1 The Jarrell Tornado of May 27, 1997 ANDREW MANKOWSKI University of Wisconsin – Madison Atmospheric and Oceanic Sciences ABSTRACT A tornado outbreak occurred over Central Texas on May 27, 1997. This outbreak included a devastating F5 tornado that hit the town of Jarrell, caused millions of dollars in damage, and killed 27 people. The tornado outbreak on this day was caused by a southward moving cold front in northern Texas, a warm and moist low-level jet flowing into Central Texas, extremely unstable air in the region, and multiple upward vertical motion forcings. High convective inhibition and a strong inversion at the PBL prevented convection from occurring early on, but daytime heating helped to break the cap and allow convection to ultimately occur. An analysis of model data, soundings, surface observations, radar and satellite data, and storm reports help piece together the cause of this deadly convective outbreak. __________________________ 1. Introduction Jarrell is located in the heartland of Texas. It is about 40 miles north of Austin and in 1997 boasted a population of roughly 1,300. On May 27, 1997 an outbreak of 20 tornadoes swept through Central Texas. The most powerful of these tornadoes, an F5 on the Fujita scale, ripped through Jarrell. This tornado had 260 mph winds, measured ¾ of a mile wide, and tracked across the ground for 7.6 miles. After the tornado had dissipated 27 lives had been lost and an estimated $120 million in damage had been done. This would rank as the fourth deadliest tornado of the 1990’s. A tornado with this strength and magnitude does not come along very often. The conditions must be just right for a super cell to spawn such a massive tornado. An analysis of the atmospheric conditions over Central Texas on May 27, 1997 indeed shows that the necessary conditions were there. A southward moving cold front was intersected by a southwest moving outflow boundary and by a warm and moist low-level jet from the Gulf of Mexico. The atmosphere was also highly unstable with soundings showing CAPE values over 6,000 and a lifted index near -13. The Jarrell tornado can also be tracked with radar. The National Weather Service was able to give a 15-minute warning about the tornado by looking at severe weather radar signatures. The severity of this tornado, however, proved to be to great even with this warning to limit the loss of life. 2. Data The data used to analyze this case study came largely from the ETA model from May 26-28, 1997. GARP and gempak were used to analyze the ETA model’s information and create maps and study different atmospheric variables. The ETA model proved to be pretty accurate in comparison with actual observations from the storm. The radar information and images came from a NEXRAD national composite at the time of the event, from May 27. The satellite information and images came from the GOES-8 satellite at the time of the event. Visible images with 1 km resolution were used along with infrared and water vapor images. Atmospheric soundings were taken in Fort Worth, TX, Midland, TX, Corpus2 Christi, TX, and Del Rio, TX throughout the duration of the event. These soundings helped to show the areas of instability and they were used to analyze actual atmospheric conditions throughout the event. Additionally, data archived from the SPC and NWS was also used to help derive, analyze, and explain the existence of these storms. 3. Synoptic Overview The main synoptic feature over Central Texas on May 27, 1997 was a cold front. Figures 1a-d show 850 mb frontogenesis and temperatures at 6Z on the 27th and 12Z on the 27th. These figures show the location of the cold front over Central Texas and its southward movement. A gradient of 15° C from the panhandle to Central Texas define this cold front at 12Z. The figures show frontogenesis is occurring at this time. This front is not that strong, but it does provide enough forcing to help the convection occur in the highly unstable atmosphere over Central Texas. The upper level jet streak associated with this front is not particularly strong. Model analysis shows about 50 to 60 knot easterly winds blowing over the region. Frontogenesis creates vertical motions. Martin (2006) explains that horizontal advection increases the magnitude of the temperature gradient, which subsequently causes an increase in wind shear and the jet core wind speed. The more intense jet results in an increase in vorticity. This increased vorticity suggests that there is divergence in the area, which implies there is also upward vertical motions. Figure 1: (a) May 27 6Z temperatures and wind (upper left). (b) May 27 6Z frontogenesis (upper right). (c) May 27 12Z temperatures and wind (lower left). (d) May 27 12Z frontogenesis (lower right).3 So Martin is saying that an increase in the magnitude of the temperature gradient requires the production of a vertical circulation in the atmosphere. This frontal feature is the main synoptic feature effecting Central Texas during the Jarrell tornado. Another synoptic feature was the existence of a weak long wave and trough over Texas. The trough is not an overwhelmingly dominant feature, but it does play a minor role in the synoptic forcings over Texas. The trough has only a slight curvature associated with it. Its location over Central Texas helps aid in additional upward vertical motions downstream of its axis. Ageostrophic wind divergence associated with the curvature of the long wave causes upward vertical motions. Though the curvature is not that strong, this feature is worth mentioning because it does provide another synoptic forcing in the region. It is a minor yet relevant feature in the synoptic overview. 4. Mesoscale Analysis There are multiple mesoscale features that play a significant role in the tornado outbreaks, particularly the Jarrell tornado. This presence of a low-level jet bringing warm moist air from the Gulf of Mexico into Central Texas is a very