I've noticed here that Spring storms are stronger and more frequent. Also, new research suggest an increase in tornadic activity when there's a La Nina yr, or a switch from El to La in the same season, like '88, '99. We started El last summer I think. Haven't checked Dec '09/ Jan '10 trend.
It did start last summer, though mid-lats continued well into June and July, just not as frequent as they were during the spring. It's nothing to be real concerned about- though in 2007 we came out out a hellish El Nino winter and we had some pretty interesting weather develop across the plains.