[Wuli]

I think the change this year is in part due to how the jet stream is tracking this year. Many other North American air currents are affected by what the jet stream is doing.

Certainly 2011 does appear to be have been unusual year in North American tornado history, and this year appears closer to norm, so far, and there was an article last year that related jet stream behavior and its seasonal variations to the occurance of tornados.

http://news.discovery.com/earth/frequent-tornadoes-a-symptom-of-jet-stream-change-110602.htm

[neverdem]

Thanks for the link.