An upper level low was located over South Dakota with winds of 45 kts over Norman, OK (per the 12z (7am CDT) analysis). At the surface, a stationary front was located to the west of Norman during the same time period. Temperatures were already in the lower 70’s with dew points in the middle 60’s. By 18z (1pm CDT), the surface temperatures were in the 80’s with dew points in the 70’s. With the combination of a dryline, a stationary front, CAPE values up to 5000 J/kg and wind shear made the atmosphere ripe for tornadoes. This atmosphere produced the Moore, OK EF-5 tornado.
For our fellow weather geeks, below is the 12z (7am CDT) observed sounding for OUN (Norman, OK):