To drop down below freezing in summer the skies must be clear with no wind. A Canadian cold front moved into Northeast Minnesota around the Laurentian Divide.
The Laurentian Divide, by the way, is the geographic area where water flows north toward the Arctic Circle and Hudson Bay instead of south toward the Gulf of Mexico.
Minnesota is a ecological mix of rich prairie, to pothole lakes, to rugged and ancient lava flows that formed amazing rocks and waterfalls. Like it's weather, you can find a vast array of geographical possibilities. Every day is a new wonder.
