Why There Have Been Fewer Large Tornadoes Since 1950
Note: increase in air travel since the 1950s seems to correlate. Boundary layer smoothness in the lower troposphere (inversion layers) that are created during periods of calm weather are the cause of tornadoes. Aircraft can/do destroy boundary layer smoothness. Accordingly, the explanation for the decrease in large tornadoes, starting in the 1960s, is the increase in commercial air traffic at about this time.
Also check out parts 1 and 2 starting here: http://www.solvingtornadoes.org/