"The forecast said it will rain" - e.g. today is 15th November, we saw/heard the forecast yesterday and it said it will rain on 16th November. So it is still in the future. "The forecast said it would rain" - today is 16th November, we saw the forecast on 14th November and it said it would rain on 15th November. So both the forecast and rain were in the past. Similarly with was/is going to rain.

@Chris // Thank you!!!!!!!!!!!! :D

Using Weronika's example: November 14 - Announcer: It will rain on November 16. There are two correct choices for reported speech: November 15 - Yesterday, the announcer said it would rain on November 16. Yesterday, the announcer said it will rain on November 16. Here is a more complete explanation. In general, all verbs are backshifted. Announcer: It will rain. [or - It is going to rain.] What did the announcer say? The announcer said that it would rain [or - it was going to rain]. However, there is a choice of not backshifting under certain conditions. Source: https://www.ef.com/ca/english-resources/english-grammar/tense-changes-when-using-reported-speech/ [excerpt] You do not need to change the tense if the reporting verb is in the present, or if the original statement was about something that is still true, e.g. He says he has missed the train but he'll catch the next one. We explained that it is very difficult to find our house.