After a fast start to the season the number of twisters has dropped back to far below average:
One reason we had so many early storms was the warmer than usual waters of the Gulf of Mexico.
During winter 2011-2012 the Arctic Oscillation kept much of the polar
cold bottled up north of the 75th latitude (except for the "gate"
opening onto central Europe, with upwards of 1000 people frozen to death
in Jan.-Feb.)
The second reason was the lingering La Niña which has teleconnection
effects on the jet stream which seem to set up temperature gradients and
wind shear conducive to storm formation.
The 1974 Super outbreak and the 2011 season which saw the EF-5 devastate Joplin Mo. occurred during La Niña.
All of this has resulted in a reversion to the mean for 2012:
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