NSSL SWAT Case Study - 15 April 1994 Saint Louis Area Bow Echo Tornadoes
During the nighttime hours of April 15, 1994 a strong MCS (Meso-Scale
Convective System) moved across Missouri. Early in its lifetime,
the MCS was mainly responsible for producing large hail and severe straight-line
winds. However, the orientation of the MCS resulted in an east -
west oriented outflow boundary. The boundary provided sufficient
local augmentation of the wind fields (and perhaps other parameters) to
produce tornadoes in an otherwise non-tornadic second MCS. A very
well organized circulation developed near the intersection of the outflow
boundary with the second MCS. This circulation produced several F0
to F1 tornadoes.
Presented here are various WSR-88D radar images from the Saint Louis,
MO (KLSX) radar. Included in some of the images is output from NSSL's Mesocyclone
Detection Algorithm (MDA) and Tornado
Detection Algorithm (TDA). A yellow circle represents a mesocyclone
as detected by the MDA. The red-in-yellow circle represents a mesocyclone
whose base is at the lowest radar scan (where it is a more likely tornado
threat). A red triangle indicates the location of a tornado as detected
by the TDA.
Description: The line of storms approaching Saint Louis
became organized into an MCS. The northern end of the strong line
of storms began to bow out, putting down a well defined gust front/outflow
boundary by 0732
UTC. As the southern end of the line was hanging back, the MCS
The storms in the center of the S-shaped line then became favored for tornado
production because they were moving across or just along the gust front/outflow
boundary. A case study from the 94-95 VORTEX project of the 2 June
95 West Texas tornadoes has shown the importance of boundaries in the production
of some significant tornadoes. For more information please read The
association of significant tornadoes with a baroclinic boundary on 2 June
1995. (Note: This paper may be removed from the web at
any time pending publication.)
BOW ECHO FORMATION:
Description: The Jefferson
City Meso was the only significant meso to form away from the boundary.
It may have been aided, however, by backing low level winds south of the
boundary. This would have increased convergence as well as storm
relative helicity. The Jefferson City meso is indicated by the tornado
signature (TVS) on the S-Shaped
INCREASED ROTATION IN THE CENTER OF THE S-SHAPED MCS:
The most favored area for enhanced storm rotation is, however, right
along the boundary. Some of the strongest convergence
and rotational signatures began to occur as cells propogated along the
boundary in the Saint Louis metro area.
UTC the well developed meso produced the first in a series of brief
tornado touchdowns near the town of New Haven.
Description: This strong meso continued to move along the
outflow boundary layed down by the northeastern portion of the MCS.
As the meso crossed Jefferson County, south of Saint Louis, it produced
three tornadoes, each lasting 5 to 8 minutes and doing F0 to F1 damage.
Relative Velocity Loop nicely illustrates the organized, stable nature
of this meso.
Description: Note the channel of weak reflectivities
at elevation 3.3° on the second
to the bow echo, the channel is in the clear-air return ahead of the
squall line and runs from the WNW to ESE with a slight S-shape. After
the squall line, the weak-echo channel is very pronounced in the stratiform
rain area and runs from NNE to SSW in a reverse S-shape. The velocity
images show that the "zero-isodop" line (actually, S-shaped) corresponds
to the exact location of the weak reflectivity channels. Is this
a product of incorrect clutter suppression? We don't know.
Has the problem been fixed? If you know the answer, email email@example.com.
ONE MORE INTERESTING FEATURE:
to NSSL SWAT Case Study Table of Contents Page.