Close Range WSR-88D Observations of Several Tornadic Storms

 
Fred H. Glass and Mark F. Britt 


NOAA/National Weather Service Forecast Office
St. Charles Missouri 63304


During the spring of 1998, the WSR-88D Doppler radar at the National Weather Service Office in
St. Louis (KLSX) observed two tornadic thunderstorms at very close ranges (<15 nm).  While 
structurally very different, both of these storms were responsible for a single short-lived tornado
(< 2 mile path length) which produced F1 intensity damage.  These were also the first tornadic 
storms observed at such close range to the radar since its installation in early 1992.  In each case,
the storm proved to be a unique challenge to the warning forecasters due to the close range
evolution, and due to the complexities associated with the interpretation of high-resolution Doppler
velocity data above and below the cloud base.

The first case occurred during the late afternoon of April 13, 1998 when a broken line of
thunderstorms formed west of St. Louis and moved through the metro area.  As the line passed
immediately east of the KLSX radar, mid-level rotation was detected with a convective cell within
the line of storms.  The mid-level rotation intensified and deepened quickly to mesocyclone criteria
within several volume scans, while strong low-level convergence developed along the storm's gust
front.  Just prior to tornado formation, the mid-level rotation increased, while the strong convergence
along the gust front become rotationally convergent.  Low-level velocity data during the time of the
tornado indicate several velocity peaks: a gate-to-gate TVS feature probably directly associated
with the tornado,  and a slightly larger rotation believed to be a tornado-cyclone.  Structually,  the
storm exhibited reflectivity characteristics of a High-Precipitation (HP) supercell.

The second case occurred during the late afternoon of May 23, 1998 when a small linear convective
cluster which formed in southwest Missouri moved through western portions of the St. Louis metro
area.  The evolution of this case was more complex and involved the interaction of the linear storm
cluster with a storm-scale boundary associated with another thunderstorm just northwest of the 
KLSX RDA.  Velocity data revealed deep convergence along the gust front as the storm moved
towards and over the RDA.  As the storm passed immediately over the RDA and interacted with the
storm-scale boudary, deep convergent rotation developed along the northern portion of the gust front
and throughtout the observable radar data.  The convergent signatures quickly evolved into a
tight pure rotational signature as the storm moved to the northeast with gate-to-gate TVS shear
at the time of the tornado.  The evolution of this tornado appeared to be more consistent with the
non-supercell variety.

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