13 May 2010 Severe Bow Eecho Event Over Northeast Oklahoma. Part 2: Characteristics of Reflectivity Structures and Mesovortex Evolution

Previous studies have shown that mesovortices formed within quasi-linear convective systems (QLCSs), including bow echoes, are often the locations of damaging winds and tornadoes. Most of the previous studies on QLCS mesovortices have focused on events that occurred during the late-afternoon and early-evening hours. In this study, the evolution of tornadic and non-tornadic mesovortices formed within a bow echo during the early-morning hours will be presented.

Because of their small scale and shallow depth, detecting mesovortices can be a significant challenge, especially at long range from a radar and within systems that are rapidly moving. As a result, forecasters often rely on radar reflectivity patterns to help identify possible regions of tornadogenesis within a QLCS. For example, previous studies of bow echoes observed over the middle Mississippi Valley and lower Ohio Valley regions have shown that tornadic mesovortices often form at the intersection of a preexisting boundary with a QLCS.

Detailed radar and damage survey analyses of a severe bow echo that moved across northeast Oklahoma during the early morning of 13 May 2010 will be presented. The QLCS consisted of a large bow echo from north-central to central sections of Oklahoma while a smaller bow echo moved across south-central Oklahoma. A mesoscale rear-inflow jet (RIJ) was clearly evident within the developing larger bow echo between 0900 and 0930 UTC. From 0930 to 1000 UTC, a well-defined weak reflectivity transition zone and enhanced stratiform rain region rapidly formed. Ground-relative radial velocities in excess of 45 ms-1 were observed within the RIJ at this time. A surface boundary generated by earlier, weaker convection extended southwestward from southwest Missouri and intersected the larger bow echo north of the apex, between Tulsa and Oklahoma City at 0930 UTC. Damaging mesovortices formed along the leading edge of the larger bow echo from the apex northward to the intersection with the pre-existing outflow boundary. A total of twelve tornadoes occurred with this event, five of which caused EF2 damage. Some of the strongest tornadic mesovortices occurred near the intersection of the larger bow echo and preexisting boundary. One of the stronger mesovortices in this vicinity spawned a series of short-lived tornadoes throughout its lifetime and caused damage rated EF0 to EF2. This mesovortex moved along and south of Interstate 44 and damaged a number of residential homes and other buildings. Additional mesovortices that formed south of the intersection point and north of the apex initially showed weaker rotational velocities but strengthened during the latter stages of their lifetimes, also spawning tornadoes. Some of the most intense damage extended from Sapulpa east-northeast to south of Claremore and south to 6 km west of Chouteau.

Time-height analyses of mesovortex rotational velocities generated from the Tulsa, Oklahoma WSR-88D (KINX) and the Tulsa Terminal Doppler Weather Radar (TTUL) will be presented to highlight differences in vortex evolution as compared to late-afternoon and early-evening mesovortices previously documented in the literature. Tornadic mesovortices within the 13 May 2010 event did not deepen rapidly prior to tornadogenesis, however they were long-lived. Previous studies have shown that tornadic mesovortices often deepen rapidly just before tornadogenesis and are longer lived relative to non-tornadic mesovortices. Some of the challenges in warning operations when dealing with rapidly moving, shallow, long-lived, and small-scale mesovortices will be discussed.