I hear what you are saying. You likley hit a weather freak that is just not all that normal down there. While 18C was the temp, the humidity and winds prone to coastlines are the factor here. I'm from Newfoundland and have lived in Mantoba for the past 12 years. I have seen both extremes.
Honestly, try to prepare for all weather but dominate the light clothes for warm to hot weather. Add a couple sweatshirts and a light jacket and I think you will likley be more than prepared. Can't predict the weather, but the odds are you will have nice weather. We had a 94 degree day in Jamaica in mid January