A Satellite Perspective of the Propagation Characteristics of a Mesoscale Convective System over Northwest Alabama and Northeast Mississippi

Geostationary Operational Environmental Satellite (GOES) Imager data, GOES Derived Product Imagery (DPI), surface and upper air data are used to diagnose the environment of a mesoscale convective system (MCS) that exhibited two modes of propagation over the southern portion of the Tennessee Valley during the afternoon and evening hours of 14-15 July, 2004. The MCS formed in an environment characterized by a nearly unidirectional wind profile and a significant amount of dry midtropospheric air, which previous studies have shown to be an environment thermodynamically and kinematically conducive for an MCS to exhibit two modes of propagation. In this case, the initial convection developed a cold pool over portions of southern Tennessee and northern Alabama. The leading edge of the cold pool helped to accelerate a line of convection into portions of east central Alabama and northern Georgia before weakening as it moved into an area of greater static stability. In contrast, the southwest portion of the outflow boundary became quasistationary over northwestern Alabama, northeastern Mississippi and southwestern Tennessee. The low-level mass convergence associated with the outflow boundary was juxtaposed with an instability axis, which helped to provide additional convective development upstream of the initial convective complex. A strong backward propagation vector nearly opposed the advection vector which resulted in a quasistationary MCS for a short time over northeastern Mississippi and northwestern Alabama. The backward propagation vector eventually overwhelmed the advection vector and allowed a westward movement to the MCS into north central Mississippi and southern Tennessee before the system began to weaken by 0600 GMT 15 July 2004. Results of the study agree with prior studies indicating that quasistationary and backbuilding convection are favored when maximum boundary relative low-level mass convergence and a minima of static stability are located upwind of the initial convection while forward propagating convection is favored when maximum boundary relative low-level mass convergence and a minima of static stability are located downwind of the initial convection.