why this sentence must be sue 'going'?

[Kritsada]

English Grammar Tests, Elementary Level

ESL/EFL Test #106 "Beginner Grammar Questions Test (1)", question 10

The forecast isn't good for tomorrow. It's ......... to rain so don't forget to bring your umbrella.

(a) having
(b) going
(c) seems

English Grammar Tests, Elementary Level

ESL/EFL Test #106 "Beginner Grammar Questions Test (1)", answer 10

The forecast isn't good for tomorrow. It's going to rain so don't forget to bring your umbrella.

Correct answer: (b) going
_________________________

why this sentence must be sue 'going'? I don't understood.
If I can use having,it's incorrect,right?

Kritsada

[Conchita]

The be going to form (+ infinitive) is a way of expressing the near future. Here it is used for prediction.