Over at the Watts Up With That blog, there is an article about the number of tornadoes. It seems that the number of tornadoes has been increasing in the last two or three years, but it is nothing out of the ordinary. There is no trend.
The weather bureau people have improved technology with which to spot tornadoes, now, especially with the Doppler radar systems. Before that, they had to depend on eye witnesses to help them with many sightings.
Take a look at the graph. You can see that they are now counting mainly the really strong tornadoes of EF3 to EF5 to look for trends.
So, if you hear of someone who says that climate change or global warming is causing more severe weather events, here is a convenient button for your use.
Now, here is the graph.