I am asking this question on behalf of a friend of mine. Do tell me, can and when or why can we use past form when we predict something in the future. I agree the phrase is not full. Anyway, the friend said that she heard it on the radio and it was the weather forecast. She was a little surprised by the past form. Unfortunately, she could not remember any sentence before or after.
The simple answer to your question is that you don't use a past construction to talk about the future. We talk about the future using will, going to, present continuous, or present simple.
The only guess I can make for what your friend heard is that she heard reported speech, and the 'is coming' became 'was coming' because of the back-shifting of the verb when we report. Could she have heard something like "The forecast said no rain was coming till tomorrow"?
Assuming that your friend reported the radio announcement correctly, and assuming that the announcer was not mistaken, then the announcer was not speaking Standard English but some English dialect or pidgin.
In Standard English neither the simple past, past continuous nor future in past are used to denote a future event or possibility of future event. However, in languages that do not have tripartite tense systems, such as Hebrew, the past tense can be used to denote future events or a condition depending on a future event. It could be that the construction that your friend heard in an English dialect is a loaned construction from a neighboring language without tripartite tense. If so, it would be interesting to know what dialect this is.