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.
Shortly before 5:30 PM on April 14, 2011 a thunderstorm that developed over Lincoln County, OK took on supercell characteristics and moved into the Tulsa metro area. The SPC Day 1 Outlook had a large part of OK in a Moderate Risk area. Specifics on the synoptic scale conditions and storm reports can be found here.
As the supercell moved closer to the Tulsa metro, it began to indicate that there was sufficient rotation within a sustained mesocyclone to warrant a Tornado Warning which included part of the Tulsa metro. The Tulsa NWS doppler radar (KINX) indicated a possible tornado near Kellyville, OK moving to the northeast.
The first image shows the supercell soon after the first tornado warning was issued. At that time, the center of rotation was located between Kellyville and Sapulpa, OK. A very distinct inflow notch can be seen along with a short flanking line and the main heavy rain/hail precipitation core to the northeast.
In the second image, the center of rotation has moved east of Hwy 75 and was moving almost due east along the 71st street corridor. The structure was still maintaining the “hook echo” structure and supercell characteristics. Many eyewitnesses to the storm claimed they saw a distinct wall cloud with rotation. In an unusual safety measure, radio station KRMG abandoned their broadcast station and offices for shelter in a basement since the wall cloud was passing over the 71st and Yale intersection. Meteorologists at the Tulsa NWS also began to notice that scans from the high resolution Tulsa Terminal doppler radar (TTUL) showed a decrease in storm inflow. This decrease likely was at the lower levels of the mesocyclone. Rotation in the mid and upper levels warranted further Tornado Warnings as the storm moved northeast into Rogers County, OK.
The conditions for tornadic potential were higher in southeastern OK. It was during this outbreak that a large supercell with a long-track tornado did considerable damage to the small town of Tushka, OK resulting in two fatalities.
The final image is the Storm Relative Velocity (SRV) image of the supercell that produced the Tushka, OK tornado as the storm was moving over LeFlore County, OK at approximately 9:27 PM. A very distinctive couplet can be seen southwest of Poteau, OK. The greens indicate air moving toward the Fort Smith, AR doppler radar site (KSRX) and reds show air moving away from the radar. SRV 0.5 level data indicated +84 kts and -82 kts respectively. At this time, a confirmed tornado was moving through LeFlore County. Conditions for long-track tornadoes and violent supercells existed in this portion of the state and indeed the strongest storms occurred in southeastern OK and southwest AR during this event. This days severe weather was one of many very prolific severe weather days that occurred during the record breaking month of April, 2011.