Tornado map's weird anomaly

Of course, our prayers go out to the people of Joplin, Missouri and other cities after the devastating tornadoes of recent days. To get a better sense of whether you are in the line of fire of an F5 tornado, check out the very cool maps at the Tornado History Project. You can see the actual paths the worst tornadoes of the past decades—in a zoomable Google map. And here's the weird part: when you look at the map of F5 tornadoes, there is an odd void over Missouri. All the surrounding states have tons of F5s, but Missouri is like a blank spot... as if a forcefield is protecting it from the worst killer twisters. Until two days ago.