This is a correction made by a native speaker from the first one to the second. Why though? I know it probably also depends on what is being emphasized in this sentence, but what I am wondering about is: isn't it more natural to have a break after では?
カナダでは、今日大雪です。
カナダでは今日、大雪です。
I think there are commas for pauses when spoken, and commas more for visual (or typographic) separation. The one suggested here seems to be the latter. In particular the four kanji sequence 今日大雪 can look like a word at a glance, when it is not. A comma between words help people scan more easily.
Also a pause between 今日 and 大雪 is not so unreasonable, although a pause between カナダでは and 今日 might be more common (or longer).
It's also possible to have both, although some may say that's excessive.
