In fact, outbreaks of this magnitude occur about every 5 years. What is unusual is for a tornado producing F5 damage to hit a large city. But a careful look at population density will reveal that large cities occupy a very small percentage of total land area in the United States.
Most strong tornadoes occur in rural areas.
There have only been two strong La Nina events. Although scientists have looked for a correlation between La Nina and tornadoes, there just isn't enough data to make any conclusions. Regarding El Nino, the NSSL's Dr. Dave Schultz has a few notes here . That means that there are as many as 20 devastating tornadoes each year. It is possible that meteorologists have underestimated the number of violent tornadoes that occur each year.