What can we expect the weather in Zhangjiajie to be between 13 to 18August this year you think?

Well, during that time, it could be hot and humid in Zhangjiajie. The average temperature can reach about 32 C. Anyway, the temperature in daytime and at night is massively different. You'd better prepare short-sleeved shirt, pants and sunglasses for daytime as well as the thin coat for night. It might rain but less than that of July. Therefore, don't worry much about the rain.