What's with all the tornadoes this year?

I've been asked this by many folks this spring.  While the process of tornado formation is rather complicated, you basically (very basically) only need three things to happen.

1.) Warm, moist air riding over cold, dry air.  This basically describes a frontal boundary.  While tornadoes are certainly possible along a warm front, they are more common along a cold frontal boundary.  At a cold front, cold, dry air pushes under warm, moist air and forces it up.  This creates the atmospheric lift needed to form thunderstorms, especially supercell thunderstorms - from which the monstrous EF2 through EF5 tornadoes are born.

2.) Wind shear.  This is basically just a fancy meteorological term for a change in wind direction and speed with height.  For a tornado to form, you need to have wind at the surface moving in a different direction that at upper levels (basically the warm, moist air needs to be going one way and the cold,dry air another) and at different speeds.  This shear creates a rotation in the atmosphere parallel to the ground.

3.) An updraft.  This is provided by the parent thunderstorm from which the tornado forms.  An updraft (or vertically rising column of air) is what feeds a thunderstorm.  When the updraft of the thunderstorm meets the horizontally rotating column of air, the updraft bends the rotation into the vertical.  This creates the rotation of the supercell thunderstorm, and ultimately the tornado.

But why so many this year?  Well, in a way, it's a "perfect storm".  I know, bad pun.  But conditions this spring were perfect for tornado development over the southern and central United States - a set up only seen once in about 30 years!  First, unseasonably warm air over the Gulf of Mexico (caused by warmer than normal water there) moved into the southern and central U.S. early this spring.  Meanwhile, people in the North Central U.S. were experiencing an unseasonably cool spring.  As these two air masses clashed in the southern and central U.S., the jet stream (a rapidly flowing current of air in the upper atmosphere) dipped far to the south, creating insane amounts of wind shear over the area where these two air masses collided.  This allowed the thunderstorms created by the clashing airmasses to grow rapidly and become extremely severe and producing large, long tracked tornadoes.  And unfortunately they passes through large metropolitan areas - thus putting a large amount of people in their paths.

As we moved into late spring, the jet stream lifted back to the north as an upper level ridge formed over the southeastern U.S.  So the southern and central U.S. now has a break from severe weather.  Hopefully this remains quiet around the rest of the nation now.