A scan of regional records reveals that for most locations in the region, the latest 70-degree days on record have happened in November. At Grand Forks, the latest 70-degree day on record was just last fall on Nov. 8, 2020. The latest in Duluth was Nov. 9 of 1999. Minneapolis and Fargo share Nov. 17, 1953, as the latest day in the 70s. Sioux Falls and Rochester share Nov. 21, 2012, as the latest 70-degree day.
For Huron, South Dakota, it got to 71 degrees on Dec. 6 of 1939. However, Rapid City has a total of seven days in the 70s during December over the years of record, including 73 degrees on Dec. 18, 1979. The obvious southerly advantage here comes from more solar radiation at lower latitudes. The western edge comes from compressional heating on days with down-slope westerly winds.