The term "normal" has a specific meaning in the science of climatology. A normal is a 30-year mathematical average of measurements made over the period 1981 through 2010. The resulting number is a measure of the central tendency of the data set, and not necessarily an "expected" outcome. Normals are recalculated once per decade.
Minneapolis/St. Paul (Twin Cities) normals are derived using data gathered at the Twin Cities International Airport.