Lowest temperatures on record have been recorded in the north and central areas of the state (Figure 18). Several areas stand out that are colder than their surroundings. These areas usually reflect the topography. The coldest area in the state so far as record low temperatures are concerned is centered in the northwest in McLean and Mountrail counties and encompasses the area where the extreme low temperature of -60° F was recorded. Other areas not quite so cold but where temperatures have dropped to about -54° F are located near the Canadian border and in the central portion of the state.
http://www.npwrc.usgs.gov/resource/habitat/climate/temp.htm
I've done pipeline facility design in North Dakota. We were required by Northern Border Pipeline company to use -50°F. It may not happen every year, but when it happens, they like things to keep running and not break down.