Short-Term Prediction of QLCS Mesovortices in the Southeast United States on 30 April 2017

A tornadic quasi-linear convective system (QLCS) occurred in a high shear, low convective available potential energy (CAPE) environment in the southeast United States on 30 April 2017. In this event, several long-lived tornadic mesovortices embedded in the QLCS moved east across the Mississippi valley and resulted in 45 tornadoes, six of which were significant (EF2), in Arkansas, Louisiana, Mississippi, and Alabama. The tornadic QLCS occurred in an environment marked by steep low-level lapse rates and 0-1 km vertical wind shear values over 35 knots based on observed soundings in the pre-storm environment at 1200 UTC 30 April. While the threat for severe thunderstorms in the region was anticipated a week prior, the possibility of a tornado outbreak was not anticipated until the event began to unfold. Given the difficulty in predicting the development of tornadic mesovortices in QLCSs in the southeast United States, the aim of this presentation is to examine the ability of short-term (0-12 h) convection-allowing numerical weather prediction forecasts from the high-resolution rapid refresh (HRRR) and National Severe Storms Laboratory Warn-on-Forecast (WoF) systems to predict long-lived mesovortices for the 30 April 2017 case.

The 0-h HRRR analyses showed 0-1 km vertical wind shear values in the pre-storm environment that were consistent with observed soundings at 1200 UTC 30 April. In closer proximity to the QLCS, however, the vertical wind shear increased to over 50 knots with the vector shear oriented almost perpendicular to the QLCS. The heterogeneity in the pre-storm environment highlights how the environment can become enhanced immediately ahead of active convection, where convection seemingly enhances its own inflow as also shown in previous works. Inspection of the HRRR forecasts suggests that there was some difficulty in accurately predicting the low-level mesovortices and attendant environment ahead of the QLCS. The 6- and 12-h forecasts verifying at 1200 UTC showed a more linear convective mode compared to the observed system with an absence of coherent long-lived mesovortices. The forecasts also had a pre-storm environment with less favorable conditions for the development of mesovortices, with lower 0-1 km vertical wind shear magnitudes and the vector shear oriented more parallel to the line (lower storm relative helicity). In the 1-h HRRR forecast, the QLCS has structure (complete with low-level mesovortices) and a pre-storm environment more consistent with observations. Emerging results from forecasts from the WoF system and analysis of the key model physical processes that are governing the accurate prediction of the pre-storm environment and convective mode will be discussed in detail.