My icing is made entirely with shortning instead of butter, and I do use meringue powder. I'm just concerned because I will probably start constructing the cake and adding decorations about 45 min. before the reception starts. I don't know if it'll start melting in that time. I live in Indy. Right now, we are having some warm and some cool days. Yesterday was 82, today is 62. I won't have any idea of the weather until at least a few days before.