Environmental Nuances and Convective Morphology During the 30 April 2017 Tornado Outbreak in the Southeastern United States

A tornadic quasi-linear convective system (QLCS) impacted the southeastern United States during the morning of 30 April 2017. This particular system was part of a multiday sequence of severe weather across the southern tier of the United States that occurred in conjunction with a Rossby wave train (RWT). The RWT originated in the western North Pacific basin on 26 April and contributed to the amplification of the upper-level flow pattern and the development of a cutoff low over the southern Great Plains on 30 April. On 29 April, several bands of convection developed to the east of the amplifying upper-level trough and moved eastward, eventually evolving into a highly complex QLCS. The southern portion of this QLCS moved through Louisiana and Mississippi during the following morning and produced 45 tornadoes, six of which reached EF2 intensity.

Owing to the strength of the synoptic forcing during this period, the potential for a severe weather event over the Southeast was identified by forecasters up to 7 days in advance. However, uncertainties in the mesoscale environment and convective mode made it difficult to anticipate whether or not a significant tornado threat would accompany this event. Inspection of global model ensemble forecasts at various lead times revealed noticeable differences in the amplitude of the RWT, development of a cutoff upper-level low, strength of the attendant surface low, and evolution of the warm sector during the event. Such differences in the synoptic environment ultimately altered the magnitude and distribution of instability, strength and orientation of the vertical wind shear, and the formation and evolution of mesoscale disturbances that affected the development, morphological evolution, and severity of convection in the Southeast. We hypothesize that the synoptic and mesoscale environment on 30 April was influenced by upscale modifications driven by widespread convection in the southern Great Plains that occurred throughout the previous day. This case highlights the difficulty in forecasting the evolution of a severe weather outbreak occurring within an extended period of active convection owing to the role that complex multiscale interactions have on governing the convective mode and associated hazards.