WIKI wrote:The most extreme tornado in recorded history was the Tri-State Tornado, which roared through parts of Missouri, Illinois, and Indiana on March 18, 1925. It was likely an F5, though tornadoes were not ranked on any scale in that era. It holds records for longest path length at 352 km (219 mi), longest duration at about 3.5 hours, and fastest forward speed for a significant tornado at 117 km/h (73 mph) anywhere on Earth. In addition, it is the deadliest single tornado in United States history (695 dead).
TOP 10 worst tornadoesFor 37 years, the most extensive tornado outbreak on record, in almost every category, was the Super Outbreak, which affected a large area of the central United States and extreme southern Ontario in Canada on April 3 and April 4, 1974. Not only did this outbreak feature an incredible 148 tornadoes in only 18 hours, but an unprecedented number of them were violent; 6 were of F5 intensity and 24 were F4. This outbreak had a staggering 16 tornadoes on the ground at the same time at the peak of the outbreak. More than 300 people, possibly as many as 330, were killed by tornadoes during this outbreak. However, this record was later broken during the April 25–28, 2011 tornado outbreak, which resulted in 325 tornadic fatalities and had 358 tornadoes touch down
So the cooling of the early 70's was the period of the worst outbreak of the time
And apparently the current plateau to agrueably cooling is the time period of the 2011 outbreak