Synoptic-scale conditions associated with the 31 May 1998 Derecho

On 30 and 31 May 1998, a derecho-producing mesoscale convective system (DMCS) raced across the upper Midwest and Great Lakes inflicting 287 million dollars in damage and killing five people. This study addresses the synoptic-scale conditions that led to the initiation and subsequent evolution of this warm season derecho. Upper-air and surface analyses for a 36-hour period have been constructed to define the large-scale features over North America that preceded convective initiation and transpired during storm evolution. Additionally, satellite and radar imagery has been acquired to further detail the track of the convective system. Analyses suggest that synoptic-scale features were important in the evolution of this DMCS.

Isobaric charts were assembled to delineate synoptic-scale influences on the DMCS. This study stresses the importance of alternative isobaric maps to decipher system development and motion, including: 700-500 hPa layer-averaged wind flow, 700 hPa thermal advection, 700-500 hPa layer-averaged differential vorticity advection, and 700 hPa omega.

Beyond isobaric analyses, isentropic charts were constructed to achieve an understanding of the pre-convective storm environment. This investigation indicates there are a multitude of applications for isentropic analyses in forecasting convective initiation. First, isentropic analysis may be used to identify the connection between the low-level jet (LLJ) and the upper-level jet (ULJ). It is hypothesized that in this case the LLJ developed as a sloped response to the upper level divergence associated with an ULJ. Second, isentropic charts may be used to identify horizontal and vertical transport of high theta-e air above an east-west quasi-stationary front. Analyses indicate that these thermal and moisture transports were key in convective initiation and the development of the system. Third, the arrangement of moisture stability fluxes and pressure advections on isentropic surfaces were used to locate convergence of moist, unstable air and regions of strong upward motion.

This study emphasizes the functionality of alternative techniques in the analysis of synoptically benign (i.e. weakly baroclinicic environment, minimal short wave interaction) environments to understand DMCS initiation and evolution.