Relationship of convection initiation and subsequent storm strength to ensemble simulated environmental conditions during IOP3b of VORTEX Southeast 2017

A 50-member convection-allowing ensemble was used to examine environmental factors influencing afternoon convection initiation (CI) and subsequent severe weather on 5 April 2017 during intensive observing period (TOP) 3b of the Verification of the Origins of Rotation in Tornadoes Experiment in the Southeast (VORTEX-SE). This case produced several weak tornadoes (rated EF1 or less), and numerous reports of significant hail (diameter >= 2 in.; >= similar to 5 cm), ahead of an eastward-moving surface cold front over eastern Alabama and southern Tennessee.

Both observed and simulated CI was facilitated by mesoscale lower-tropospheric ascent maximized several tens of kilometers ahead of the cold-frontal position, and the simulated mesoscale ascent was linked to surface frontogenesis in the ensemble mean. Simulated maximum 2-5 km AGL updraft helicity (UHmax) was used as a proxy for severe-weather-producing mesocy-clones, and considerable variability in UHmax occurred among the ensemble members.