Exploring the Operational Utility of Entraining CAPE in Supercell Tornado Forecasting

The intensity of updrafts and downdrafts in thunderstorms is particularly influenced by the mixing of dry air within the ambient environment into the storm’s cloud core through a process known as entrainment. This dilution reduces the potential buoyancy available to the storm and thus is directly related to vertical velocity, which itself can influence storm severe hazards and is related to many parameterizations in modeling schemes. The deleterious effect of entrainment depends on both environmental and storm characteristics, which are of course intertwined. Peters et al. (2020) and Peters et al. (2023) introduced analytical formulas to relate these internal and external characteristics of entrainment with the maximum updraft speed in thunderstorms, measured in terms of convective available potential energy (CAPE). These studies provided robust evidence that entraining CAPE (ECAPE) can better predict storm intensity than the more traditionally used undiluted CAPE.

This study picks up on the previous theoretical research on ECAPE by pursuing a logical next step, assessing the operational utility of ECAPE in severe supercell thunderstorm forecasting. Using a large dataset of nontornadic and tornadic supercell thunderstorms curated by the NOAA/NWS Storm Prediction Center, we will compare the forecast skill of undiluted CAPE versus ECAPE individually and as a component of the significant tornado parameter (STP). In particular, these comparisons will focus on fringe and marginal storm environments, such as high shear-low CAPE (HSLC), which is a part of the parameter space that has particularly low forecast skill and a higher incidence of false alarms and missed events. Results thus far indicate that the use of ECAPE in the STP yields a small improvement in discriminating between nontornadic and tornadic supercells across the whole dataset, with most of the gains occurring in fringe environments as hypothesized. In HSLC regimes, the maximum vertical velocity when accounting for entrainment can sometimes exceed the undiluted CAPE. This somewhat counterintuitive result might explain a HSLC storm’s ability to produce severe weather in what could otherwise be considered unfavorable conditions. In total, the use of ECAPE has the potential to be a fruitful real-time forecasting tool but requires further scrutiny before transitioning from research to operations.