Observational study of a Midwestern severe wind Mesoscale Convective System (MCS) on 29 June 1998: a single Doppler analysis study

A line of severe convective storms traversed across central Iowa during the afternoon of 29 June 1998, producing widespread wind damage and isolated tornadoes at discrete locations along the leading edge of the convective line The storms evolved in a highly unstable, deep-layer sheared environment where magnitudes of Convective Available Potential Energy (CAPE) exceeded 3500 J kg^-1 and 0-6 km shear values exceeded 20 m s^-1. Discrete convective storms resembling hybrid supercells were observed 80 km northwest of Des Moines (DSM), Iowa and 100 km northeast of DSM near Marshalltown, Iowa. However, the line gradually evolved into a nearly solid convective line as the system approached DSM. Scattered wind damage was reported across many sections of the entire line. However, there were two preferred locations where there was a nearly continuous line of intense wind and tornadic activity: 40 km northwest of DSM through the DSM area and 70 to 80 km northeast of DSM.

A hybrid supercell northwest of DSM contained descending vortices where the strongest cyclonic shears were detected in the mid-levels of the circulation. As the storm approached the DSM metro area, an outflow-dominated storm evolved as several non-descending tornadic and non-tornadic vortices occurred along a very progressive outflow boundary.

In this study, the pre-convective environment will be shown to play an important role in the intensification and the transition from supercell to line structure. Single Doppler WSR- 88D radar data from Des Moines are used to document the storm reflectivity and velocity structures as the storm approached the area. Time-height rotational-velocity (Vr) traces will be used to show the characteristics of the circulations and illustrate the differences between tornadic and non-tornadic vortices. Specifically, we will show that several deep vortices near the northern flank of the hybrid supercell appeared to be responsible for strengthening the convective-scale outflow. As the outflow accelerated, several non-supercell tornadic and non-tornadic vortices that initially developed from low-levels and then rapidly deepened and intensified, appeared to be responsible for the tornadic activity northwest of DSM. These later findings are consistent with those reported earlier from other cases studied across the Mid-Mississippi Valley region.