I planned to visit Hong Kong in September 2015. Should i bring a raincoat? ^^

According to the previous weather records of Sep., there will be some rainfall in that month probably, and most of them is light rain, which will not last for a whole day, so you do not need to worry about it. If you take too much luggage, the raincoat is not that necessary.

I'd recommend you bring along raincoat. The weather records of the past years show that frequent rain may take place. As well as that, it is still during the typhoon season. Considering all, the raincoat can be needed.