Ok, speaking as someone who does not live in a tornado zone, but who would not be remotely surprised if global warming changes that, one would be strongly inclined to revette the shelter with at least 2' of re-barred concrete overhead.
The reason for that is that you have no idea how strong the tornado is or how much debris it is going to be carrying.
Worse case is it picking your shelter up whole and dumping it elsewhere. And I don't mean in The Land of Oz either! Even a small lift and drop of oh, say 20', would be fatal to the occupants.