P 4.72 A PRELIMINARY INVESTIGATION OF DERECHO-PRODUCING MCSs IN ENVIRONMENTS OF VERY LOW DEWPOINTS Stephen F. Corfidi* 1 , David A. Imy 1 , Sarah J. Taylor 1 and Allen Logan 2 1 NOAA/NWS/NCEP/Storm Prediction Center, Norman, OK 2 University of Missouri, Columbia, MO 1. INTRODUCTION Because of the damaging winds and other severe weather threats with which they are associated, derecho-producing mesoscale convective systems (MCSs) pose an important challenge to forecasters (Wakimoto 2001). In late spring and summer, derecho MCSs typically occur in environments of substantial convective instability, with very moist boundary layer inflow (Johns and Hirt 1987). During the cool season (October through April), when derechos are more commonly associated with amplifying disturbances in the westerlies, they occasionally occur in environments of only modest convective instability (e.g., Wolf 1998, Evans and Doswell 2001, Burke and Schultz 2004). Nevertheless, lower tropospheric moisture content in such situations is typically above seasonal norms. More rarely, derecho-producing MCSs occur in environments of very limited moisture (surface dewpoints at or below 10 C and/or mean precipitable water at or below 1.25 cm) and correspondingly low convective available potential energy (CAPE). Such systems have been observed throughout the year and over much of the continental United States. Because low dewpoint derechos (LDDs) develop in environments not commonly associated with widespread severe convective weather, these events sometimes catch forecasters by surprise (e.g., Fenelon 1998, Corfidi 2003). This paper examines the synoptic and mesoscale environment associated with eight LDDs that have been identified over the continental United States since the mid 1970s. Emphasis is placed on those factors that appear to be most strongly associated with LDD initiation and sustenance in an attempt to better anticipate these uncommon events. 2. METHODOLOGY The cases studied were selected on the basis of data availability and knowledge of the event by the authors (Table 1). A more exhaustive search will be performed at a future date to identify other LDDs undoubtedly present in the Storm Prediction Center ___________________________________________ * Corresponding author address: Stephen F. Corfidi, NOAA/NWS/NCEP/Storm Prediction Center, 1313 Halley Ave, Norman, OK 73069; email: stephen.corfidi@noaa.gov Figure 1. Paths taken by LDD events studied. Numbers refer to cases listed in Table 1. (SPC) severe weather database. Care was taken to eliminate any cases for which the convection did not appear to be surface-based. The surface dewpoint criterion used --- aerially averaged values at or below 10 C --- excluded cases involving strongly forced convective bands occurring in environments of intense low level shear and nearly moist adiabatic thermodynamic profiles along cold fronts associated with cool season extratropical cyclones. In accordance with Johns and Hirt (1987) and Coniglio et al. (2004), each LDD produced a continuous swath of non-random, convective wind damage and/or measured convective gusts in excess of severe limits (≥ 58 kts (26 ms -1 )). The path length criterion, however, was reduced to 200 km to include a sufficient number of cases to create meaningful composites. The data set includes both warm and cool season LDDs that affected wide-ranging parts of the country (Figure 1 and Table 1). The systems collectively caused at least a dozen injuries in addition to significant damaging wind. Measured gusts in three cases exceeded 80 kts (40 ms -1 ). Average surface dewpoints were 17 C for the 5 July 1997 event over Tennessee and North Carolina. The case was nevertheless included as moisture was unusually sparse (precipitable water values at or below 1.5 cm) for a day with significant severe convection, given the location and time of year. While average event duration was approximately four hours, two of the LDDs lasted more than six hours. Three of the systems which affected the eastern United States were producing damaging winds as they moved into