MESOSCALE CONVECTIVE COMPLEXES AND PERSISTENT ELONGATED CONVECTIVE SYSTEMS OVER THE UNITED-STATES DURING 1992 AND 1993

Large, long lived convective systems over the United States in 1991 an d 1993 have been classified according to physical characteristics obse rved in satellite imagery as quasi-circular [mesoscale convective comp lex (MCC)] or elongated [persistent elongated convective system (PECS) ] and cataloged. The catalog includes the time of initiation. maximum extent. termination, duration. area of the -52 degrees C cloud shield at the time of maximum extent, significant weather associated with eac h occurrence. and tracks of the -52 degrees C cloud-shield centroid. B oth MCC and PECS favored nocturnal development and on average lasted a bout 12 h. In both 1992 and 1993, PECS produced -52 degrees C cloud-sh ield areas of greater extent and occurred more frequently compared wit h MCCs. The mean position of initiation for PECS in 1992 and 1993 foll owed a seasonal shift similar to the climatological seasonal shift for MCC occurrences but was displaced eastward of the mean position of MC C initiation in 1992 and 1993. The spatial distribution of MCC and PEC S occurrences contain a period of persistent development near 40 degre es N in July 1992 and July 1993 that contributed to the extreme wetnes s experienced in the Midwest during these two months. Both MCC and PEC S initiated in environments characterized by deep. synoptic-scale asce nt associated with continental-scale baroclinic waves. PECS occurrence s initiated more often as vigorous wales exited the inter mountain reg ion. whereas MCCs initiated more often within a high-amplitude wave wi th a trough positioned over the northwestern United States and a ridge positioned over the Great Plains. The low-level jet transported moist ure into the region of initiation for both MCC and PECS occurrences. T he areal extent of convective initiation was limited by the orientatio n of low-level features for MCC occurrences.