May 21
- 1918 – A major tornado outbreak impacted Iowa and Wisconsin, killing 27 people. An F4 (possibly F5) tornado devastated Boone, Iowa and completely swept away two farms, killing nine people. Another F4 tornado destroyed most of Lone Rock, Wisconsin, killing four people there, and four more on farms. An F5 tornado killed four people as it destroyed at least 20 farms between Densison and Adaza, Iowa.
- 1949 – A tornado outbreak, continuing from the previous day, killed 51 people across the Midwestern United States. An F4 tornado destroyed more than 200 homes in Cape Girardeau, Missouri, killing 23 people and injuring 130. Another F4 tornado devastated Shelburn, Indiana, killing 14 people, injuring 251, and destroying 160 homes.