Rule reminder
'A meno che (non)' = 'unless'. It always takes the subjunctive. Present/future meaning → present subjunctive; past narrative → imperfect subjunctive; completed past action → present perfect or pluperfect subjunctive. The 'non' after 'a meno che' is expletive and does not negate the sentence.
Usciamo dopo cena, a meno che non … (piovere) ancora.