I copied it from longman site
Choose: The weather forecast says it (will - is going to) rain tomorrow. I think the correct answer is "is going to" because it is prediction based on evidence. The weather forecast doesn't give personal opinions. Do you agree?
First, let me say that grammar about different forms of the future is very complex and often more than one form can be used in a sentence, sometimes depending on what the speaker intends to emphasize. So while we teach students to follow certain rules, you might hear native speakers use different forms.
Instead of using the word “evidence” in these rules, use Michael Swan’s term, “present reality.” “If we say that something in the future is going to happen, it is usually already planned or decided, or it is starting to happen, or we can see it coming now.” (Swan, page 188)
“We prefer will for predictions when there is not such obvious outside evidence — when we are talking more about what is inside our heads: what we know, or believe, or have calculated.” (Swan, p 191)
Weather forecasts are based on data that is gathered and analyzed, not on “present realities.” The weather forecast itself is more likely to use will -- or a mix of going to and will -- but the sentence in question here is not the weather forecast itself but a repetition of what the forecast says. When I am reading the forecast, I might take that as "evidence" and use going to.
In the case of talking about the weather, both will and going to are often possible, and we should learn to accept that on tests. However, there are other cases where the difference bewteen the two forms is clearer.
Let’s look at a few other examples to clarify what Swan means by “present reality” or “evidence.”
Don’t lend your car to Ahmed. He’ll crash it. (a prediction based on my knowledge of his driving habits.)
Look out! We’re going to crash! (present reality: we are seconds away from hitting something)
Some day my daughter will marry and have a baby. (a prediction based on my knowledge of what people usually do)
My daughter is going to get married. (present reality: she’s engaged)
My daughter is marrying next week. (arrangements have been made)
My daughter is going to have a baby. (present reality: she’s pregnant)
My daughter and her husband both have blue eyes, so their baby will have blue eyes. (prediction based on my knowledge of genetics)
(Michael Swan. Practical English Usage, 3rd ed. Oxford, 2005.)
The weather forecast says it (will - is going to) rain. I think the best answer is "is going to" because their prediction is based on evidence. However, I frequently find "will" used in this sentence. What do you think?
The grammar of the future forms is one of the most complex areas of grammar. Linguists find it difficult to define the uses of the various forms, and often more than one is possible. This is especially true of the choice between will and going to.
If you have read the forecast, you can take that as evidence and say that it is going to rain. Others may choose will because they don't see any "present reality" such as dark clouds in the sky -- or simply because in this case the two forms are both used.
Forecasters themselves base their predictions on calculations. They sometimes use will, sometimes going to. They might say, for example, "It's going to be a rainy day tomorrow. Temperatures will be in the low 20s and there will be showers throughout the day." In a forecast, will is shorter and a bit more formal than going to, so those may also be reasons why it is used.
There are other cases when one form of the future is almost always used instead of others, but talking about the weather is more problematic unless we accept that there can be more than one correct answer.