"Until an outbreak of tornadoes in the past week, this year had been a relatively quiet one for twisters in the Midwest and Plains states."
"The reason, weather experts said, had much to do with a weather phenomenon that also caused much of the East Coast to shiver through colder-than-normal temperatures this spring: the high-altitude winds known as the jet stream brought Arctic air farther south, and for longer, than in a typical year. In the central United States, that prevented warm moist air from the Gulf of Mexico — a key ingredient in the formation of tornadoes — from moving north.
'The jet stream was stuck in place,' said Jeffrey Masters, director of meteorology for the Web site Weather Underground. 'It kept funneling cold air down.'
The jet stream finally started shifting north this month. 'The pattern broke, and then wham,' Dr. Masters said."