As mentioned in my previous blog on the 1997 Central Texas tornado outbreak, a question still frequently asked is why the storm system moved toward the south-southwest for over 3.5 hours. In the aftermath of the 1997 events, this question (including the influence of the gravity wave) has led me to hours and hours of research, and in this blog I will share what I have found. At the outset, I will tell you this: the south-southwest motion of the storm and the tornadoes that it spawned were not a direct result of a gravity wave, although it is clear that a gravity wave moved across the area where the stalled dryline feature was located before the initial tornadic supercell formed near Waco.
The AMS Glossary provides a technical definition of a gravity wave: “A gravity wave is a wave disturbance in which buoyancy (or reduced gravity) acts as the restoring force on parcels displaced from hydrostatic equilibrium.”1 Put in simpler terms, a gravity wave is a result of a force that has disturbed the effect that gravity exerts on the Earth’s atmosphere. Think about tossing a pebble into an undisturbed water pond. The impact of the pebble disturbs the fluid (water) which reacts by generating waves that move outward away from where the pebble entered the water. Gravity waves in the atmosphere are generated by something strong enough to temporarily disturb the gravitational force that holds the atmosphere in place against the Earth. The gravity wave on May 27, 1997 was generated by a cluster of intense thunderstorms occurring in eastern Oklahoma and western Arkansas during the preceding evening and overnight period. The gravity wave then moved toward the south-southwest across Texas during the morning and early afternoon hours.
Gravity waves can be seen on satellite imagery, and often also detected by the WSR-88D radars, but they do not have the ability to directly influence storm motion. At the most, gravity waves may aid in storm initiation by accelerating the removal of a capping inversion that might otherwise delay storm development for several additional hours. On May 27, 1997, atmospheric conditions in Central Texas were very unstable and primed for explosive storm development, with or without the gravity wave passing over the area. You can find further discussion regarding the role of that gravity wave in the sequence of events in a conference paper written by Stephen Corfidi, who was a forecaster at the Storm Prediction Center on that day.2 In a recent private communication to me, he confirmed that “erosion of the cap likely was the primary role that the gravity wave played”.
Having said that, we are still left with the question of why the storm moved toward the south-southwest for several hours, and produced several strong or violent tornadoes. In the aftermath of the 1997 tornado outbreak, many theories were proposed, including the influence of the gravity wave mentioned above. If you are inclined to read additional research on the movement of the 1997 “Jarrell” storm, I recommend that you read the papers published in the Monthly Weather Review (an AMS journal) by Adam Houston and Robert Wilhelmson. Two articles were published in 20073,4, and one in 20125. (These are available without charge online even if you aren’t an AMS member; the not so good news is that they are very technical.) Houston and Wilhelmson created computer simulations to examine various possibilities of storm initiation and storm motion using data from May 27, 1997. They found that the intersection of the stalled dryline and the slowly-approaching cold front (in the presence of extreme instability) led to a “zippering” effect (their terminology) in which the intersection slowly moved south-southwest along the dryline. That’s where the supercell storm initiated, and then moved south-southwest in concert with the cold front-dryline “zipper”. While their research likely explains the evolution of the May 27, 1997 event, a question that remains is whether it may also account for other cases of extreme south or southwest supercell deviation.
It is important to remember that supercell storms are not “entities”, but rather are ongoing physical processes that are continuously evolving in time and space. The maintenance of a supercell storm requires a vigorous, uninterrupted updraft that ingests warm, moist, unstable air into the storm. In weak mid-level flow regimes, supercells often appear to be moving “hard” to the right, toward the area where the most unstable air is typically located. Within a couple of years of the 1997 tornadoes, two additional events occurred in Central Texas where supercell thunderstorms moved toward the south-southwest and produced strong to violent (and deadly) tornadoes. One event occurred in 1999 and the other in 2000, and both occurred in May. The common denominators shared by those events and the 1997 outbreak are 1) the presence of weak low-level boundaries oriented north-northeast to south-southwest, very high levels of convective available potential energy (CAPE), and relatively weak mid-level flow (typically from the west or southwest). In both cases, there was evidence of an intersection of a weak cold front and a dryline at or near the point of storm initiation.
May 11, 1999 – West Central Texas
After producing several weak, short-lived tornadoes near San Saba, a large supercell emerged from a cluster of strong storms (some with radar signatures of rotation), and moved toward the south-southwest, producing a large, violent tornado (damage rated F4) in Mason County, and a subsequent strong tornado (damage rated F3) in Gillespie County at Harper. The total path length of this storm was ~55 miles. One death and several injuries occurred near Loyal Valley, in Mason County. Although there were no deaths reported at Harper, damage to property was estimated at one million dollars.
May 12, 2000 – North Central Texas
A supercell thunderstorm developed over Lake Whitney near the intersection of a cold front and dryline. The tornado developed over the lake, 25 miles northwest of Waco, and caused two deaths as it moved ashore in Bosque County on the west side of the lake. The most significant damaged occurred at the Lakewood Harbor Subdivision, three miles northwest of the Whitney Dam, where 38 homes were destroyed and 27 others were damaged. Damage was estimated at three million dollars. Although only one tornado occurred, the supercell continued moving south-southwest for 50 miles to near Killeen in Bell County, attended throughout its journey by a well-defined wall cloud, and radar-indicated updraft rotation. The storm also produced hail as large as baseballs (2.75″ diameter) at Clifton in Bosque County and as large as golf balls (1.75″ diameter) at several locations in western Bell County.
I have accumulated considerably more information in this general area and I am currently working on an oral presentation for the Texas Weather Conference (Sept. 21-22, 2018) in Arlington, Texas. The title of that presentation is “Texas Tornadic Supercells Exhibiting Highly Deviant Motion”. I have created a webpage at my personal website where I will be adding additional information and graphics on this general topic as I prepare for the conference. You are invited to follow my work there: http://centxwx.net/Deviant_Supercell_Motion.htm
1 American Meteorological Society, cited 2018: Gravity wave. Glossary of Meteorology. [Available online at http://glossary.ametsoc.org/wiki/Gravity_wave ]
3 Houston, A.L. and R.B. Wilhelmson, 2007: Observational Analysis of the 27 May 1997 Central Texas Tornadic Event. Part I: Prestorm Environment and Storm Maintenance/Propagation. Mon. Wea. Rev., 135, 701–726, https://doi.org/10.1175/MWR3300.1
4 Houston, A.L. and R.B. Wilhelmson, 2007: Observational Analysis of the 27 May 1997 Central Texas Tornadic Event. Part II: Tornadoes. Mon. Wea. Rev., 135, 727–735, https://doi.org/10.1175/MWR3301.1
5 Houston, A.L. and R.B. Wilhelmson, 2012: The Impact of Airmass Boundaries on the Propagation of Deep Convection: A Modeling-Based Study in a High-CAPE, Low-Shear Environment. Mon. Wea. Rev., 140, 167–183, https://doi.org/10.1175/MWR-D-10-05033.1