(a) and (b) are fine.
(c) is fine, but used in a subtley different situation to (a) and (b). (c) sounds like you already have an expectation of rain, and that you are expressing regret that the rain will upset your plans. It is similar to the regret expressed in the sentence, "I wish you wouldn't go". In both of these cases, the understanding is that the expected action will take place, and it will have a negative effect. The phrase expresses regret at the future action, or the outcome of the future action.