America's Funniest Tornadoes is a TV show on FOX which seems to be a clip show of funny things happening during tornadoes. From what little was shown of the clip, A farmer is trying to go down to a field during a tornado (presumably to do farmwork involving a shovel.), but then a stop-sign (presumably being forced off the sign pole from the tornado) suddenly hits him on his groin.
The Simpson family saw a snippit of this show while looking through the mid-season lineup, with Bart proceeding to laugh when the farmer was hit in the groin.
Behind the Laughter
America's Funniest Tornadoes is an obvious parody of "America's Funniest Home Videos" on ABC.