Flood of May 1st and Late Season Cold

On May 1st, this enormous trough produced a tornado outbreak and historic flooding in western and central Tennessee.Dewpoints in the lower Mississippi valley were in the mid to upper 70s.
CAPE was over 5,000 J/kg:
About a dozen supercell thunderstorms quickly developed over Arkansas.
I don't ever remember seeing more tornado warnings in such a small area.
This was the beginning of the historic flooding in Tennessee, were they were already on the nose of a broad low level jet:The 13.57 inches of rain that Nashville received on May 1st & 2nd not only set new records for 6 hour, 12 hour, 24 hour, calendar day, and two day rainfall, but also broke the old record of 11.04 inches for the entire month of May! The highest rainfall total reported was near Camden, TN with 19.41 inches.

A late season cold spell prompted frost and freeze advisories across the Midwest and Northeast from May 8th-11th. On May 8th there were wind advisories for the Northeast, gale warnings for the East Coast and all of the Great Lakes, and winter weather advisories for the upper Great Lakes.
On May 24th, Salt Lake City had their latest measurable (0.2") snowfall on record, beating the old record by 6 days, and a record low of 33F.

In addition, up to 20 inches of snow fell in Vermont in late April and snow continued into mid-May in Colorado, where the snow was reported to be pink and red on April 29th because of a dust storm in northeast Arizona the previous day.

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.