Tornadoes in the United States
The Midwestern states are very prone to tornado activity, as they are part of "Tornado Alley." States included in the area that are hit by tornadoes the most are Oklahoma, Kansas, Missouri, Nebraska, Iowa, and South Dakota. The Midwestern States are often hit by tornadoes during the late spring and early summer months, especially the months of May and June. This is due to the aforementioned air mass moving northward into the Midwest and combining with another air mass that move southward from Canada. This creates unstable air, creating the potential for storms to strike the most during these months. The frequency begins to decrease in the middle of the summer, as the air mass moves northward into the Great Lakes states.
Climate of the United States
In the lower Midwest (and southern Plains states, especially), temperatures can rise or drop rapidly; winds can be extreme; and clashing air masses, including hot, dry air of Mexican and/or Southwestern origin, warm, moist air from the Gulf of Mexico and cold, dry air from Canada can spawn severe thunderstorms and tornadoes, particularly from April to June. The "dryline," separating hot, dry air of Mexican/Southwestern U.S. origin from warm, moist air from the Gulf of Mexico, often causes severe, occasionally violent, thunderstorms to fire in central and eastern Texas, Oklahoma and Kansas; these sometimes contribute toward the hailstorms and tornado outbreaks that the Southern Plains are well known for. Reflecting these air-mass conflicts, central Oklahoma, including the Oklahoma City and Moore-Norman areas, has the highest frequency of tornadoes per unit land area on planet Earth, with May the highest-risk month for tornadoes throughout "Tornado Alley," from northern Texas north-northeastward toward western and central Iowa.