Eight tornadoes were recorded on July 31, 1966 across the Dakotas. The strongest was later rated F3. It tracked from south of Ashley, ND and into North Dakota, north of Long Lake.
The Storm Data entry for the date states that approximately 20 funnel clouds were reported in this area and 8 “touched down”. There is a chance that more than 9 South Dakota tornadoes developed on this day but knowing where they began and ended can’t be determined.
The Spencer, SD F4 tornado of May 30, 1998 was 1 of 5 tornadoes produced by one supercell that tracked about 30 miles. This destructive tornado had a path length of 14 miles and hit the small town of Spencer (population at the time of 320) destroying most of its 190 buildings. 6 people were killed and 150 were injured. The tornado was estimated to reach a width of a mile.
This is the only F5 on record for the state of South Dakota.