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.
Tornadoes in the United States
In the Southwestern United States, New Mexico reported 485 tornadoes during the 1950 to 2006 period (NCDC figures), California reported 355, Arizona reported 209, and Nevada reported 75. New Mexico borders the notoriously tornado prone states of Texas and Oklahoma, hence the noticeably higher figures. Most of the tornadoes here happen in the very eastern part of the state. New Mexico's region and the desert landscape of the state help prevent the amount of twisters that happen in Oklahoma and Texas. Arizona and New Mexico experience regular summer thunderstorms during their monsoon season. These are sometimes tornadic but rarely produce violent tornadoes.