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.In the early evening,it's going to rain in downtown Los Angeles.But in the mountains,it's going to snow.

不会存在be 后面加动词原形的。除非这个动词的过去分词跟原形是一样，表示被动。
动词分为系动词、实义动词、情态动词等。
be 动词的原形就是 be..

如果用be的话 后面得就跟形容词 名词
如果用动词的话，就是动词原形 将要发生什么或者做什么
明白了吗？
希望能采纳，谢谢

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.