Following the Boundaries:
May 3 & May 12, 2000 Texas Chases

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Wall Cloud Southeast of DFW on 5/3/2000

Mesoscale Madness: Chasing the Boundaries in Texas

Web Posted May 21, 2000 6:00 PM
by Amos Magliocco

  The importance of mesoscale boundaries in storm initiation and tornadogenesis is one of the most important discoveries in storm research during the 1990's.  Stormchasers have learned the value of performing a hand analysis of current conditions or analyzing radar or visible
satellite imagery to find these features.  One May 3, an outflow boundary from previous convection in Southern Oklahoma was draped across North Texas and moved southeast.   Vertical wind profiles were supportive of severe storms, and CAPE values south and east of the DFW metroplex approached 3000 j/kg as a day of good solar insolation and high dewpoints created an unstable boundary layer.  I turned in the last assignment for my graduate fiction class and raced east towards Terrell, Texas around 3:00 PM.
  The Storm Prediction Center issued a Tornado Watch for the metroplex and areas south all the way to Waco, and I moved back to the southwest, hoping to catch storms as they moved out of the metro area and the crushing rush hour traffic.  Around 4:00 PM, a storm fired in Denton County, north of the DFW area, and traveled south through Tarrant County along the boundary.  I caught the storm on Interstate 20 as it pushed south and east through Arlington, Grand Prairie, and Duncanville.  I encountered the large wall cloud depicted in these photographs   just west of Highway 67.  As the storm approached and strengthened, I retreated onto 67 and moved south, keeping pace with the southern flank of the HP storm which was displaying weak midlevel rotation at the time, not enough for the Fort Worth NWS to issue more than a severe thunderstorm warning.

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Wallcloud seen from Interstate 67 looking Northwest 5/3/2000

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Wallcloud seen near Corsicana looking West 5/3/2000

  Soon I caught up with Steve Miller, and were soon joined by Glenn Dixon, Scott Eubanks and James Clarke.  We stayed in excellent position relative to the storm, following smaller and smaller roads as the storm nearly trapped us in Northwest Navarro County.  The Roads of Texas map came through, however, and we observed this storm and very persistent wall cloud until the cell became disorganized after sunset.  A rewarding and interesting chase, watching this storm glide along the boundary and maintain rotation in an environment of only marginal vertical shear.
  For more information on mesoscale research see the Vortex Educational Series here:
http://www.nssl.noaa.gov/noaastory/
 

May 12, 2000: All the Way to Waco
Web Posted May 21, 2000 6:00 PM
by Amos Magliocco

  By early morning, a cold front moved through North Texas at around 20 miles per hour, slowing later than the models predicted.  When it finally stalled, it hung from Fort Worth to the south-southwest and allowed the air to the southeast of the area time to destabilize.  Wind shear was  marginal for rotating cells, but CAPE was estimated near 6000 j/kg and so Robert Hall and I left Denton under the premise that very high instability will often compensate for other parameters which may be less than ideal.  This would prove to be the case.
  We headed for Terrell, Texas (again) to move east of the boundary and out of the area traffic.  Steve Miller called and said he'd analyzed a mesolow forming south of Tarrant County, and that surface winds in that area were backing.  He also reported a thin line of persistent cumulus in Erath County.  Steve said he was targeting this area for initiation as the best chance for storms to become severe.
  Within a few minutes, Eric Nguyen called from Norman, OK where he was watching radar for us, and reported a storm firing quickly in Johnson County.  Eric said the echo was up to 57dbz in only two volume scans!  Robert and I combined to misread signs and maps in Ennis, so that we found ourselves moving due west rather than southwest.  We thought this was a terrible mistake, but we would later understand that it was the best thing that could have happened to us.  There was confusion about the movement of the storm from the beginning with several reports listing east or southeast as the storm's heading.  At this time, we were joined by Steve Miller, Glenn Dixon and Scott Eubanks.
    The storm was backbuilding along the cold front, so that the updraft was propagating southwest, leaving a large, flared anvil to the north, driving rain underneath.  We drove south on 35 through this rain, wondering when we would escape it and if we would emerge near the core of the storm in time to see anything.  We felt sure the storm was gusted out, but continued to pursue. 
  When we  realized the actual course of the storm, we closed on it quickly and intercepted the base, lodging ourselves in the notch to watch a beautiful wallcloud just south of Moody.  As with the 5/3/2000 storm described above, this wallcloud continued for some time, but exhibited few signs of rotation.   We followed the storm into Belton where the setting sun lit the base with an orange glow, and trailing scud cast diffuse colors in every direction.  In the foreground of breathtaking scene, a small lowering hovered as the storm drifted south and became outflow dominant.  Most of these images I captured with my new Sony video camera, and have yet to pull the shots off the machine to post.  I will do so soon, and post those here.