What tense is the sentence "It is going to rain."?

"It will rain." is pretty formal sounding. In everyday speech, people rarely say that.

Here's a brief excerpt from " Grammarly Blog "

How to Form the Simple Future

The formula for the simple future is will + [root form of verb].

I will learn a new language.
Jen will read that book.
My brothers will sleep till noon if no one wakes them up.
You will see what I mean.

It doesn’t matter if the subject is singular or plural; the formula for the simple future doesn’t change.

But…

There is another way to show that something will happen in the future. It follows the formula [am/is/are] + going to + [root form verb].

I am going to learn a new language.
Jen is going to read that book.
My brothers are going to sleep till noon if no one wakes them up. You are going to see what I mean.

The “going to” construction is common in speech and casual writing. Keep in mind though that it’s on the informal side, so it’s a good idea to stick to the will + [root form] construction in formal writing.

There is a great deal of information on their website. You might want to check it out:

**https://www.grammarly.com/blog/verb-tenses/**

The structure for Present Continuous Tense is to use the present tense of the verb "to be" followed by the present participle.

I can write:

I am going to the store - this means that I am either on my way to the store or will be doing so shortly.

And so in the same way we can say:

It is going to rain - this means that, while it may not be raining in the present moment, it will in the near future.

Present Continuous vs Simple Present

It is going to rain (Present Continuous) is different from It rains (Simple Present) in that while there is some indication as to time frame with the first sentence, there is simple a statement about the ability for rain to fall in the second sentence.

Present Continuous vs Future Continuous

It is going to rain (Present Continuous) is different from It will be raining (Future Continuous) in that with the first we are saying it will rain soon, with the second we are setting up the fact that while someone is doing something in the future, it will also be raining. For example:

It will be raining when the movie gets out.