2001 National Weather Association Annual Meeting
13-19, October 2001 Spokane Washington

Brief Overview of Two Isolated Cell - Convective Line Mergers
Leading Towards Tornadogenesis: Two Very Challenging Cases

Ron W. Przybylinski and Gary K. Schmocker
NOAA/National Weather Service
12 Research Park Drive
St. Charles MO 63304

    Since the beginning of our study of organized Mesoscale Convective Systems (MCSs) which
produced damaging downbursts winds and non-supercell tornadoes across the Mid-
Mississippi Valley Region,  we identified a number of events where convective lines merged
with nearly stationary isolated cells located near the vicinity of a old outflow or warm frontal
boundary.  In many of these cases, the isolated cell rapidly intensified just prior to merger,
often exhibiting strong low-level reflectivity gradients along the western flank of the storm,
and became part of the convective line just after merger.  A few cases showed the presence
of a weak and shallow circulation along the forward flank of the convective line, upwind of
the isolated storm.  During the period of merger, two evolutions were frequently observed:
1) the vortex rapidly intensified - deepened but the core diameter increased in size (greater
than 10 km) and distorted the convective line echo resulting in wind damage near and
south of the core or 2) the vortex rapidly intensified - deepened but the core diameter
remained relatively small (less than 6 km) and tornadogenesis occurred within ten or fifteen
minutes from the time of merger. 
    Two MCS events documented during the 2000 spring - early summer convective season
revealed isolated cell - convective line mergers which were extremely challenging during the
period of severe weather warning operations.  One could classify them on the extreme
left side of the severe MCS spectrum.  In each event,  tornadoes occurred within ten minutes
after isolated cell - convective line mergers and caused F1 damage with damage paths
greater than 5 miles in length.  Comprehensive ground surveys were conducted for both
events.
    Using WSR-88D Doppler data from St. Louis (KLSX), this presentation will focus on the
storm and circulation evolution of two MCS events (May 27, 2000; June 24, 2000) which
occurred during the late evening hours while the second event evolved in the late afternoon.
Synoptic and mesoscale settings varied in each case.  The first event occurred under a
progressive shortwave trough and east-west warm frontal boundary across central Missouri,
while the second case evolved under the presence of an upper-level low center and surface
trough extending from a weak surface low over far west-central Illinois south through
south-central Missouri.  Magnitudes of surface-based Convective Available Potential Energy
(CAPE) and 0-3 km Bulk Shear significantly varied in each case, with the May evening event
(KSGF 0000 UTC - 2500 J/kg / 16 m s-1) and the late afternoon June event (KILX 0000 UTC -
1576 J/Kg / 10 m s-1).  
    In each case, the convective system already reached the mature stage of MCS evolution
over our County Warning Area (CWA).  Our goal is to show the complexity and difficulty that
each case exhibited preceding and during the period of isolated cell - convective line merger
and tornadogenesis.  Characteristics of each tornadic circulation under study will be shown
through time-height cross-sections of Rotational Velocities (Vr).  We will try to explain why
tornadocyclones were observed in the May event while a strong mesocyclone occurred with
the June case.  It is hoped that the preliminary observations from each case will provide a
greater understanding on the topic of isolated cell - convective line merger process and
attempt to improve our capabilities in the issuance of more timely and accurate tornado
warnings to the public.

 

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