Instead of looking at temporal statements as playing out in a time-indexed classical logic, it is more realistic to look at temporal logic as a variety of (time-indexed) modal logic.
I disagree that your statement most naturally breaks down to "If it is tomorrow, it is raining." There is no world in which such statements have any purpose. What is missing is not logical segmentation of this sort. What is missing is the mood or modality in which you are putting forward the otherwise meaningless statement.
One simply cannot meaningfully say "Tomorrow, it will rain", where the will is an expression only of the future tense of 'is'. You cannot ever know this fact, so it is not a realistic approach to the use of language. Instead, here, 'will' is a modal verb, the future tense, not directly of 'is', but of 'must', the complement of 'can', indicating your prediction or belief, and not an indicative connector. You mean that for some refinement of 'can', it cannot fail to rain.
But in that translation, 'some refinement' is very important. "If the laws of physics are correct, it cannot fail to rain tomorrow" is very far from "If the wind does not shift unexpectedly, it cannot fail to rain tomorrow." or "If the way my knee aches right now is just the right way that faithfully represents a given barometric trend, and I have correctly assessed the degree of the pain, it cannot fail to rain tomorrow." Or even "(Without external basis) I believe that it will rain tomorrow."
There is an intermediate position between modal and ordinary logic that considers all modal propositions vacuously true. But only until enough context is supplied. One has enough context once the premises are supplemented enough to express the mode asserted at least well enough to give a notion of probability to the statement.
For some notion of 'can' -- in which you personally simply cannot every be wrong -- it is surely true that whatever you say, including 'It will rain tomorrow' is true. That world accords with the non-modal world where the statement is vacuously true. But if you live in that world you are megalomaniacally psychotic. Instead, all of us are supposed to guess by context the particular refinement of the meaning of 'can' involved in making sense of your statement.
That refinement is made up of a bunch of premises, the likelihood of each of which we can assess.