Rob, I think I may have neglected to mention that you might consider a
dual-purpose design: root cellar and storm cellar. There is not far
from here an old concrete cellar built like that, for going into when
tornadic activity is predicted but for food storage most of the year.
Door faces downwind, steps down into a hillside for coolth.
Friends here recently purchased a cast concrete structure as a tornado
shelter. Serious money.