"Its (city) average temperature is more than 20 degrees below zero and only when you are there can you feel a true winter of ice and snow." This is technically correct, but the end rings a little untrue to me. Something a little more natural would be "...a truly snowy and icy winter." The second sentence is definitely wrong to me. You can say something like "The average temperature is more than 20 degrees below zero so you can feel a truly snowy and icy winter only while you are there." That sounds fine to me, but something about that adverbial phrase in the middle makes it sound off.