Well, geographically central US America would be what is commanly referred to as the Plains states . . .Kansas and Nebraska. But "the Midwest" is aculturally defining term, like "The South." Nevada is in the southern portion of the US, but it isn't considered to be a part of "The South." And the aforementioned states as stated by the OP are Rockies states. Having only barely lived west of the Mississippi, I'm not as familiar. But I'm certain that Utah has never been considered a portion of the Midwest. I'm relatively certain that it is typically known as either Southwest or Rockies, depending on context. In fact, it is far to the west of the central divide, and the term Midwest typically refers to bits much further east; although they are not justly defined in terms of geographically, a long history makes the capitalized term carry a certain meaning. Of course, such is only my cultural perspective.