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.
Tornado outbreak of April 13–15, 2018
An outbreak of tornadoes unfolded across the region as an expansive upper-level trough pushed into the Central United States, along with a 100kt jet streak pushing into Oklahoma and Missouri. A surface area of low pressure tracked from North-Central Kansas toward the Missouri-Iowa state border, supporting a warm front arced across southern Iowa, and a dryline extending from Kansas into Texas. Within the warm sector of the low, surface dewpoints rose to the mid 60s. A swath of 850mb winds at or above 50kt persisted underneath an elevated mixed layer, yielding an unstable environment characterized by mean-layer Convective Available Potential Energy (CAPE) of 1500–2500 J/kg. Throughout the afternoon, confluent low-level flow caused several discrete thunderstorms to form across Arkansas, where the SPC had issued a Particularly Dangerous Situation (PDS) tornado watch. The most significant supercell of the day developed over LeFlore County, Oklahoma and eventually progressed into Crawford County, Arkansas, where it produced a half-mile-wide EF2 tornado that caused substantial damage in and near Mountainburg.