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.