Tornado formation

I was wondering why the US is having such violent storms lately and found this great explanation for the WaPo blog:

Thanks to the 139 tornadoes reported (this number will change as the National Weather Service conducts their damage assessments), April 2011 has now almost certainly seen more tornadoes than any other April on record since 1954 when an estimated 407 tornadoes descended from the heavens.

Yesterday, the South, Southeast and mid-Alantic had the necessary ingredients for severe thunderstorms and tornadoes, but in varying degrees.  In the South and Southeast, all the ingredients came together to create a once-in-a-generation outbreak of severe weather and tornadoes. Unstable air had been in place for several days (leading to tornado outbreaks in TX, AR, MS, and LA in days prior), and an approaching cold front. Normally, this type of set up would create severe weather, but not to this effect. This time, a strong jet stream drove deep into the southeastern states with westerly winds of over 100 mph. At the same time, a deepening surface low created low-level winds from the south/southeast, creating a severe turning of the winds with height or wind shear. With this extreme wind shear and instability, storms had little trouble starting to rotate and producing tornadoes.