Leigh,

It's not likely the direct passage of the funnel over these shelters would pull them out of the ground. The lateral pressure of the tornado is what causes the debris field to be created. Since these shelters are buried the surface area exposed to the wind is relatively small. My only suggestion if someone was to install one of these would be to have a hydraulic jack of considerable strength and some 4X4 lumber stowed in the shelter. If the debris from the tornado is piled up on the door there is every possibility that you may not be able to open the door. The jack and lumber would allow egress.

My thoughts...