Scale-Interactive Processes in the Evolution of Multiepisode Tornado Outbreaks in the Southeastern U.S.

Previous studies have found that the culmination of a prolific tornado outbreak often results from a complex sequence of multiscale process interactions. This presentation discusses several of the multiscale processes that contributed to the evolution of the 27 April 2011 tornado outbreak and contrasts this event with other major historical outbreaks in the Southeastern U.S. The April 27^th event was part of a longer outbreak that occurred in advance of a slow-moving upper-level trough that became increasingly amplified with time. Moreover, this event comprised three separate episodes, which all exhibited different convective modes and together produced 199 tornadoes during a 24-h period. The first episode resulted from afternoon convection that grew upscale into an extensive quasi-linear convective system (QLCS), which consisted of several embedded mesoscale vortices that produced numerous tornadoes of up to EF3 intensity during the early morning. A second weakly-tornadic QLCS moved throughout northern Mississippi and Alabama during the late morning, which was followed to the south by multiple parallel lines of tornadic supercells that developed during the afternoon. All three of these convective episodes were located predominantly within the “warm sector” (i.e., at and on the warm side of synoptic fronts), as is often observed during synoptically-active tornadic events in the Southeast.

Despite the fact that organized severe convection is often observed in the warm sector during these outbreaks, the specific mesoscale processes governing its development are not well understood and may be highly case-dependent. An improved physical understanding of the processes leading to the manifestation of these warm sector lines and how these processes are represented by NWP is essential to improving forecasts of convection initiation, morphology, and severity within this region. However, this is complicated by the fact that outbreaks within the Southeast tend to culminate following multiple days of active convection, which can dramatically alter the large-scale environment through upscale interactions. It has been postulated that the latter portions of multiday tornadic periods tend to be more severe owing to these upscale feedbacks. On longer time-scales, upscale dynamical adjustments can promote the development or intensification of upper-level and low-level jet streaks and lead to the amplification of the upper-air pattern owing to the enhancement of downstream ridging. Such large-scale adjustments act to modify the thermodynamic and shear profiles within the environment and may influence the movement, location, and strength of frontal boundaries. On shorter time-scales, convection can lead to the development of unbalanced mesoscale circulations, gravity waves, and low-level thermal boundaries, which then may interact with ongoing convection or influence the development and severity of subsequent convection. Because the multitude of scale-interactive processes that might arise during multi-episode events depends upon the nature of the convection by which they were generated, forecasts of these phenomena are expected to be highly sensitive to the behavior of various model parameterization schemes and would thus be difficult to properly represent. This presentation will discuss the sensitivity of the scale-interactive processes during the April 27^th outbreak to convectively-induced latent heating and to the influence of different microphysics and planetary boundary layer schemes.