The Severe Bow Echo Event of 14 June 1998 over the
Mid-Mississippi Valley Region: A Case of Vortex Development
near the Intersection of a Pre-existing Boundary and a
Convective Line

Gary K. Schmocker and Ron W. Przybylinski
NOAA / National Weather Service
12 Research Park Dr.
St. Charles, Missouri 63304

Erik N. Rasmussen
NOAA / National Severe Storms Laboratory
Boulder Colorado

One of the most destructive damaging wind events of the spring and early summer convective season
of 1998 over the Mid-Mississippi Valley region occurred during the early morning hours of 14 June 1998
across eastern Missouri and a small part of southwestern Illinois.  Extensive property damage occurred
across the northern sections of the St. Louis metro area and in the vicinity of the confluence of the
Mississippi, Illinois, and Missouri Rivers.  Peak surface wind gusts ranged between 40 to 50 m s-1
causing countless trees to be snapped or uprooted, falling on numerous homes, vehicles, and other
structures.  Damage resulting from a least three non-supercell tornadoes was also identified after a
detailed storm survey was conducted.

The synoptic and mesoscale environment of early 14 June 1998 was characterized by strong
southwesterly low-mid level winds ahead of a trough in the Plains, and moderate instability with
a convective available potential energy (CAPE) of 2000 J Kg-1.  The first mesoscale convective
system (MCS) moved southeastward across the Mid-Mississippi Valley region between 0600 and
0900 UTC and laid an outflow boundary which extended northwest to southeast across east-central
Missouri.  This boundary moved relatively quickly northeastward toward St. Louis and a secondary
developing MCS over northeastern Missouri.  The St. Louis WSR-88D VAD wind profile depicted
a dramatic increase in the low-level (0-3 km) wind shear and SR helicity between 0930 and 1130 UTC
as the boundary approached and moved through the WFO St. Louis (KLSX) site.

The second MCS developed between 0900 and 1000 UTC, as two parallel lines of convection,
oriented northeast-southwest, rapidly formed 90 to 120 km northwest of St. Louis.   The second
line gradually merged with the leading line after 1030 UTC resulting in the intensification of the
leading convective line.

After 1045 UTC, one convective cell along the southwestern part of the leading convective line
intersected with the northwest-southeast oriented outflow boundary produced earlier by the first
MCS. Three vortices rapidly formed just north of the intersection of the convective line and the
outflow boundary (on the cool side of the boundary), with the second circulation (Circ #2) becoming
the strongest and longest lived of the three.  This vortex initially formed below 2 km, rapidly
intensified within the lowest 2 km and gradually deepened to an overall depth of 5 km midway
through its lifecycle.  This circulation lasted over one hour and spawned an F(0) intensity non-
supercell tornado.

We will show that Circ #2 aided in the acceleration of a bowing segment south of the vortex and
helped to focus clusters of intense damaging winds across the northern parts of the St. Louis metro
area.  We will also underscore the importance of identifying external outflow boundaries intersecting
convective line segments from WSR-88D reflectivity data, and how these boundaries can change
the local low-level wind shear profile leading to dramatic increases in 0-3 km SR helicity.  Such
boundaries can augment the horizontal vorticity and can help promote convective cell intensification,
vortex initiation, and subsequent non-supercell tornadogenesis and/or the production of enhanced
wind damage.