f you plan for the worse scenario, right down to the finest detail then no one is going to get upset if there is a last minute change of plan.
Consider buying some umbrellas that will complement your outfits; they will always come in useful afterwards.
I always have a wet weather plan, for where the photographs are going to be taken, I will have discussed this with the couple beforehand and got their approval and checked with the venue to make sure that they are on board.
When photographing an outdoor wedding ceremony, I will be in constant contact with the venue to agree timings for going ahead outside, or moving under cover.
That said, some venues have great indoor locations…