I will be Zhangjiajie on Sept. 24th for 3 days.

According to the weather forecast, it will be cool and windy. The average temp will be about 70F. The temp in the mountain area would be lower than that, about 65 F. But you do not need to wear jackets, just long-sleeved shirts for cold hours. There is also light shower, but it will only last for a few hours. If necessary, you can prepare some rain gear.