The area of destruction for a tornado ought to be narrow enough that if you SURVIVE, you'll be able to walk out of it to unaffected civilization or otherwise get help in short order. Seems to me that a near-miss is the thing to be more worried about - something that doesn't destroy your house, but does take out the utilities and/or your access route. Much the same problems you could have with a severe thunderstorm, really. If those are your main worries, I'd expect a week of food and supplies to be more than ample, as services are likely to be restored fairly quickly. It's not the same sort of lingering and wide-spread problem that you might get from a hurricane, flood, or earthquake (or even a blizzard).

Anyone disagree?