During the afternoon and evening of 22 April 2011, a number of severe weather events occurred across OK to KY. An overview of the days events from the Storm Prediction Center can be found here.
One supercell that produced a tornado in Garvin county was not exhibiting the typical supercell structure on radar. In spite of its structure, it did display a pronounced hook echo.
The Base Reflectivity image show above clearly indicates a hook echo on the McClain and Garvin county line. The polygon indicates the area covered by the Tornado Warning issued by the Norman, OK National Weather Service.
The Storm Relative Velocity image taken at the same time shows a distinct couplet in the same location as the reflectivity image hook echo.
At 0104 UTC, the center of rotation has moved slightly to the southeast and is now just south of the McClain county border. The reflectivity image indicates the supercell may have begun to wrap precipitation around the mesocyclone at the lower levels.
The Storm Relative Velocity image at 0104 UTC indicates the mesocyclone has maintained its previous intensity. Also evident is outflow and/or a strong Rear Flank Downdraft (RFD) running from the southern part of the couplet to near Pauls Valley, OK.
By 0113 UTC the hook echo has become even more pronounced as the storm begins a slow drift to the southeast into the inflow. As displayed, the new Tornado Warning now covers an area to the southeast of the hook echo. The SPC Storm Data does have a record of a tornado in this area but it’s not known if any damage occurred.