1 JOURNAL OF AOS SPRING-2007 CASE STUDY 3: JARRELL TEXAS TORNADO OUTBREAK ON MAY 27, 1997 John Paul Argenti Department of Atmospheric and Oceanic Science, University of Wisconsin-Madison, Madison, Wisconsin ABSTRACT This Case Study is focused on a supercell event that occurred over central Texas on 27 May 1997, which ultimately spawned an F5 tornado in spite of a forecast for no tornadoes on this day. Emphasis will be given to the synoptic pattern and mesocale mechanisms that contributed to the event. Model, Observational, Radar, and Satellite data primarily centered around 18Z 27 May 1997 are presented to help in this process. Surface convergence, upper level divergence, extreme CAPE values, and an MCS generated gravity wave are among the key ingredients that allowed for a deadly and unforeseen event to transpire. _________________ INTRODUCTION jet streak located above 300mb moved eastward and oriented itself in a position On the afternoon of 27 May 1997 an putting the right entrance region over the unusual supercell outbreak took place area affected by the supercells. This across central Texas. Unusual in that the may have created some upward vertical synoptic weather pattern lacked some of motion and divergence aloft. Supercells the classical features associated with typically require many favorable severe weather in the Texas region conditions all-occurring simultaneously during this time of year. The event took in order to be produced. This paper will place after a forecast for just severe attempt to provide reasonable evidence thunderstorms rather than tornadoes and and arguments as to why the Jarrell supercells. Extremely high CAPE and Tornado event occurred despite what low environmental shear characterized appeared to be an absence of critical the environment associated with the features such as low-level shear and an tornado outbreak. A stationary boundary uplift mechanism. that was present may have allowed for a There were 34 reports of severe long-lived rotating supercell to develop weather including 12 for large hail and in the absence of the environmental 22 for tornadoes from this event. The shear. most violent (F5) tornado that was Radar and Satellite imagery both spawned occurred near the town of show that the supercells did form along Jarrell in Williamson County TX the stationary boundary associated with causing 27 of 30 total fatalities during a weak cold front, dry line, and strong the outbreak. moisture convergence at the surface. A Data from the storm are summarized 2 JOURNAL OF AOS SPRING-2007 in section 1 and listed throughout this forecast for 18Z 27 May 1997. The two- paper in figures where applicable. The panel plot showing 250mb isotachs and overview of the synoptic pattern will be wind barbs in one panel and 250mb presented in section 2, with specific streamlines and divergence in the other emphasis on the distributions of panel was created using GARP and moisture, instability, divergence, Meso-Eta model 6-hour forecast for 18Z boundaries, and large-scale wind 27 May 1997. The four-panel plot of patterns. The mesoscale convective Upper Air soundings taken over Fort initiation and the genesis of the Jarrell Worth, TX and Corpus Christi, TX supercell are presented in section 3. represents data from 0Z 27 May to 0Z 28 Section 4 summarizes the important May 1997 and was taken from the atmospheric factors that produced this University of Wyoming sounding unusual supercell event. archive. The cross-section plot of mixing ratio values between Norman DATA OK, Fort Worth TX, Corpus Christi TX, and Brownsville TX was created using The data presented in this case study upper air sounding text data from those has been gathered from several sources. cities respectively taken from the The four-panel plot of Synoptic scale University of Wyoming sounding weather at key atmospheric levels (300, archive for 12Z 27 May, 1997. The 700, 850, 850-500 mb) represents Eta four-panel plot of GOES-8 visible Model analysis for 12Z 27 May 1997 satellite imagery was taken from and was copied from the Unisys Weather archived data via the GARP program Archive. The Miller Diagram was hand showing data from 1615Z to 1915Z over drawn using Upper Air Sounding Texas at 4km resolution. The four-panel Observations at levels of 300, 500, 700, plot of radar reflectivity images from the and 850mb from the GARP program at Nexrad site at New Braunfels, TX 12Z 27 May 1997. The four-panel plot covering 2043Z to 2100Z was taken of the surface dryline, frontal boundary, from a website related to the NWS surface low, surface observations, and covering the Jarrell Tornado and gravity wave position from 16Z 27 May contains the time frame of the Jarrell F5 to 19Z 27 May 1997 was taken from a tornado touchdown at 2048Z. figure on a paper written by Stephen F. Corfidi, further noted in references. The SYNOPTIC OVERVIEW two-panel plot displaying wind barbs and moisture convergence at 975mb as On 12Z 27 May a weak upper level well as a streamline analysis at 975mb trough was centered over the central was created using the Eta model 6-hour plains states accompanied by a jet streak forecast for 18Z 27 May. The 500mb oriented southwest to northeast over streamline and temperature advection north Texas through Missouri. This plot over Texas was created using GARP orientation of the jet put north central and the Meso-Eta model 6-hour forecast Texas under the right entrance region for 18Z 27 May 1997. The surface to typically known to allow for ascent at upper atmosphere plot of Convective mid levels and divergence aloft, two Available Potential Energy was created conditions favorable for a supercell using the Meso-Eta model 6-hour outbreak. A 12Z 27 May Eta model 3 JOURNAL OF AOS SPRING-2007 Figure 1) Four-panel plot from the 12Z 27 May 1997 Eta Model analysis of a) 850 mb Temperature (C), Geopotential Heights (m), wind velocity (arrow), b) 300 mb wind speed (knot), Geopotential Height (m), wind velocity (arrow), c) Surface pressure (mb), 1000-500mb thickness (m), and d) Lifted Index. analysis of several synoptic scale convection for a few days and produce a features is shown in Fig 1 to highlight loaded gun sounding of warm moist air the favorable conditions for supercells beneath an EML that only needed a mentioned. In the first panel of Fig 1 trigger (vertical motion) to produce (top left) we can see the jet streak north convection. of Texas, and on the southeast side of a The third panel of Fig 1 (bottom left) trough at 300mb. The wind vectors at shows a forecast of very warm this level indicate there is divergence temperatures from Del Rio up toward south of the right entrance region. The Dallas Texas that relates to the cap second panel of Fig 1 (top right) inversion mentioned earlier having showing 700mb vertical velocity suppressed convection for a few days. confirms that upward vertical motion is The fourth panel of Fig 1 (bottom right) occurring over central Texas underneath displays the lifted index (LI), which is a the area of divergence. These features useful indicator of the potential for are very important considering Texas severe weather. Values below -6 are had an onshore flow pumping moisture usually taken to represent a significant and high temperatures across the surface risk of severe weather and/or supercells. beneath an elevated mix layer (EML) Values over central Texas at 12Z range cap (Corfidi 1998). The cap was from -6 to -12 in the Eta model and this probably strong enough to suppress analysis was done for 7am before 4 JOURNAL OF AOS SPRING-2007 Figure 2) Miller Diagram for 12Z 27 May 1997. Purple color is 250mb, Blue color is 500mb, Brown color is 700mb, and Red/Green colors are 850mb. Used as a 3-D aide for thinking about the environmental set up for a severe weather event. daytime heating has a chance to decrease the lower atmosphere whatsoever. Shear these values. The same panel shows a is widely considered a necessary layer of air between 850 and 500mb with ingredient for supercell development, low relative humidity covering Texas making the formation of supercells on 27 indicative of an EML and remaining May an eye-opener in many respects. presence of the cap inversion that can A good way of piecing together how a produce severe weather. severe weather event occurred is to The wind patterns at all levels of the combine features throughout the atmosphere at 12Z 27 May were atmosphere into one visual aide. Fig 2 is somewhat atypical for a tornadic a hand drawn Miller Diagram that gives supercell event, except at jet-stream a 3 dimensional picture of the level. There was not a well-defined low- atmosphere above north central Texas at level jet the day of the outbreak, rare 12Z 27 May, five hours before the first considering the spawning of a F5 thunderstorm kicked off. This Miller tornado. Winds on the surface were Diagram contains features from 250, generally onshore, while at 850 they 500, 700, and 850mb that are color were disorganized and weak. At 700 coded in the figure. At upper levels the and 500mb winds were out of the west flow is southwesterly with divergence across Texas and rather weak. In fact immediately to its south and east over one of the unique aspects of this case north Texas. At 500mb and 700mb there study was the lack of any strong shear in is straight-line westerly flow along with 5 JOURNAL OF AOS SPRING-2007 Figure 3) Four-panel plot covering 16-19Z respectively 27 May 1997 showing the evolution of a frontal boundary, dry line, gravity wave, and surface low in conjunction with surface observations.