Pardon me while I dig up a post from October. Ok October 2006.

Blast,

There is a shelter designed at the wind lab of Texas Tech, that is really what you are looking for. Generally they are built at the time of new construction because they are tied into the foundation beams and piers of the slab on grade foundation.

I am a custom homebuilder in DFW area, and have put one in almost every home I build. They are MUCH less expensive than DuPont's offering and secured by more than 1/2 inch drop-in anchors drilled and epoxy'd into the concrete slab 6 inches.

As a recent retrofit to a home, I built an "out building" that looked like a pool equipment room / storage for a customer. The damn thing should withstand even close 105mm HE rounds much less a cat5 tornado. (he really had this one over engineered).

Susan,

The May 9 1999 tornado is the one in Oklahoma you referred to hearing about. It is the reason the Fujita scale (F1 - F5) was changed. There was no Fujita F6 so it is now the Enhanced Fujita scale (EF1-EF5). All tornados are now measured against this one which is EF5. The only other EF5 since then was Greensburg, Kansas.

We were visiting my DW's family in that area when it hit. I was the last into the underground shelter as I wanted one more look. For the rest of my days on this earth, I shall never forget what was to be seen.

Two of the aforementioned (From Texas Tech) shelters were directly hit by that tornado. When it was over, the occupants opened the door to find their shelters were the only things standing from horizon to horizon.

Blast PM me for any questions, the really aren't that hard / expensive and perfect for your area.