It's going to 后面一般是动词原形,为什么有些还加be 再加原形?还是都可以

In downtown Los Angeles,it's going to be sunny and clear tomorrow morning.
In the early afternoon,it's going to be windy.
In the late afternoon,it's going to be cloudy这三个句子都是说天气情况的,sun、wind、cloud、都是名词,不可以直接跟在it's going to 后,所以要借用be动词,be就是动词原形,这样才符合语法.
In the early evening,it's going to rain in downtown Los Angeles.
But in the mountains,it's going to snow.这两个句子虽然也是说天气,但是rain、snow都可以作动词使用,故不用借助be动词.