The Role of an Upper-level Potential Vorticity Anomaly in a Severe Weather Outbreak During MPEX

The Mesoscale Predictability Experiment (MPEX) took place from 15 May to 15 June 2013 with the intention of sampling mid- and upper-level disturbances crossing the complex terrain of the Intermountain West with a high density of dropsondes in order to improve the accuracy of subsequent runs of convection allowing numerical weather prediction models. A potent potential vorticity (PV) anomaly that originated over the eastern Pacific and crossed the Rocky Mountains was sampled during MPEX by the project G-V dropsonde aircraft on two occasions: once on the morning of 11 June 2013 as it began to cross the Rocky Mountains in Utah, and a second time on the morning of 12 June 2013 as it ejected into the Northern Plains and began circumnavigating a continental anticyclone centered over the Oklahoma panhandle. This potent PV anomaly aided in initiating convection that produced numerous tornado and hail reports across Iowa, Illinois, Indiana, and Ohio as well as a low-end derecho that trekked across Indiana and northern Ohio on the evening of 12 June and into the early morning hours of 13 June 2013.

The purpose of this presentation is to characterize the influence of the aforementioned potent PV anomaly on the 12-13 June 2013 severe weather outbreak with the aid of high-density dropsonde observations provided by MPEX. A preliminary analysis shows enhanced upper-level winds on both the PV anomaly's poleward and equatorward sides produced a coupled jet structure, which created enhanced upper-level divergence and allowed for modest cyclogenesis on 12 June 2013. Enhanced vertical motion associated with the resulting surface cyclone in conjunction with favorable dynamic and thermodynamic parameters allowed convection to flourish along the cyclone's warm front. The convection was initially discrete in nature but later organized into a bowing progressive derecho with a well-defined bookend vortex on its poleward side. Particular attention will be paid to the vertical structure of the atmosphere in and around the PV anomaly leading up to convective initiation. A companion presentation by Bosart and Guastini (separate abstract) will document the life of the aforementioned PV anomaly from its incipient stages as a PV streamer over the Pacific to its interaction with the Rocky Mountains as a closed vortex.