Abstract Central Iowa NWA
Severe Storm and Doppler Radar Conference
March 2001
Observational Study of a Midwestern Mesoscale Convective System (MCS) on
29 June 1998 Across Central Iowa: A Single Doppler Analysis Study.
Jason T. Martinelli*, Ron W. Przybylinski**, and Yeong-Jer Lin*
*Department of Earth and Atmospheric Sciences
Saint Louis University
St. Louis, Missouri MO 63103
Tel: (314) 977-3116
Fax: (314) 977-3117
e-mail: martinel@eas.slu.edu
**Weather Forecast Office
12 Research Park Drive
St. Charles, MO 63304
Tel: (636) 447-1876
Fax: (636) 447-1769
e-mail: ron.przybylinski@noaa.gov
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 and 0-6 km shear values exceeded 20 m/s.
Discrete convective storms resembling hybrid supercells and observed 80 northwest and
100 km northeast of Des Moines (DSM) Iowa were embedded within the larger line of storms.
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 nearly a continuous swath
of intense wind and tornadic activity occurred; 40 km northwest of DSM through the DSM
area and 70 to 80 km northeast of DSM. A hybrid supercell 100 km northwest of DSM
contained descending vortices where the strongest cyclonic shears were detected and
persisted at mid-levels of the circulation. As the storm approached the DSM metro
area
from the northwest, the outflow dominated High Precipitation (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 will be shown to play an role in the intensification,
and the transition from supercell to line structure. Single Doppler WSR-88D radar
data from
Des Moines (DSM) are used the 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
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 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.
