Just because it is below zero in Northern Florida does not mean it will be below zero in Yellowknife. Check out the complete weather systems over North America, highs and lows, including the jet stream and the current polar vortex excitement.
When you drive from Toronto to Ottawa in autumn, you often meet a very nice invisible barrier about 60km to the West of Ottawa, where the weather pattern is coming down from Hudson's Bay and temps drop about 6 degrees celsius in a 15km stretch, and the difference is startling. Sometimes it is there, sometimes it is not, but it is very noticeable and just one of those things ;-)
When it is reverse the temperature change can be in the positive direction but usually around that same place.
