The historic trend for strong tornadoes has been in decline the last fifty years or more. We may have an uptick this year, but the trend is down. There is a slight uptick for mild tornadoes F1 and F2, considered to be due largely to improved detection.
Tornadoes are caused by the convergence of cold air and hot air. Due to global warming there is no more cold air anymore, hence the reason the trend is down. (Send me my Global Warming research grant check!)