Most tornado related deaths are from projectiles.
There is a FEMA design for a "Safe Cell" concept (many details and suggestions) over the past 20 years and is based on history and testing. Basically, it is a concrete or reinforced concrete block room with a NECESSARY concrete roof that can be built in a basement, used as a closet or bathroom or as a separate structure above or below grade. Wood is really inadequate unless the 3/4" plywood sheets (closely spaced studs) are laminated with steel plate to eliminate projectile penetration. The acceptance test is to prevent any penetration through the wall or roof by a 12' long 2x4 fired by an air canon at about 150 mph, so that explains why it took so long to come up with an unbuildable wood/steel system.
Dick
The standard is very good and give suggestions on on doors, door swing direction (into the safe cell), acceptable hardware (hinges, latches and number of each)and ventilation.
Most are built in basements or for a slab on grade home they become a multi-use room (closet, bath, gun storage, etc.). The are commonly built into new homes in tornado-prone areas. They are created to protect from tornado winds (up to 250) and much stronger than hurricane winds (125-135) once they a landed on shore.
Even a basement is not absolute protection from a tornado because of the extremely high winds. There were 2 children in the basement about 10 miles from here and they were sucked out through the walk-out end 50 feet away (one was drowned in a lake). If there was a safe cell, they would be alive today, but that is hind sight.
Tornadoes can occur in many places other than Kansas (Wizard of OZ) or Oklahoma as everyone has seen. Even in MN, we had the most tornadoes in the country 2010 (115). They are unpredictable, form quikly and are unbeleivable strom and not like hurricanes that are predictable, so a near by safe area