A Survey of Thunderstorm and Tornado Occurrence in High-End Environments

A high risk for severe thunderstorms was issued by the Storm Prediction Center on 6 May 2024 in central Oklahoma and south-central Kansas with the anticipation of a tornado outbreak in the region. The environment parameter space was high-end as shown on the 0000 UTC 7 May sounding at Norman, Oklahoma, with surface-based convective available potential energy near 4500 J kg^-1, storm-relativity helicity in the 0–1 km layer over 300 m² s^-2, and vertical wind shear in the 0–6 km layer near 30 m s^-1. These explosive environment conditions were represented by fixed-layer significant tornado parameter (STP) values over 8, which brings with it the expectations of several long-track, violent tornadoes. In all, 15 tornadoes were documented across Oklahoma by the National Weather Service (NWS), with 5 tornadoes identified by NWS Tulsa in eastern Oklahoma and 10 by NWS Norman in central Oklahoma. For the latter, 5 supercell storms moved into central Oklahoma in the late afternoon into an environment characterized by a large patch of STP over 8. Yet, these supercells produced only weak tornadoes, with 5 rated as EF1 on the Enhanced Fujita Scale, 2 rated EF0, and 3 rated EFU. The lack of significant (EF2-3) and violent tornadoes (EF4-5) in central Oklahoma, despite extreme STP values, motivate the development of a climatology of STP “patches” to address the following science questions: 1) what percentage of high STP environments fail to produce EF2+ tornadoes? 2) what are the statistically significant differences between high STP environments that produce EF2+ tornadoes and those that do not? 3) Are there repeatable high STP “failure” modes?

Composite mean model analyses from the Warn-on-Forecast System (WoFS) convection-allowing ensemble are used to identify STP mesoscale “patches” during 2017–2023. Patches are identified by areas where STP exceeds a threshold over a >=300 km² region. Several STP thresholds (1, 3, 5, and 10) are tested. An automated tracking algorithm is used to identify linear and cellular convection objects that move through or along the immediate periphery of the STP patch. Tornado reports are then assigned to convection objects. Preliminary results from examination of 4627 hourly WoFS analyses available, mesoscale patches that exceeded an STP of 10 (similar to what occurred on 6 May 2024) occurred in only 2% (91) of all the analyses. A survey of thunderstorm and tornado occurrence relative to STP patches and comparison of STP patches that produced EF2+ tornadoes with those that do not will be presented.