But is this actual state of affairs is the same state of affairs as the state of affairs which will obtain when it rains tomorrow? This seems to be the argument.
1. A statement must be true or false, and so there must be conditions under which it is true.
2. To state the conditions when a given proposition is true, is to say what IS the case, if it IS true. Merely to state the truth conditions is to specify something that exists now.
3. Thus statements about the future can be reduced to statements in the present tense. For if the truth condition obtains, the statement that it obtains, is true now. If it does not obtain, the statement that it does not obtain, is true now.
But why should the truth conditions be in the present tense? I agree that a proposition like 'Tom thinks it will rain tomorrow' says something about the present. It says that Tom has a certain thought, right now, in the present. Similarly 'the weather forecaster says it will rain tomorrow' says something about what the person on the TV says, now. Likewise 'It is causally determined that it will rain tomorrow'. Similarly also for 'it is possible that it will rain tomorrow', which says something about the speaker's present state of knowledge.
But 'it will rain tomorrow' doesn't say anything about the present. 'It will rain tomorrow' is true if and only if it WILL rain tomorrow.