Like the others are suggesting, I would look into a shelter that's not technically below the ground level, even though you cover it with earth by making a hill over it. It can be sunken a few feet but then you have to be very careful to water-proof everything well. With tornadoes obviously you want it as low to the ground as possible, and a very solid support structure as well. But this is good thinking because I know of people who have built underground shelters but then they flood every time it rains. If there is a tornado, they won't be able to use them without drowning.