Summary

The 27 May 1997 central Texas tornadic event has been investigated in a two part observational study. In the first part of the examination (Part I), the prestorm environment and the role of the preexisting boundaries on storm maintenance/propagation were addressed. As demonstrated in Part I, the 1D environment associated with this event, based solely on (1D) vertical profiles of state variables, was unfavorable for significant (>= F2) tornadoes. Yet, the storm complex produced at least 6 significant tornadoes, including one rated F5. The overarching objective of this article is to reconcile the differences between the expected tornado potential and the observations.

As in Part I, storm analysis relied heavily upon the WSR-88D level II and level III data from three Texas radars. It is shown that each of the six observed tornadoes that produced greater than F0 damage formed along the storm-generated gust front, not along preexisting boundaries. Half of these tornadoes formed on the distorted gust front, the portion of storm-generated gust front whose orientation was deformed largely by the horizontal shear across the cold front. The remaining three tornadoes developed at the gust front cusp (the persistent gust front inflection located at the northeast end of the gust front distortion). Unlike the tornadoes south of the gust front cusp, these tornadoes are found to be associated with antecedent mesocyclones located in the low-levels above the boundary layer. Furthermore, these mesocyclonic tornadoes are found to be larger and more destructive than the three non-mesocyclonic tornadoes.

Numerous convective cell mergers are identified but are found to be poor predictors of tornadogenesis. However, the observed intensification of the Jarrell tornado near the time of a merger associated with its parent cell is offered as a possible indication that mergers occurring during ongoing tornadoes might have resulted in tornado intensification.

The role played by the extreme values of CAPE in place during this event (values exceeded 6500 J kg^-1), is also examined. It is suggested that extreme CAPE may have made both tornadoes and midlevel mesocyclones more likely than a comparable event with smaller CAPE because of the increased capacity for vorticity generation through increased tilting and/or stretching afforded by the stronger updrafts that are theoretically supported in higher CAPE environments. It is further proposed that midlevel mesocyclones might have been favored in the extreme CAPE conditions present during this event as a result of increased updraft longevity. Updraft longevity is posited to increase in higher CAPE environments because the associated stronger updrafts might be able to transport precipitation higher into the thunderstorm where it can be more readily ventilated from the updraft by the upper-tropospheric winds.

Images
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Map of the seven tornadoes examined in this work

Mesocyclones at the gust front cusp

Participants

Adam L. Houston and Robert Wilhelmson

Publications

Houston, A.L., and R. B. Wilhelmson, 2006: Observational analysis of the 27 May 1997 central Texas tornadic event. Part II: Tornadoes. Mon. Wea. Rev., Submitted.

Houston, A.L., 2004: The role of preexisting airmass boundaries in the maintenance and rotation of deep convection in a high-CAPE, low-shear environment. Ph.D. thesis, Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, 275 pp.

Houston, A.L and R.B. Wilhelmson, 2002: The role of the preexisting boundary on tornadogenesis in the 27 May 1997 Central Texas event Preprints, 21st Conf. Severe Local Storms, San Antonio, TX, Amer. Meteor. Soc., 469-472. Extended abstract, Slides from talk