was issued at 5:20 p.m. when Nesat was about 510 km south-southeast of Hong Kong. Local winds strengthened from the east in the evening and strong winds were recorded over most areas in Hong Kong, occasionally reaching gale force offshore and on high ground. As Nesat continued to move closer in the early hours of 29 September, local winds continued to strengthen and the No. 8 SE Gale or Storm Signal was issued at 4:40 a.m. when Nesat was about 360 km to the south-southwest. Nesat was closest to Hong Kong at around 7 a.m. that day passing about 350 km to the south-southwest. Under the influence of the large circulation of Nesat, gale force easterlies generally affected the southern part of Hong Kong that morning, occasionally reaching storm force on high ground. Gusts of over 120 km/h were recorded at Tates Cairn, Tai Mo Shan, Green Island, Cheung Chau, Ping Chau and Ngong Ping in the morning. As Nesat moved further away from Hong Kong and made landfall over Hainan Island in the afternoon, local winds turned to the southeast and gradually weakened. The No. 8 SE Gale or Storm Signal was replaced by the No. 3 Strong Wind Signal at 4:10 p.m. when Nesat was about 450 km southwest of Hong Kong. Local winds remained generally strong during the evening and gradually weakened later that night. The No. 3 Signal was replaced by the Standby Signal No. 1 at 0:20 a.m. on 30 September. All signals were cancelled at 6:25 a.m. as winds subsided further. At the Hong Kong Observatory Headquarters, the lowest instantaneous mean sea-level pressure of 999.0 hPa was recorded at 3:29 a.m. on 29 September when Nesat was about 370 km south-southwest of Hong Kong.
The weather in Hong Kong was mainly fine and hot with some haze on 27 September. It continued mainly fine on the morning of 28 September, but became cloudy with squally showers in the afternoon. Squally showers affected Hong Kong on 29 September with more than 60 millimetres of rainfall recorded over Lantau Island and the western part of the New Territories. Under the influence of the outer rainbands of Nesat, the weather remained mainly cloudy with a few showers on 30 September.
During the passage of Nesat, there were 418 reports of fallen trees and 15 reports of collapsed scaffoldings in Hong Kong. A total of 25 people were injured. In particular, a large sheet of scaffolding was blown down in Prince Edward Road East in San Po Kong, hitting a taxi and injuring its passenger. Over the seas off Chai Wan, a crane barge drifted across the harbour after its anchor cable was snapped. The barge first hit a pier at an oil storage depot in Chai Wan and then slammed into a sea wall at the Heng Fa Chuen promenade. At one point, the barge's extended crane arm came close to an apartment block, prompting the evacuation of more than 50 residents. At the Hong Kong International Airport, over 40 flights were cancelled, around 490 flights affected and 44 aircraft were diverted due to adverse weather.