Friday, 28 October 2005
Alvarado F and Atria (Hotel Albuquerque at Old Town)
A line of severe convective storms traversed central Iowa during the afternoon of 29 June 1998, producing a wide swath of straight-line wind damage (several gusts exceeding 50 m/s) and isolated weak-to-moderate intensity tornadoes (F0-F2). The storms evolved in a highly unstable, deeply-sheared environment where magnitudes of convective available potential energy (CAPE) likely exceeded 3500 J /kg prior to convection and 0-6 km bulk shear values approached 30 m/s. Several discrete convective storms resembling hybrid high-precipitation (HP) supercells were observed 80 km northwest through 100 km northeast of the WSR-88D radar located near Des Moines, Iowa (DSM). During the early period of organization, these discrete storms were embedded within a larger convective region. However, as the system approached DSM, the convective mode evolved to a more solid linear structure. Although scattered wind damage was reported across many sections of the entire line, this study focuses on the evolution of one particularly active portion associated with a nearly continuous swath of severe straight-line winds and tornadic activity 40 km northwest of DSM through the metropolitan area. One hybrid HP supercell, originally detected 100 km northwest of DSM, contained several mid-level, rotating centers. During the early part of the analysis (1716-1806 UTC), the observed vortices exhibited descending vortex characteristics; their strongest cyclonic shears were detected and persisted at mid levels of the circulation. As the storm neared the DSM metropolitan area (1806-1833 UTC), the outflow-dominated hybrid HP supercell further matured and several non-descending tornadic and non tornadic vortices occurred along a very progressive outflow boundary. In this study, the pre convective environment (specifically the vertical wind shear) will be shown to play a role in the system=s intensification and transition from a supercellular to a linear structure. WSR 88D data from DSM are used to document the storm reflectivity and velocity structures as the storm approached the DSM area. Time height rotational velocity (Vr) traces will be used to show the characteristics of the circulations and illustrate the differences between the 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 enhancing the mesoscale rear inflow and convective scale outflow. As the outflow accelerated, several non supercell tornadic and non tornadic vortices, which initially developed from low levels, rapidly deepened and intensified. One of these non-descending vortices appeared to be responsible for the tornadic activity northwest of DSM. These latter findings are consistent with those reported earlier from other cases studied across the Mid Mississippi Valley region.
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