P1.15 DISCRIMINATING AMONG NON SEVERE, SEVERE, AND DERECHO-PRODUCING MESOSCALE CONVECTIVE SYSTEM ENVIRONMENTS Ariel E. Cohen* 1 , Michael C. Coniglio 2 , Stephen F. Corfidi 3 , and Sarah J. Corfidi 3 1 Atmospheric Sciences, The Ohio State University, Columbus, OH 2 NOAA/National Severe Storms Laboratory, Norman, OK 3 NOAA/NWS/NCEP/Storm Prediction Center, Norman, OK 1. Introduction Organized clusters of thunderstorms meeting particular spatial and temporal requirements are known as mesoscale convective systems (MCSs) (e.g. Zipser 1982; Hilgendorf and Johnson 1998; Parker and Johnson 2000). Knowledge of the environmental parameters that govern MCS intensity is essential in operational meteorology. This is especially true of convective systems that produce widespread damaging surface winds. Herein, we examine a subset of organized, long-lived systems of this type referred to as derecho-producing convective systems, or DCSs. One of the first detailed examinations of DCSs was Johns and Hirt (1987). This work was based on a data set of 70 MCSs occurring during the warm season (May-August) of the years 1980-1983. The study discussed the relationship between DCS position, motion, synoptic scale boundaries, and environmental parameters. They found that large convective instability, and the presence of dry air at mid levels above moist air in the low levels, were characteristics common to many DCS environments. The authors inferred that the dry- over-moist moisture profiles allowed for the development of large negative buoyancy in the lower levels that fostered development of strong downdrafts and severe surface winds. Johns and Hirt (1987) also suggested that relatively strong mean mid- and upper-level wind speeds were associated with DCSs. Evans and Doswell (2001), meanwhile, suggested that strong system-relative winds in the low-levels and weak system-relative winds at mid-levels were important to DCS development. * - Corresponding author address: Ariel E. Cohen, The Ohio State Univ., Depart. of Geography – Atmospheric Sciences, Columbus, OH 43210-1361; e-mail: cohen.274@osu.edu Additionally, they emphasized that the convective available potential energy (CAPE) and vertical wind shear vary widely among derecho events. Recently, Coniglio et al. (2004) showed that the shear often extends through a large depth and weakens as derechos decay. To build on this work, we present a study on the variables that discriminate among non–severe MCSs, severe but non derecho-producing MCSs, and DCS environments. Accordingly, the purpose of the present work is to examine the differences in meteorological variables derived from proximity soundings among three categories of MCS intensity and to discuss the physical implications of these results. Section 2 describes the data set of MCSs considered in this study, the scheme used to rate the MCSs in the data set, and the statistical analyses applied to the data set. Sections 3, 4, and 5 describe the use of the kinematic, instability, and moisture variables, respectively, used in the MCS environment discrimination. Results are summarized in section 6. 2. MCS Data Set, MCS Intensity Rating Scheme, and Statistical Analyses Using archived radar images provided by the University Corporation for Atmospheric Research (UCAR) and the Storm Prediction Center (SPC) (available online at http://locust.mmm.ucar.edu/case-selection/ and http://www.spc.noaa.gov/exper/archive/events), 269 MCSs were identified for this study that had an associated proximity sounding from upper air observations (see Coniglio et al. 2005 for a description of this data set). Each MCS exhibited a contiguous line of leading convection at least 100 km long for at least five continuous hours. These MCSs occurred east of the Rocky Mountains between May and early September from 1998 through 2004. The MCSs were selected if the nearest part of the 50 dBZ radar reflectivity contour of the MCS was no more than 200 km and three hours