من موقع لونجمان بشأن الإجابة على نفس السؤال
(c) Weather forecasts are based on data that is gathered and analyzed. That may constitute a form of "present reality" or "evidence". In the reports themselves, however, the form will is often used because it is more formal and less cumbersome than saying going to repeatedly. Frequently weather reporters will begin with going to and then continue with will.
(42) It's going to be a beautiful day tomorrow. It will be sunny and breezy all day. The high will be 32 and the low at night will be 22.
But when we talk about tomorrow's weather based on what it is now, not on a published forecast based on data, the "present reality" is not so clear. The fact that the weather has been bad for three days does not necessarily mean that it will continue to be bad. Weather can change quickly. Either future form can be used. See also section 1f above.
(43) It's been rainy for three days. It probably will be the same tomorrow.
(44) It's been rainy for three days. It's probably going to be the same tomorrow.
In contrast to the above, where either form can be used, there are two examples in the Sec 2 textbook, page 22, where one form is the better choice.
The first is someone reading a brochure about England.
(45) I think the weather in England will be cold and rainy.
She is not reading a weather forecast about the very near future (the next day or two), but a brochure about travel facts. Based on her reading, she predicts what the weather will be when they travel. Will is the better choice here.
In the other example on that page, someone is looking at the sky and sees that it is clear.
(46) It's going to be another very sunny day.
She is basing her statement about today based on the present reality of a clear sky. Going to is the better choice here.
The present continuous is not used for predictions, so it is not used to talk about future weather conditions.
(47) *It's raining tomorrow.
Conclusion
Once again, teachers and test writers must be aware that the future forms are very complex in English and that often more than one form can be used in a given sentence. The choice of form might depend on what the speaker intends to emphasize. While there are some situations in which one form or the other is clearly correct, there are many other situtions in which more than one form can be used.
ed 19/05/11
References:
L. G. Alexander. Longman English Grammar. London: Longman, 1988.
Raymond Murphy. English Grammar in Use. Cambridge: Cambridge Unversity Press, 1985.
Michael Swan. Practical English Usage, 3rd ed. Oxford: Oxford University Press, 2005.