Mesoscale Processes Influencing Convective Morphology during the 26–27 April 2011 Tornado Outbreak

The 2011 “Super Outbreak” is most notoriously recognized for the sheer number of long-track supercells and associated violent tornadoes that impacted the Southeastern U.S. during the afternoon on 27 April 2011. However, the afternoon supercell outbreak was preceded by two tornadic quasi-linear convective systems (QLCSs), the first of which (hereafter QLCS1) impacted the Southeast during the early morning and alone produced more than 100 tornadoes of up to EF-3 intensity throughout its lifetime. The organization and evolution of this overnight QLCS were highly complex, and several of the tornadoes were associated with long-lived mesovortices embedded within the line. QLCS1 was succeeded by a second, primarily elevated and weakly tornadic QLCS (hereafter QLCS2) that moved through the Southeast during the late morning. QLCS2 also exhibited an incredibly complex evolution and evolved from a coherent, organized system into a disorganized conglomeration of convective bands and clusters. Concurrent with the disorganization of the QLCS2, multiple bands of discrete supercells erupted within the warm sector (i.e., away from any discernible surface boundaries), culminating in one of the most prolific tornado outbreaks ever documented.

This focus of this presentation will be on the role that various mesoscale processes had on the development, organization, and evolution of convection during the 26-27 April 2011 outbreak. This particular outbreak occurred in association with a highly amplified synoptic pattern and an anomalously strong baroclinic system that slowly moved eastward over the Rocky Mountains. Throughout the course of the event, several disturbances (e.g., potential vorticity anomalies, jet streaks, mesoscale gravity waves, elevated baroclinic zones) moved into the lee of the Rockies, which supported episodic convective development and interacted with ongoing convection. Additionally, the progressive disorganization of QLCS2 and onset of the supercell outbreak occurred following a period of pronounced upper-level frontogenesis over the southern Great Plains and the subsequent movement of a deep tropopause fold into the Southeast. The hypothesized significance of this process on the evolution of the warm sector supercell outbreak and some considerations for improving the depiction of warm sector convective development in NWP models will be emphasized.