The temperature in Great Falls, Montana dropped more than fifty degrees in six hours on Friday, going from 54°F at 2:53 PM down to 2°F at 8:53 PM. Now that's a cold front.
Here's a graph from the city's airport observing station, showing the sharp plunge as the cold front passed through:
The Rocky Mountains in southern Alberta and northern Montana can produce incredible cold air damming events such as this one. Very cold, dense air presses up against the Arctic-facing bases of the mountains, where it sits and pools up because it's too dense to ride up the face of the mountain and reach the valleys on the other side. The result is Great Falls sitting near zero while Helena, just seventy miles southwest as the crow flies, is more than forty degrees warmer.
The blast of Arctic air is a result of a high pressure system over central Alberta transporting bitterly cold air from the north; a weak low pressure system to the south is giving the cold air an extra nudge towards Great Falls courtesy of enhanced northeasterly flow.
The chill will head east along the international border through the weekend, with high temperatures in the single digits reaching the Upper Midwest by Monday.
While Great Falls' wild temperature swing is impressive, it's got nothing on the record for the fastest temperature drop in the world. If your grandparents or great-grandparents were in Rapid City, South Dakota on January 10, 1911, they saw temperature readings fall from 55°F at 7:00 AM to 8°F fifteen minutes later—the fastest recorded temperature drop on record. Two days later on January 12, temperatures rose back up to 49°F only to fall down to -13°F two hours later, a 62-degree swing.