I've lived in Mid MO all my life. Hid in the tub a few times as a kid with a green sky. My folks split and Dad moved to NashVegas and lived there for 20+ yrs. Went down to visit some friends of his that lived on an old homestead about 45mins South of Nashville. 100+ yr old cabin was still there and an old Barn. But in the old family grave plot, early 1900's were 5 graves, all the same family and date, "Killed by Twister" and I never forgot that. Can't imagine . . .
Current House does not have a basement. We go hide at the neighbors a few times. But now we are adding on, garage slab is spec'd out thickened in the corner. Plan was to build concrete block "vault" and fill the cores with grout, steel door. BUT I found a house being demo'd with a F5 rated steel SafeRoom for $1200. All modular, 1/4in plate bolt together pieces so we will anchor that for now.
ANY shelter is better than NO Shelter. Below ground might be best, but consider flooding and going outside to access. An above ground, grade level steel box, inside the garage that you can walk/roll into rated for an F5 will do just fine 99% of the time. If the finger of god comes down with an F5+ it was likely my time to go anyway. Sure beats hiding my kids in the tub.