The South China region is partly within the tropics and is the warmest and wettest part of the country during the summer.
Rainfall is very heavy between May and September, along both the coast and adjacent inland. Winters are mild and mostly frost free. However, summer heat and humidity can be rather uncomfortable.
Typhoons are frequent here. They can be violent, bringing very heavy rains and strong winds for several days at a time along the coastal regions. The Typhoon season is from July to October.
Since the most important Trade Fairs run during April and October, the weather conditions may not affect you too much. Here is a link to the Hong Kong Observatory where you will find current and forecast weather and climate information for Hong Kong.
Hong Kong Weather and Climate http://www.hko.gov.hk/contente.htm
Here are links to the World Meteorological Organization (WMO) and World Climate Index. These provide updated weather and climate information for the following Chinese cities:
Other cities in China