A Numerical Investigation into the Presence of Veer-Back-Veer Vertical Wind Profiles in Supercell Environments: A Case Study of 30 and 31 May 2013

With recent advances in research, forecasters are becoming increasingly adept at predicting severe weather outbreaks, sometimes days in advance. However, while forecasts have become more accurate in predicting storm mode, such as linear or supercellular convection, discriminating between tornadic and nontornadic supercell environments continues to present a challenge. This research focuses on two severe weather events from May of 2013 in which one of the main forecast challenges was determining whether the environments were supportive of strong tornadoes.

Both 30 and 31 May 2013 featured remarkably similar synoptic and thermodynamic environments. Large instability, abundant low-level moisture, the presence of a dryline and triple point, strong deep-layer vertical wind shear, and strong low-level shear, on both days favored robust isolated convection. On 30 May, despite the occurrence of multiple supercells across central Oklahoma, no violent tornadoes were reported, whereas on 31 May, the El Reno, OK, supercell and subsequent 4.2-km wide tornado took place. Since similar environments were sampled on both days, the fact that a violent tornado occurred on one of the days, and not the other, may be a bit surprising. Examination of the vertical wind profiles reveals that a veer-back-veer (VBV) vertical wind profile was present in the middle troposphere on 30 May, but not on 31 May.

Numerical simulations conducted with the Bryan Cloud Model (CM1) suggest that the VBV wind profile at least partially inhibited the formation of violent tornadoes on 30 May. Simulations initialized with observed soundings from 30 May produced convection, but these simulated storms struggled to persist. With the 31 May environment, a much different solution unfolded with two significant storms being produced, one of which was a long-lived right-moving supercell. A simulation with the 30 May VBV vertical wind profile superposed with the 31 May thermodynamic environment produced an intriguing result; supercells initiated in the model, but quickly dissipated. The presence of the VBV vertical wind profile is thus hypothesized to be the main environmental limiting factor as to why violent tornadoes were not observed across central Oklahoma on 30 May. A conceptual model of the VBV vertical wind profile, and its effect on supercell dynamics, will be presented to hopefully improve convective forecasts in the future when this wind profile is observed or predicted.