A Study of Storm and Vortex Morphology during the 'Intensifying
Stage' of Severe Wind Mesoscale Convective Systems

Ron W. Przybylinski, Gary K. Schmocker
NOAA/ National Weather Service
12 Research Park
St. Charles, MO

Yeong-Jer Lin
Dept. of Earth and Atmospheric Science
Saint Louis University
St. Louis Missouri

Since 1993, several staff members from National Weather Service St. Louis and Saint Louis
University continue to be actively involved in the study of mesoscale convective systems
(MCSs) which produce severe wind damage and non-supercell torndoes across the
Mid-Mississippi Valley Region.  The study of seventeen severe wind MCSs have been
completed, while an additional five cases are currently being investigated.  All of the severe
wind MCSs occurred during the warm season (May - September). Damage surveys were
completed for 80% of the events studied.  Meteorological data for all of the events which
produced severe weather were analyzed to determine their morphology, evolutionary
characteristics, and atmospheric environments in which they evolved.  Six of the seventeen
events met the criteria to be classified as a derecho while the remaining eleven events
evolved into bow echo patterns. Over fifty convective-scale (tornado and non-tornadic)
vortices were analyzed within 150 km of the KLSX of adjacent WSR-88D radar sites.  Time-
height rotational velocities (Vr) traces were used to show their evolutionary characteristics.
We specifically focused our study on the 'intensifying stage'  of MCS evolution, where
isolated or groups of convective cells begin to fill-in to form a nearly solid linear echo
preceding the bowing of the convective line.   Based on storm reflectivity patterns during this
stage, we were able to classify our severe wind MCS events into four groups or types. 
Our initial findings show that the external boundaries from earlier convection or a quasi-
stationary frontal boundary intersecting the convective line appeared to play an important
role in the early stages of convective-scale vortex formation in three of the four groups. 
Such boundaries were identified by reflectivity fine lines,  a line of small (usually weak)
isolated cells oriented orthogonal to the larger more intense cells of the convective line,
visible satellite imagery, or mesoscale surface analyses.  In nine of the seventeen cases
studied, external boundaries intersected either the far northern (5 cases) or south-central
part (4 cases) of the convective line.  In five other cases, the convective line did not
appear to intersect an external boundary, but remained well north of the boundary,
travelling parallel to it during the MCS's life span.  Nearly 40% of the vortices documented
at the convective line-external boundary intersection became tornadic. Observations have
shown that the second circulation to develop near the intersection becomes one of the
strongest and longest-lived vortices of the group and appears to play a role in enhancing
wind damage just south and southeast of the vortex center during the MCS's lifespan.  
Vortices at this intersection often preceded the formation of other convective-scale vortices,
near the apex of a bowing segment south of the intersection.  We will show comparisons
of evolutionary characteristics of convective-scale vortices which intersect external
boundaries to those which do not intersect this feature.  Additionally,   convective-scale
vortices which form at the intersection of an external boundary and a convective line will be
compared to vortex evolution studies completed by Burgess et al. 1997.  It is hoped that
these findings will provide forecasters a greater insight into the understandings of external
boundary - convective line intersections and vortex evolution.

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