
OK, here's how tornadoes and hurricanes cause casualties. High wind speeds (100-300 MPH) generate sufficient stresses on buildings that they often fail and collapse, crushing the occupants. These high winds also pick up pieces of debris, which then act as missiles which can do catastrophic damage to anything they hit. These high winds, and more specifically the destruction they cause, are usually measured using the Enhanced Fujita Scale.
http://en.wikipedia.org/wiki/Tornado_intensity_and_damage
This house was destroyed by roughly an EF-3.
50 yards away I found this chicken. Imagine getting hit by a chicken going 200 mph... Tornadoes can generate odd missiles. (This also confirms the old wives' tale about chickens being de-feathered by tornadoes.)
So, the goal is to have a structure that is sufficiently hardened that it will withstand the wind and remain intact and shield you from the missiles the storm is launching in your direction. Hardening an entire home to make it proof against these threats is cost prohibitive. But you CAN build or buy a small structure that can take the abuse of a tornado or hurricane. The question is, do you need a storm shelter?
Take a look at the map; it shows regions of the USA based on potential wind speeds in tornadoes. Even a "puny" EF-2 (113-157 mph wind) can shred a mobile home and cause extensive damage to weaker structures. An EF-3 (158-206 mph) will start demolishing better made homes. And an EF-4 (207-260 mph) leaves little standing after it passes.
So, if you live in a mobile home in Zone 2 I think you should have a storm shelter. If you have a solid home but live in Zone 3 or 4 and no basement, you should have a storm shelter.
Now, basements can be a pretty safe place to shelter--certainly safer than nothing. But even in a basement you can still be very vulnerable as the home atop you comes to pieces.
Now, you can buy a storm shelter from any number of merchants (e.g. www.missouristormshelters.com). But most of the commercially made shelters are tiny and expensive. A 5x7 is a common size and will cost you over $3,000. These shelters are better than nothing but in many instances will provide zero actual protection. Let me explain. When the weathermen see signs that a storm is developing to the point that it may spawn a tornado they issue a tornado watch. If the storm continues to develop to the point where a tornado DOES spawn, or where it is incredibly likely that one will, they issue a tornado WARNING.
This is when you and your family need to take cover - because if a tornado is coming for you it will be upon you like Smaug on Laketown or an anvil on a Looney Toons character - suddenly and with little warning. At the point that you can see and hear the actual tornado you have VERY little time to take cover and it may be too late.
So, when a tornado warning is issued you need to take cover. When it's lifted you can come out. This is the safe policy to have in place.
And that's the problem with these small pre-made shelters: they may be nice but sheltering in one for an hour or two at three AM with two toddlers and a dog doesn't sound like a lot of fun, which means you will do it once and never again.
The shelter I built for $3,000 has 200 square feet of interior space, a far cry from the 30-50 square feet most small shelters have. This gives plenty of space to stretch out; you could even have furniture and bedding in there. And as a (kind of unpleasant) bonus, if your home is wiped out you have somewhere to stay. Try camping for a few days in a micro shelter.