Oklahoma, Alabama, Joplin: Why we're seeing so many tornadoes and superstorms

More powerful tornadoes can see wind speeds above 200 m.p.h. and typically stay on the ground longer and cut longer paths, increasing the likelihood they can strike a populated area.

What has been triggering this Spring's spate of severe storms and chart-busting tornadoes?

The North Atlantic Oscillation, a see-saw circulation pattern over the North Atlantic, is as good a place to start as any, some researchers say.

The NAO has fallen into general pattern, during the past 23 months, that acts as a roadblock for the jet stream, a high-speed river of air energized by the temperature difference between polar air to the north and warmer air to the south. The jet stream in effect steers storm systems from west to east across North America.

Over the past 14 months, the jet stream's typical path across the Atlantic "has been blocked more often than it has in recorded history," says Jeff Weber, a researcher at the University Corporation for Atmospheric Research in Boulder, Colo., which oversees the National Center for Atmospheric Research there.

This roadblock pattern has remained strong and persistent through much of April and May, putting deep, south-dipping kinks into the jet stream. Storms move across the country more slowly, with more meanders than usual. The south-dipping kinks push the lingering storms

Meanwhile, powerful, cold storms have continued to roll in from the North Pacific. As these storms cross the Rockies, they are borne south along these kinks. The slowed jet stream allows storms to linger in the south, drawing more and more moisture and energy from the Gulf of Mexico.

When the cold and warm air masses meet, violent thunderstorms can develop along the storm front accompanying the system, especially in late spring, when the sun is higher in the sky and warms the surface longer than it does in winter, sharpening the temperature contrast.

For tornadoes to form, another ingredient must be present – wind shear, set up by changes in speed and direction of winds with altitude.

On May 24, for instance, the Storm Prediction Center forecast the strong likelihood for tornadoes in sections of Arkansas, Kansas, Missouri, and Oklahoma during the afternoon and evening, thanks in no small part to a strong jet stream aloft – moving in an east-northeasterly direction – atop lower-altitude winds, driven by the storm system itself that flew up from the south.

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