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An Unusual and Interesting Event
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An unusual event occurred in southern Dallas County on the
evening of Wednesday,17 January 1996.  An unusual
"supercell" produced a series of small, F0 to F1 tornadoes.

From a review of the damage tracks, it appears that
a series of small vortices formed sequentially and briefly swept
along the southern side of a meso and then dissipated as the
next formed slightly to the NE.  Each had a short narrow track
of less than two miles in length.  I found evidence of six.

NWS said that on the WSR-88D, the parent low level circulation
(meso- or miso-cyclone?) with 60 kts of shear, was detectable
to only 8,000 feet AGL! The storm had a forward flank updraft
and on the reflectivity display appeared to persist in a quasi-
stable state, shaped somewhere between an HP and a bow
echo.

The storm was cyclic.  There was a "hand-off" near I-35 in De
Soto (southwestern Dallas County)  and another farther east
that occurred southeast of the I-45 and I-20 interchange.

As the storm crossed I-35, it appeared that one circulation
(meso) dissipated and another took over.  The new meso
formed about 1.3 miles north of the previous one.  This was
apparent on radar as well as from a review of the damage
tracks.  I witnessed the second hand-off farther east as I
intercepted the storm from the north.

Notes on the Chase
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I intercepted the storm in southeast Dallas County, near the
intersection of highway 175 and I-20.  As I approached this
intersection, the dissipating tornado produced a series of blue-
green power flashes several miles to my southwest, giving me
a false sense of security.  (NWS radar and spotters reported
storm movement as ENE.)  I was startled by a quick series of
bright flashes less than two miles to my south and several
miles east of the tornado.  The flashes to my south occurred
under a broad funnel shaped lowering under a ragged rain free
base as the new circulation blew down power lines.

I had not considered that a new circulation might form NE of
the older one and had been visualizing a bow echo storm
structure based on NWS radar reports.  Although I had
anticipated that the circulation might eventually evolve
northward along the eastern side of the bow, I had not
considered that the storm might be cyclic.

The rapid formation of a new rotating updraft occurred
between volume scan sequences, so no timely warning for a
new circulation could be given to the spotters by NWS.

The new "meso' produced intermittent F0/F1 damage along its
track from west of Seagoville to Terrell, a distance of about 15
miles. Due to a lack of roads and structures along this track, a
thorough damage survey could not be made.  As a result, there
is no firm confirmation of tornadoes with this circulation.

Conclusion
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It was a heck of a storm to intercept at night.  The blue-green
power line flashes were spectacular.  Intercepting an extremely
fast moving tornadic storm at night in a city with blinding
street lights and traffic was quite a challenge.  When combined
with a minimum five minute interval between radar updates
and a shortage of spotters in that part of the city, it was a little
unnerving.  (At 60 mph, in five minutes, it moved five miles.
That's a little faster than normal experience supports.)

Observation
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The ham radio operators at the NWS are not trained to
interpret radar, so all updates to the spotters must be
relayed by them from the NWS PUP operator to the spotters.
Because of the feverish pace of activity at NWS during severe
weather events, updating radar info for the spotters is a slow,
clumsy and inexact process.  Having said that, I must point out
that after having chased storms at many locations across the
southern plains, it is easy to conclude that the Fort Worth
NWS/spotter team is among the best and most efficient in
existence.  (They were working several storms within their
area of responsibility. Maybe I am expecting a little too  much.)
The ATV (amateur radio television) radar feed was not
working, so I didn't have on-board radar and had to rely on
their inputs, other spotter reports and what I could see
visually.

On-board radar is essential when spotting in a city, especially
when visibility is limited or at night. If I had not had on-board
radar the evening of May 5, 1995, I would likely have been a
participant in the big Fort Worth May Fest hail event. But by
having on-board radar images, I was able to get out of the way
and stay to the southeast of the meso and the hail.

Suggestion
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There have been several events in Dallas county similar in
nature to this one (shallow circulations and tornadoes).  They
seem to occur when the low level shear is extreme.  This storm
was moving at 60+ mph. Earlier, the 500 mb winds were
reported to be around 90 kts. A case study of an event like this
would make a great paper for a researcher to publish.

Sam Barricklow K5KJ

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