15. This to me is the key sentence in the entry though:
"The four westernmost states of the Midwestóthe Dakotas, Nebraska, and Kansasówere in the late 19th century very much part of the "Old West"."
This is the area that, when I was a kid, we called "the great plains." That term seems to have gone into disuse, but I don't know why. As far as weather goes, and therefore crop production, it is different from those other states.
I also don't count Missouri as the midwest, because it was a slave-state, which in my mind puts it in the South.
I think there are also some dialect maps that bear out the distinction I'm making: