going to Jiuzhaigou and Zhangjiajie in December, how about the weather and scenery?

Hi all, I'm planning going to Chengdu from Hongkong in December 2011. From Chengdu I have a plant to visit Jiuzhaigou and Zhangjiajie. Is it very cold to visit these sites in that time? how about the scenery in December, is it still enjoyable?

Thanks for your kind information.