Unless it said very clearly "black tie"
formal typically means "suit and tie with dress shoes"
I could be wrong, because technically "formal attire" can mean black tie (or white tie), but no one says this anymore meaning black tie because formal to most people simply means "jacket and tie"
so you should be fine with any suit and tie.
and i just reread and it you say "semi forms"... so yea, just where that suit you will be fine.