During the Central Texas Tornado Outbreak of May 27, 1997
Carl R. Morgan
NWSFO Austin/San Antonio, Texas
Texans will remember Tuesday, May 27 as one of the state's saddest days. The most violent and deadly tornado outbreak in recent history claimed 28 lives as portions of central Texas were devastated. An extremely unstable airmass produced a series of thunderstorms, beginning with a south-southwestward moving supercell which developed just south of Waco. As the afternoon progressed, this storm produced several mesocyclones, spawning as many as five tornadoes with maximum intensities ranging from F3 to F5.
The purpose of this paper is to describe the pre-storm environment, how it was altered by the evolution and motion of the parent supercell, and to discuss methods that may help indicate when a ground-relative wind environment, which is considered unfavorable for supercell development, can become favorable due to storm-scale contributions.
The surface map on the morning of May 27 presented a complex set of features (Fig. 1). A slow-moving cold front lay across Texas, extending from near Dallas southwestward to the Big Bend. In the region south of the front, temperatures were generally in the upper 70s to low 80s (F), with dewpoints ranging from the mid to upper 70s in central Texas to the low 80s along the immediate coast. A ridge of slightly higher dewpoints extended from the southeast coast into central Texas. Visible satellite imagery (Fig. 2) revealed two additional ingredients; an outflow boundary progressing south and southwest through northeast Texas and central Louisiana, and a theta-e ridge, suggested by a field of cumulus ahead of the front.
The outflow boundary, which originated from collapsed convection over Arkansas, was intersecting the front, and the resulting 'triple-point' was moving into the theta-e ridge. The morning sounding from Del Rio revealed that an extremely unstable airmass was in place south of the frontal boundary. Unmodified, the Del Rio sounding (Fig. 3) produced a CAPE of 5629 Jkg-1 and a lifted index of -13C. Also revealed was a relatively weak environmental wind field. The 0-6 km ground-relative mean wind was from 250 at 6 kt, and SHARP yielded a 0-3 km storm-relative helicity of -27 m2s-2.
Modified Storm Environment
Modifications were made to the Del Rio sounding (Fig. 4) to accomodate adiabatic heating, both at the surface, and in the lower and mid levels of the dry air aloft. The resulting CAPE value increased to 6725 Jkg-1, and the lifted index fell to -15C, reflecting an additional decrease in stability. The supercell began to develop between 1700 UTC and 1800 UTC near Waco, at the intersection of the cold front and outflow boundary. As the storm developed, it propogated to the south-southwest along the cold front. This unusual storm motion appeared to be the result of the momentum of the outflow boundary combined with a significant layer of north-northwest winds above 11,000 ft, as revealed by the VAD Wind Profile from the New Braunfels 88D Radar (Fig. 5).
With extremely large CAPE, once an updraft is established and air parcels begin to accelerate through the area of positive buoyancy, it follows that the storm inflow will increase in a proportional manner. This effect was well documented by spotters during the height of the storm, as storm inflow was measured from the east-southeast at 25-30 kt. In addition to the enhanced storm inflow, the storm-relative winds above the inflow layer were more than double the magnitude of the ground-relative mean wind, due to the large component of the storm motion vector which opposed the mean wind. Combining the increased inflow with the unusual storm motion nearly 125 to the right of the mean wind, the storm-relative wind field takes on a much more ominous character.
Using a combination of the 1200 UTC sounding data from Del Rio, the VAD Wind Profile from New Braunfels, and surface based reports from spotters, the SHARP program was used to generate a hodograph (Fig. 6) believed to be representative of the storm-relative wind field during the evolution of the supercell. SHARP, by default, calculates storm-relative helicity based on a storm motion vector 30 to the right of the 0-6 km mean wind, and at 75% of its magnitude. Storm motion in this case, however, was from approximately 15 at 15 kt. After modifying the wind field to account for the enhanced storm inflow and observed storm motion, SHARP calculated a 0-3 km storm-relative helicity of 243 m2s-2.
Davies-Jones et al. (1990) suggested that when helicity values approach or exceed 150 m2s-2, then there is greater potential for mesocyclone-induced tornadoes, given adequate instability and forcing. However, helicity alone can be a misleading value. Johns et al. (1993) has shown that strong and violent tornadoes can and often do form in opposing extremes of helicity and instability. The Energy-Helicity Index (EHI) was developed by Hart and Korotky (1991) as a way to combine these parameters into one value for forecasting purposes. It is defined as:
where H represents the 0-2 km storm-relative helicity.
Studies by Davies (1993) suggest that the following set of guidelines may be helpful when assessing the threat of supercell-induced tornadoes, assuming adequate support from storm-relative winds:
< 2.0 - significant mesocyclone-induced tornadoes unlikely
2.0-2.4 - mesocyclone-induced tornadoes possible, but unlikely to be strong or long-lived
2.5-2.9 - mesocyclone-induced tornadoes more likely
3.0-3.9 - strong tornadoes (F3) possible
> 4.0 - violent tornadoes (F4) possible
SHARP calculated the EHI from the Del Rio observed 1200 UTC sounding to be 0.88. When adjusted for the storm-relative parameters, this value increased to 5.63.
As this case shows, storm motion can be determined by any number of factors other than the prevailing flow. Since storm-relative helicity can be maximized within a possible range of storm motions, it becomes operationally important to recognize the limits of that range (Korotky et al. 1993). A forecaster can utilize the SHARP program to quickly determine the range of storm motions which represent the greatest threat for supercell development. If convection develops and begins to propogate within this higher threat range of motions, and additional severe weather ingredients exist, the storm environment may be changing to, or may have changed to, a state favoring supercell growth.
May 27, 1997, presented a complex set of environmental parameters; extreme instability, a weak environmental wind field, and a slow-moving cold front intersected by a subtle outflow boundary. Although severe convection was anticipated, the family of violent tornadoes was not. The wind environment, unlike one normally associated with violent tornado outbreaks, was transformed by a unique storm motion.
In a classic severe weather environment, strong vertical wind shear is needed to generate horizontal vorticity, which is then tilted into the vertical and stretched. Davies-Jones and Burgess (1990) note that environmental winds have little effect on storm structure when they are light in comparison to storm-induced winds. In this case, since strong wind shear was not a part of the pre-storm environment, the storm created its own favorable wind field. As a result of an extremely large CAPE, storm-relative inflow was enhanced. Storm motion, with a large component directed into the prevailing environmental wind, resulted in stronger storm-relative winds from the southwest, above the inflow layer. The final product was a storm-relative wind field which bore little or no resemblance to the environmental wind field, and was therefore able to produce and sustain a family of violent tornadoes.
The author would like to thank Jim Ward, NWSFO Austin/San Antonio Science and Operations Officer, for thoughtful input and review.
Davies, Jonathan M., 1993: Hourly Helicity, Instability, and EHI in Forecasting Supercell Tornadoes. Preprints - 17th Conf. on Severe Local Storms, St. Louis, Missouri, Amer. Meteor. Soc., 107-111.
Davies-Jones, R. and D. Burgess, 1990: Test of Helicity as a Tornado Forecast Parameter. Preprints - 16th Conf. on Severe Local Storms, Kananskis Park, Alta., Canada, Amer. Meteor. Soc., 588-592.
Hart, J.A. and W.D. Korotky, 1991: The SHARP Workstation -v1.50 A Skew T/Hodograph Analysis and Research Program for the IBM and Compatible PC. User's Manual. NOAA/NWS Forecast Office, Charleston, WV, 62 pp.
Johns, Robert H., J.M. Davies, and P.W. Leftwich, 1993: Some Wind and Instability Parameters Associated With Strong and Violent Tornadoes, 2. Variations in the Combinations of Wind and Instability Parameters. The Tornado: Its Structure, Dynamics, Predictions and Hazards. Amer. Geo. Union, Wahington, DC. 583-590.
Korotky, William, R.W. Przybylinski, and J.A. Hart, 1993: The Plainfield, Illinois Tornado of August 28, 1990: The Evolution of Synoptic and Mesoscale Environments. The Tornado: Its Structure, Dynamics, Predictions and Hazards. Amer. Geo. Union, Wahington, DC. 611-624.
Figure 1. Surface conditions at 1400 UTC
on May 27, 1997
Figure 2. Visible satellite image from 1815 UTC on May 27, 1997
Figure 6. Modified hodograph representing central Texas on May 27, 1997