because "llover" is the verb and "lluvia" the noun. Both words are translated as "rain".
You may know that after the "to be going to" you always need a verb in English. Well, in Spanish "to be going to" is translated as "ir a" which is also the translation of "to go to". The way of difference both meaning is knowing the word after the "ir a":
If it is a verb, then it is "to be going to", while if it is a noun, it is "to go to". And the other way round, if the original verb is "to be going to" then there will be a verb after the "ir a" and if it is "to go to", there will be a noun.
Furthermore, if you use "lluvia", the sentences doesn't make sense, he can't go to rain ("va a lluvia") because rain isn't a place.
Ps.- I suppose this comment answers your question, but i am not really sure. If not, please reply it explaining your question ;)
I have to admit that I didn't real of th separation of the word in the audio until you said it. I do not really know why it's pronounced like that. In normal speech that word would be pronounced faster and altogether.
PS.- In this case, the word este has not the accent because it means "this" (determinant) instead of "this one" (pronoun) which has the accent
I don't love this translate. I typed "This Saturday is going to rain." Which is correct imo when making a declaration. Such as, "Is it going to rain this week?", "This Saturday is going to rain."
The pronoun "it" in this sentence refers to which antecedent? "This Saturday"? Clouds? The sky? If the answer is "This Saturday", then surely, the word "it" is unnecessary.