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 became S-shaped.  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.)
  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 image.

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.

Around 0924 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.
The Storm 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 and third images above. Prior 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 stumpf@nssl.noaa.gov.

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