Environment and early evolution of the 8 May 2009 derecho-producing convective system

This study documents the complex environment and early evolution of the remarkable derecho that traversed portions of the central United States on 8 May 2009. Central to this study is the comparison of the 8 May 2009 derecho environment to that of other mesoscale convective systems (MCSs) that occurred in the central U.S. during a similar time of year. Synoptic-scale forcing was weak and thermodynamic instability was limited during the development of the initial convection. Nonetheless, several mesoscale features of the environment appeared to contribute to initiation, including a mountain wave, a mid-level jet streak, a weak mid-level vorticity maximum, a “Denver cyclone”, and a region of upper-tropospheric inertial instability.

The subsequent MCS developed from several convective elements that formed along mesoscale features of the environment. An unusually strong and deep low-level jet (LLJ) transported exceptionally high amounts of low-level moisture northward very rapidly, destabilized the lower troposphere and generated frontogenetical circulations that appeared to aid convective development. The thermodynamic environment ahead of the developing MCS contained unusually high precipitable water (PW) and very large mid-tropospheric lapse rates. Values for some environmental parameters that are often associated with strong MCSs and severe surface winds, like downdraft convective available potential energy (DCAPE), mean winds, and 0 – 6 km vertical wind shear, were not as anomalously large as the PW and lapse rates. In fact, the DCAPE was lower than what is typically found in the environment of developing MCSs. The importance of each of these factors in the development of the MCS is discussed.