Typhoon Koppu passed Hong Kong over 14-15 September 2009; came within around 120km (to southwest) overnight. Major thunderstorm the evening before - see this video. In post below this, there is video from Cheung Chau, on 14th and 15th.
It was very hot on 13 September - to 34.5C on Cheung Chau; after hot few days following Hong Kong's hottest August since 1963. With a tropical storm forming near Luzon, and headed towards Guangdong, seemed thunderstorms were likely. Even so, this storm was remarkably severe - for amounts of lightning, and tremendous thunderclaps. Phew!