First widespread below 0°F in the Continental United States. (Photo Credit: NWS Grand Forks)

DISCUSSION: On the week of December 8, a trough moved over parts of North Dakota and Minnesota. The trough brought with it a dose of a very dry cold polar air from Canada into the U.S. leading to the first widespread below 0 degrees Fahrenheit temperatures across much of the Northern Plains of the 2019-20 Winter season. The trough also brought with it a strong wind which made the wind chill much more dangerously cold. However, some locations had below 0 temperatures in November but they were not as widespread. The wind was strong due to a tight pressure gradient in the upper atmosphere. A pressure gradient is a quantity that describes the change of pressure over a distance and is used to help determine wind speed at high levels of the atmosphere.
Wind chill is an apparent temperature developed to give a sense of how the temperature feels with how windy it is. It is used to help determine how long one should be outside before developing hypothermia and frostbite. However, wind chill is not always accurate as it does not factor in moisture especially if a person’s skin is wet or how many layers a person is wearing.  In addition, wind chill is not consistent due to the minute time often varied in wind speed.
The trough slowly moved out before another one moved in with a warm front bringing warmer temperatures and snow to the area including up to an inch at Minneapolis-St. Paul International Airport. In a normal year, places like Minneapolis-St. Paul and St. Cloud, Minnesota are not below 0 degrees until late December or early January as winter progresses. The coldest areas are usually the rural counties in North Dakota and Minnesota with International Falls, Minnesota being the most consistently, coldest metropolitan area during the winter.
 
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©2019 Meteorologist JP Kalb