Observed Characteristics of the Tornadic Supercells of 27–28 April 2011 in the Southeast United States

• Published in: Monthly weather review Vol. 150; no. 11; pp. 2883 - 2910
• Main Authors: Lyza, Anthony W., Flournoy, Matthew D., Rasmussen, Erik N.
• Format: Journal Article
• Language: English
• Published: Washington American Meteorological Society 01.11.2022
• Subjects: Convection; Documentation; Documents; Evolution; Fatalities; Life cycle; Life cycles; Mesoscale phenomena; Outbreaks; Quality control; Radar; Remote sensing; Shear; Storms; Supercell thunderstorms; Supercells; Thunderstorms; Tornadoes; Mississippi; Georgia; Alabama; United States > US; Tennessee
• ISSN: 0027-0644; 1520-0493
• DOI: 10.1175/MWR-D-21-0274.1

Abstract An historic outbreak of tornadoes impacted a large swath of the eastern United States on 26–28 April 2011. The most severe series of tornadoes was associated with numerous classic supercell thunderstorms that developed across the Southeast during the afternoon and evening of 27 April and continued into the predawn of 28 April. This study documents characteristics of these storms with respect to tornado production and mesocyclone strength during different periods of each storm’s life cycle. The supercells initiated in four quasi-distinct spatiotemporal regions, with each cluster exhibiting slightly different evolutionary traits and tornado production. These included differences in the mean times between convection initiation and the time of first tornadogenesis for each supercell, as well as variations in overall and significant tornado production. This suggests that mesoscale environmental differences, such as proximity to a mesoscale boundary, and/or storm-scale events strongly influenced the variety of supercell evolutionary paths that were observed during this event, even in the presence of a synoptic-scale background environment extremely favorable for supercell and tornado production. The azimuthal shear products from the Multi-Year Reanalysis of Remotely Sensed Storms database perform well in discriminating between mesocyclones associated with ongoing weak, strong, and violent tornadoes during the event. Furthermore, mean azimuthal shear values during pre-tornadic (e.g., within 30 min of tornadogenesis) and tornadic phases are significantly larger than those during nontornadic phases. This warrants further study of azimuthal shear characteristics in different environments and its potential usefulness in aiding real-time forecasting efforts. Significance Statement This study documents the prolific supercell tornado outbreak that occurred in the southeastern United States on 27–28 April 2011. We associate tornado families with their parent supercells and use a radar-derived database to quantify changes in mesocyclone strength. We show that a variety of supercell evolutionary paths occurred during the event that were somewhat distinct based on where and when each supercell initiated. We also find significant differences between supercell intensity, characterized using azimuthal shear as a measure of mesocyclone strength, during nontornadic periods as opposed to the 30-min window prior to tornadogenesis. These findings are relevant for both researchers and operational forecasters and motivate future work to better understand relationships and processes influencing supercells and their background environments.