Extended Abstract

and PS each accounting for about 20%. The key factor determining theorganizational structure was the vertical wind shear. TS systems exhibited frontto-rearstorm-relative flow throughout the troposphere, LS systems rear-to-frontstorm-relative flow aloft, and PS systems contained primarily line-parallel stormrelativeflow aloft. Many systems did not retain one mode of organizationthroughout their life cycles, but rather transitioned between modes, the TS modebeing the most-frequent final state.Fig. 2. Three modes of organization of MCSs in the central United States: trailing,leading, and parallel stratiform precipitation systems (Parker and Johnson 2000).Not all of the modes of convective organization shown in Fig. 2 lead to heavyrainfall. Unique patterns of convective organization and cell propagation arenecessary to produce extreme rainfall (Doswell et al. 1996). Such patterns haverecently been documented by Schumacher and Johnson (2005).A study of 184 heavy-rainfall cases for the period 1999-2003 reveals threedominant modes of MCS organization associated with extreme-rain-producingstorms (Schumacher and Johnson 2005, 2006). These modes (Fig. 3) arereferred to at training line/adjoining stratiform (TL/AS), back-building (BB), andtrailing stratiform (TS). The first type, comprising 34% of the MCSs, ischaracterized by a typically east-west convective line along a quasi-stationaryfrontal boundary with west-to-east training convective cells on the cool side of theboundary and an area of stratiform precipitation displaced to the north. Thesecond type, comprising 20% of the cases, is typically smaller in overall size andfeatures back-building cells along an outflow boundary, with repeated cellformation over the same location and an area of stratiform precipitation typicallydownstream. The third type, also comprising 20% of the cases, has the overallstructure of commonly occurring trailing-stratiform (TS) MCSs, but in the heavyraincases, the southern end of the line becomes oriented in an east-west-5-