Tornado Outbreak of April 24 2010

The first large tornado outbreak of 2010 came very late - April 22nd-24th. A major reason for the late start was the lack of moisture due to unusually cold Gulf of Mexico waters through mid-April:
However, water temperatures at buoy 42035 near Galveston jumped from 63.5F (17.5C) on April 5th to 73.9F (23.3C) on April 24th, allowing dewpoints to reach 70F+ in Mississippi.
The most significant tornadoes were on April 24th especially in Mississippi.
A massive 1.75 mile wide EF4 tornado tracked 149.25 miles across Louisiana and Mississippi, killing 10, and injuring 146. (More info, images here.)
Notice the very clear skies immediately following the tornadoes:
A radar loop of the entire event is available here and storm relative velocity here.

High resolution radar imagery from 1714Z (1214 CDT) shows debris enhanced reflectivity near Yazoo City and a very intense rotational couplet.
High resolution loop here.

Although strong instability was present; lifted index -8C/CAPE 2,000 J/kg, it was not exceptional.

What was exceptional was the trough which produced wind speeds over 130 knots at 250 mb, and over 90 knots at 500 mb.Even more impressive was the storm relative helicity values, which were literally off the charts.
Notice that the scale only goes to 500 m2/s2, while the high in western Tennessee is over 1,500 m2/s2! Even 0-1 km SRH is over 1,400:
To see how this can occur, look at how rapidly winds increase and veer from the surface to 700 mb (or even just to 850):
Here is the sounding from Jackson, MS.