The Role of an Upper-level Potential Vorticity Anomaly in a Severe Weather Outbreak During MPEX
Corey T. Guastini and Lance F. Bosart
Department of Atmospheric and Environmental Sciences
The University at Albany/SUNY
1400 Washington Avenue
Albany, NY 12222
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 tracked from Indiana to the New Jersey coast from 0000 to 1500 UTC 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. Analysis shows enhanced westerly mid-level winds on the PV anomaly s equatorward side advected an elevated mixed layer eastward and southerly flow ahead of a surface cyclone induced by the PV anomaly advected moist air northward from the Gulf of Mexico. The combination of the elevated mixed layer and low-level moisture created large instability in the surface cyclone s warm sector, and isentropic lift along the surface cyclone s warm front initiated severe convection in northern Illinois around 2100 UTC 12 June 2013. The convection was initially discrete in nature but later organized into a bowing progressive derecho.
The described synoptic set-up was more dynamic than the typically benign warm-season progressive derecho pattern described in the literature. A May August, 1996 2013 progressive derecho climatology along with subjectively identified progressive derecho synoptic regimes will be presented to illustrate the spectrum of synoptic environments in which warm-season progressive derechos may form. Particular emphasis will be placed on where the 12 13 June 2013 progressive derecho falls within this spectrum.