The sentence that presents a counterclaim to the claim "Winter tornadoes are scarier than those that occur at other times during the year" is:
"Tornadoes that happen in the spring are generally more powerful than those that occur in the winter."
This sentence argues against the idea that winter tornadoes are scarier by suggesting that tornadoes at other times of the year (specifically spring) are more powerful, which could imply they are more dangerous or frightening.