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Case #1: 25 May 1996 Severe Convective Windstorm / Tornado case
In this event we key upon the evolution of:
- Isolated cell - convective line mergers
- Growth of vortices (tornadic / non-tornadic) just after merger.
As the linear convective line evolved into a bow echo, the line intercepted a number of isolated convective cells downwind from the system. The convective complex produced widespread wind damage over 60 mph and two tornadoes (F1 intensity). We will review the lifecycle of this convective line, including the straight-line winds that produced wind damage, and the evolution of tornadic (non-tornadic) vortices near the time of isolated cell - convective line merger.
Fig. 1. Damage map associated with the 25 May 1996 severe MCS. (W) represents locations of wind damage; (T) signifies tornado damage track. (F1) F-scale devised by Dr. T.T. Fujita represents the degree of structural damage caused by the wind speeds associated with the tornado. The tornado initially touched down at 2240 UTC. Damage survey was conducted by the SOO the following day.
Fig. 2. Surface chart for 2000 UTC, 25 May 1996.
- A stationary
frontal boundary extended from north-central Kansas through central Missouri (slightly
north of Columbia (COU) and St. Louis (STL) and eastward into southeast Indiana / northern
Kentucky.
- Old outflow boundary from earlier morning convection stretched from
just south of COU through Nashville (BNA) TN.
- An axis of 69 to 70°F
surface dewpoints pooled along the frontal and old outflow boundaries from central
Missouri southeastward into western Kentucky. Note the presence of a strong temperature /
dewpoint gradients across north-central Missouri.
Fig. 3. 500 mb Chart for 0000 UTC, 26 May 1996.
- Weak shortwave trough over the Mid-Mississippi Valley region was moving through the mean 500 mb ridge axis. An axis of upper-level moisture stretched from the Great Lakes through the Mid-Mississippi Valley Region.
Fig. 4. SHARP Sounding
for 0000 UTC, 25 May 1996 Springfield (KSGF) Missouri with select thermodynamic and
kinematic parameters.
- Degree of Instability (P max) CAPE = 2796 J/Kg / CIN
-14 J/Kg (Highly unstable environment)
- Vertical Wind Shear (0 - 4 km) 11 m/s (Weak vertical
shear)
- The sounding
revealed a relatively dry layer of air from 750 - 550 mb. Such a layer would support the
production for damaging winds. (See COMET
MCS module for more information).
- Slightly stronger vertical shear was noted across much of northern
and central Missouri during the afternoon. Lathrop MO profiler at 2000 UTC revealed 13 m/s
shear within the 0 - 3 km layer.
