Tornado Alley
Although the official boundaries of Tornado Alley are not clearly defined, its core extends from northern Texas, Oklahoma, Kansas, Nebraska, into South Dakota. States such as Missouri, Iowa, Minnesota, Wisconsin, Illinois, Indiana, and western Ohio are sometimes included in Tornado Alley. Some research suggests that tornadoes are becoming more frequent in the northern parts of Tornado Alley where it reaches the Canadian prairies.
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.