I live in a quite hot area and I am going to visit a mountainous place in its humid period. The temperature ranges from 5 degrees (celsius) to -10 degrees (celsius), but I don't have any waterproof coat... But is it necessary?

I am thinking about going with two sweaters and a thick coat over them in order to keep me warm, would this prevent me from getting HYPOTHERMED.com ? Should I stop being a selfish little fuck and buy a waterproof/winter vest instead? I don't want to waste my shekels bros