I just listened this morning to a piece on NPR, a discussion with a research meteorologist, who opined that any single piece of real estate was likely to be struck by a tornado once in 4,000 years.
Using those stats, if you live there for 30 years in one house, your house has a 30 in 4,000 chance of being struck by a Tornado. With those odds, I'd definitely spend some money on a reinforced room! Plus, home insurance would probably go down.