It seems that you have missed the necessary options for this, but anyway, as I searched for it, here is the answer. What is so unique about the north american midwest that results in having more tornadoes than anywhere in the world is that the northward movement of warm moist air from the gulf of Mexico; the southward movement of dry cold air from Canada; and the lack of mountains preventing the convergence warm gulf air with cold Canadian air. Hope this answers your question.