More than two decades after the event, some aspects of the 1997 F5 tornado at Jarrell, Texas remain enigmatic. One of the frequent questions concerns the unusual south-southwest movement of the parent supercell and the tornado. I have been working on these issues and will be sharing that information here in this and a subsequent blog. At the outset, I will tell you this: the south-southwest motion of the storm was not because of a gravity wave. But first, let me set the stage by recalling the weather situation on May 27, 1997.
The S.P.C. Convective Outlook had much of Central Texas in a moderate risk for severe thunderstorms.
The southwestern anchor point of the moderate risk area was Austin. Surface analysis at 7am CDT (Fig. 2)found a weak, slow-moving cold front over western Central Texas. East of the cold front, a quasi-stationary boundary (defined best by surface dewpoints) was located just west of IH-35 from near Dallas to west of Waco to southwest of Austin. This boundary was likely the dryline from the previous afternoon. (This feature was not analyzed on the NCEP map shown.)
The weak cold front was approaching the dewpoint boundary from the northwest. Immediately east of the dryline boundary, surface dewpoints were pooling and were unusually elevated for Central Texas, with readings from 75 to near 80 deg F. West of the boundary, the surface dewpoints were much lower. As the surface cold front moved slowly east and southeast, it eventually began overtaking and merging with the ‘day-old’ dryline, first near Dallas, then gradually overtaking it farther south.
During the morning, the air west of the day-old dryline warmed more rapidly while the moist air east of the boundary warmed, but less quickly. The initial radar detection of the developing storm as seen on the New Braunfels WSR88D radar at 12:01pm CDT. Figure 3 shows the developing storm cell near Lake Waco, drifting southward.
The storm developed rapidly in an environment characterized by CAPE (convective available potential energy) of at least 5,000 J kg-1, but perhaps as high a 7,000 J kg-1. My first view of the developing storm was from 27 miles to the south in Temple, and my first impression (as vivid today as when it happened) was that it looked like an atomic bomb had exploded. The developing cumulonimbus was utterly erect and looked like it was boiling as it expanded vertically and laterally. Except for a few small cumulus clouds elsewhere, the developing storm dominated the northern quadrant of the sky.
The NWSFO in Fort Worth issued a severe thunderstorm warning about 12:50pm, stating that the storm was moving slowly toward the east-southeast. If so, that was a very brief motion. Only minutes later, the Storm Prediction Center issued a Tornado Watch that included Bell and McLennan counties and extended far to the east to Shreveport and Texarkana. By the time I approached the storm not long after 1pm CDT, it was moving slowly toward the south-southwest and would continue that motion for the ensuing three hours. The thunderstorm warning was upgraded to a tornado warning about 1:15pm as I encountered the first tornado of the day. It was about midway between Moody and Lorena, near the Spring Valley community. As I watched from a mile away, the tornado destroyed a mobile home. Figure 4 shows the tornado immediately before it began roping-out. Damage from that tornado was rated F2.
That first tornado occurred 43 miles north-northeast of Jarrell. The ‘parent’ supercell produced at least five more tornadoes prior to the devastating tornado at Jarrell. In sequence, the tornadoes prior to Jarrell were: a) one mile east of Moody (F3 damage), b) at Morgan’s Point Resort on Lake Belton (F3 damage), c) simultaneous with (b), a tornado (not surveyed) about 3.7 miles west of Belton Dam on the Fort Hood military reservation, d) a tornado immediately southeast of Stillhouse Hollow Dam (southwest of Belton), and e) a tornado at Prairie Dell, four miles north-northeast of Jarrell.
Figure 5 shows the Lake Belton tornado, Figure 6 shows the small tornado at Prairie Dell, Figure 7 shows the multi-vortex stage of the Jarrell tornado as it was developing, and Figure 8 shows the intensifying tornado over the town of Jarrell.
The Jarrell tornado was extremely powerful, and also moved very slowly. As a result, anything in its path was subject to the very strong winds for a longer period than had the tornado been moving faster. Asphalt was stripped from roadways, site-built houses were demolished and debris removed from the foundations, cattle and horses were dismembered and rendered unrecognizable. Twenty-seven people died and twelve were injured, many very seriously. A reasonable conclusion from these casualties is that if you were in the path of that tornado, the odds were that you would die.
After the Jarrell tornado had become rain-wrapped, another brief tornado occurred between Jarrell and Georgetown, and then two additional strong to violent tornadoes occurred, one at Cedar Park (F3) and one between Lakeway and Bee Cave, west of Austin (F4). This latter tornado occurred more than 80 miles south- southwest of the location of the first tornado southwest of Waco. Additional details about the events on May 27, 1997 can be found in the NOAA Service Assessment located at this URL: https://www.weather.gov/media/publications/assessments/jarrell.pdf
As mentioned at the outset, my goal is to discuss the south-southwest motion of the parent storm and the tornadoes it produced. What may surprise you is that there are other documented cases involving similar supercells and related tornadoes that have exhibited similar motion in or near Central Texas. If you are a regular reader here, you probably read my recent blog about the two simultaneous tornadoes in Austin, Texas in May of 1922 (Read that post here), both of which moved toward the south-southwest. By now I expect many of you are wondering why these tornado-producing storms moved toward the south-southwest. In the next blog on this topic, I will discuss other similar tornado events and delve into what might cause unusual supercell-tornado motion.
Lon Curtis Bio: Although educated as an attorney, and serving 24 years as a Texas prosecutor, Lon Curtis pursued his passion for weather as a serious avocation before realizing his dream of becoming an on-air meteorologist at KWTX-TV (Waco) in 1997. That opportunity came about, in part, because of his role in storm spotting during a tornado outbreak in May of 1997, including the tornado at Jarrell that killed 27 people. Lon is a member of the American Meteorology Society and the National Weather Association, and holds the AMS Certified Broadcast Meteorologist certification (on inactive status since his retirement from broadcasting in 2011). Lon authored a peer-reviewed research article published in the AMS journal Weather and Forecasting (2004), on mid-level dry air intrusions as a factor in tornado outbreaks associated with landfalling tropical cyclones. Since retiring, he has continued his research interest in severe convective storms, and is building a database of all tornadoes in Texas since 1880 that have caused one or more deaths. He also maintains a personal website on weather conditions and hazards affecting Central Texas.