6.1 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 PREEXISTING BOUNDARY AND A CONVECTIVE LINE
G.K. Schmocker1, R.W. Przybylinski1, E.N. Rasmussen2
1National Weather Service Office
12 Research Park Drive
St. Charles, MO 63304
2CIMMS
National Severe Storms Laboratory
Boulder, CO
1. INTRODUCTION
The recent work of Markowski et al. 1998a and
Rasmussen et al. 2000 from data collected from the Verifications of the Origin of Rotation
in Tornadoes Experiment (VORTEX) found that the majority of significant tornadoes (>F1)
in supercell storms occurred near (within 30 km), and on the cool side of external,
preexisting, low-level boundaries. These external, preexisting boundaries include synoptic
fronts, thunderstorm outflow boundaries, and boundaries due to differential heating
(low-level baroclinity along the edge of anvil shadows; Markowski et al. 1998b). Low-level
boundaries have been shown to enhance the low- level convergence, vertical vorticity
(Maddox et al. 1980), CAPE, moisture depth, boundary layer humidity, horizontal vorticity,
and storm-relative (SR) helicity. The baroclinically generated horizontal vorticity at
boundaries augments the horizontal vorticity already present due to the larger scale mean
vertical wind shear. In the following case study, we will show how an outflow boundary
appeared to cause a dramatic change in the local low-level vertical wind shear profile and
SR helicity, leading to a more favorable environment for low-level mesocyclogenesis along
a convective line as it intersected this outflow boundary.
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 (Fig. 1). 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. A church lost its entire roof, while a school, an apartment complex
and several homes lost part of their roofs. A large airport hanger was destroyed along
with the aircraft inside. While most of the damage was the result of intense straight line
winds, there was also damage due to at least three non-supercell tornadoes (F0-1),
identified after a detailed storm survey was conducted.
Fig. 1. Map of severe wind gusts (W, >50kts),
tornadoes (with F rating),
and circulation tracks across east-central Missouri and southwestern
Illinois. Squall line position is denoted with times in UTC. Time of initial
circulation detection is also depicted in UTC, and denoted by a marker
specific to each circulation track every five minutes thereafter.
2. SYNOPTIC AND MESOSCALE ENVIRONMENT
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. Convection developed
well north of a warm frontal boundary which extended from an area of low pressure over
eastern Kansas southeast through northeastern Arkansas. At 850 mb, a 40-50 kt
southwesterly low-level jet brought high (12- 16 degree) dewpoints into eastern Missouri,
as well as strong warm air advection. A 50-60 kt wind max at 500 mb advected low
humidities into the region, as evidenced by the pronounced drying noted between 800 and
600 mb on the 1200 UTC Springfield, Missouri (SGF) sounding (Fig. 2). A steep lapse rate
of 8 C km-1 was present in the elevated mixed layer between 800 and 550 mb. This low-mid
level temperature and moisture stratification produced strong convective instability with
a e from the surface to 700 mb of 26 K, typical of a wet microburst environment (Atkins
and Wakimoto 1991). The SGF sounding contained convective available potential energy
(CAPE) of 2006 J kg-1, although values of CAPE were likely lower than this across east
central Missouri, northeast of the warm front. There was moderate low-level ( 0-3 km) wind
shear of 17 m s-1at SGF, with a SR helicity of 294 m2 s-2 due to strong storm inflow and
veering of the wind with height in the lowest 1 km.
Fig. 2. Skew-T plot (Skew T/Hodograph Analysis and Research Program,
Hart and Korotky, 1991) from Springfield, MO (SGF) at 1200 UTC
14 June 1998.
3. INITIAL RADAR ANALYSIS, LOCAL WIND PROFILE AND MARC SIGNATURES
The first mesoscale convective system (MCS) to develop 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 outflow boundary
moved relatively quickly (about 20 m s-1) northeastward toward St. Louis and a second
developing MCS over northeastern Missouri. The VAD (velocity azimuth display) wind profile
from the St. Louis WSR-88D (KLSX) depicted a dramatic increase in the local, low-level
(0-3km) wind shear and SR helicity between 0930 and 1130 UTC as the boundary approached
and moved through KLSX. The 0-2 km SR helicity increased from 106 m2 s-2 at 0928 UTC to
783 m2 s-2 at 1111 UTC (near the time of boundary passage at the radar site), while the
0-3 km helicity also increased dramatically from 155 m2 s-2 at 0928 UTC to 884 m2 s-2 at
1111 UTC. This increase in the SR helicity was due to the strengthening of the wind below
3 km and backing of the wind below 1 km from 0930 to 1111 UTC, leading to a clockwise
turning hodograph in the lowest 3 km as depicted in Figure 3.
Fig. 3. Hodograph generated from the WSR-88D KLSX VAD (velocity
azimuth display)
wind profile at 1111 UTC 14 June 1998. Black dot represents storm motion vector.
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. The mid- altitude radial convergence
(MARC) velocity signature (Schmocker et al. 1996) was briefly identified along the leading
edge of the convective line after 1030 UTC, about 10 minutes prior to the first wind
damage report in the WFO St. Louis county warning area. Values of MARC intensified from 25
m s -1 (convergent velocity difference) at 1040 UTC to a maximum value of 35 m s -1 (at a
height of 3-5 km) at 1101 UTC during the time of the first wind damage reports. The MARC
velocity signature identified in this nocturnal case was not as strong or long lasting as
compared to MARC identified in several afternoon/early evening bow echo cases studied by
the authors.
4. CIRCULATION EVOLUTION NEAR INTERSECTION OF CONVECTIVE LINE AND PRE-EXISTING
OUTFLOW BOUNDARY
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. Several small, weak and isolated convective cells, oriented
northwest-southeast, from the southern portion of the convective line to just south of the
radar site signified the location of the outflow boundary (Fig. 4). 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 (Fig. 1). The first circulation formed at 1050
UTC in the intensifying storm's inflow notch. Throughout most of it's life cycle this
circulation was a shallow vortex with the strongest cyclonic shears (rotational velocities
up to 21 m s-1) remaining in the lowest 2 km. This circulation did not meet mesocyclone
criteria (Andra 1994) due to its lack of persistent depth and relatively weak shears.
Circ. #2 developed at 1101 UTC, just south of the first circulation, and also in the
storm's inflow notch (weak reflectivity notch on the northeast or left forward flank of
the storm). The northeastward moving outflow boundary extended from this inflow notch
southeast to just east of KLSX at 1111 UTC. This vortex initially formed below 2 km,
rapidly intensified (rotational velocities of 25 m s -1 ) within the lowest 2 km and
gradually deepened to an overall depth of 5 km midway through its life cycle (Fig. 5). A
non- supercell tornado (F0 intensity) occurred in the vicinity of Circ. #2 (within 10 km
and on the cool side of the external, outflow boundary) as the vortex deepened between
1135 and 1145 UTC. A tornado vortex signature (TVS ; small gate-to- gate velocity couplet
at the lowest elevation slice) was observed with a V of 23-43 m s-1 between 1121 and 1142
UTC on the north side of Circ. #2 and along the outflow boundary, serving as a radar based
precursor to this weak tornado along with the deepening trend of Circ. #2.
Fig. 4. WSR-88D reflectivity image at 0.5º elevation angle for
1056 UTC. The outflow boundary is identified by weak convective
cells between KLSX and the community of Montogomery City MO.
Fig. 5. A time-height series plot of the rotational velocities
(Vr; in m s-1) of circulation #2. Delta-V [ ].
Shortly after the development and rapid intensification of
Circ. #2, the convective line accelerated ("bowed out") just south of this
circulation (Fig. 6). Much of the wind damage occurred just south of the path of Circ. #2
as can be seen in the storm damage/circulation tracks map (Fig. 1). This cyclonic vortex
appeared to cause an intensification of the low-mid level winds (rear inflow jet current)
south of the vortex, and aid in the transfer of momentum from the storm's lower-mid level
region to the surface. In several bow echoes examined by the authors, wind damage or the
strongest wind damage was often observed along the southern periphery of the second core's
(the second circulation to develop along the convective line) path (Przybylinski et al.
2000). In many cases studied (including this case) the vortex was a relatively strong
convective-scale cyclonic circulation located in the updraft region of the storm,
following typical mesocyclone evolution (Burgess et al. 1982). In other events this
circulation evolves into, or becomes part of a developing cyclonic line- end vortex (a
broad but strong and long lasting circulation located mainly within the downdraft region
in the northern part of the bowing line; Weisman 1993).
Fig. 6. WSR-88D reflectivity (left) and storm-relative mean velocity (SRM -
right) images at
1.5º elevation at 1117 UTC 14 June 1998 from KLSX. Circ #2 is located at the 315º
radial
48 km.
After 1142 UTC, Circ. #2 began to broaden with the diameter growing to 15 km by 1202
UTC. Rotational velocities also began to decrease, although they may have been
underestimated after 1142 UTC, as the location of Circ. #2 north and northeast of the
radar site did not favor an optimum viewing angle.
5. CONCLUDING REMARKS
In this case study, we have shown that an outflow boundary (even one with a
ground-relative speed of 20 m s-1)
intersecting a convective line can locally augment the SR helicity, leading to an
environment more favorable for
low-level mesocyclogenesis. The local 0-2 km SR helicity increased seven fold (106 m2s2 to
783 m2s2) in less
than two hours from the time the outflow boundary approached KLSX from the southwest at
0928 UTC to the time of boundary passage around 1111 UTC. As the boundary
intersected the storm on the southern end of the convective line, this storm intensified
and the inflow into this storm horizontally stretched the horizontal vorticity and SR
helicity generated along the boundary. This horizontal vorticity (SR helicity) was
further stretched and tilted in the vertical by the storm's updraft leading to low-level
circulation development near and just north of the convective line-external boundary
intersection. The strongest and most persistent circulation, Circ #2, initially formed
below 2 km, and revealed non-descending characteristics (Trapp et al. 1999) during the
organizing stage of vortex evolution. As the vortex entered the mature stage,
it appeared to partially aid in the acceleration of the bowing line segment south of this
vortex, leading to enhanced wind damage. A weak tornado developed on the northern
periphery of Circ #2, along the external boundary, shortly after the cyclonic shears of
Circ #2 intensified in the low-levels (lowest 2 km) and as Circ #2 deepened to 5 km in
height (intensifying mid-level rotational velocities).
This case underscores the importance of identifying
outflow boundaries from radar data, satellite data, surface mesoanalysis, and any other
methods which may infer the existence of a low-level boundary. Even with detailed surface
mesoanalysis, an older boundary may be missed as it loses it's horizontal temperature
gradient, although the horizontal vorticity and helicity can remain until dissipated by
turbulence (possibly hours after a boundary ceases to exist in the temperature field;
Rasmussen et al. 2000). Not only must the existence of the boundary by determined, but its
orientation with respect to the on-going convection (an organized convective line in this
case) it intersects with, and its movement both in a ground and storm relative framework
must also be considered as they may be important factors in the ability of the boundary to
enhance new convective growth and low-level mesocyclogenesis along the convective line.
The warning forecaster needs to watch for rapid circulation development near the
convective line-preexisting boundary intersection, as well as subsequent intensification
and deepening of any circulations in this region which may lead to straight line wind
damage or possible tornado development in the vicinity of the larger circulations.
6. ACKNOWLEDGMENTS
The authors are grateful to Steve Thomas (MIC) WFO St. Louis for his support in this
study. This paper is funded from a subaward under a cooperative agreement between NOAA and
the University Corporation for Atmospheric Research (UCAR).
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