1997 Central Texas tornado outbreak

A surface weather analysis of Texas at 2:00 p.m.[lower-alpha 1] on May 27 showing the cold front (solid line with pennants), dry line (dashed line with semicircles), gravity wave (solid line), and low-pressure area (L symbol). The alignment of these features produced the local environmental conditions enabling the tornado outbreak.

The severity of the 1997 Central Texas tornado outbreak was predicated on an unusual spatial and temporal alignment of weather features at highly localized scales. The broader environment was not particularly anomalous for the late springtime over Central and East Texas.[4] Unlike conventional tornado outbreaks in Texas, the 1997 outbreak was not associated with a strong trough of low pressure and strong density gradients in the lower troposphere.[5] Instead, the most prominent weather system over the central U.S. at the time was a distant upper-level low centered over Nebraska imparting little influence on atmospheric conditions over Texas.[4]

Consequently, winds in the mid-levels of the troposphere over Texas on May 27 were weak and westerly, ranging from 30 kn (35 mph; 56 km/h) over North Texas to below 15 kn (17 mph; 28 km/h) over Central and South Texas.[4] Winds closer to the surface were also quiescent;[4] the average storm-relative winds within the lowest 6 km (3.7 mi) of the atmosphere as measured in Del Rio, Texas, prior to the onset of storm development was only 6 kn (6.9 mph; 11 km/h).[6] Further aloft at the level of the jet stream, there were two nearby wind speed maxima over northern Mexico and the central Mississippi Valley; this led to strong divergence of air over Texas.[lower-alpha 4] Wind shear was negligible within the lowest 6 km (3.7 mi) of the atmosphere contrary to typical tornado outbreaks.[lower-alpha 5][4][10]

Despite generally meager winds, the atmosphere was nonetheless thermally energetic. The combination of several days of onshore wind flow from the Gulf of Mexico and the persistence of a capping inversion aloft led to an accumulation of moist air over the state. Dew points soon exceeded 70 °F (21 °C) within the boundary layer,[4] representing ample moisture in the lower levels of the troposphere.[11] Radiosondes launched from Corpus Christi and Fort Worth on the morning of May 27 detected the presence of a robust "elevated mixed layer" of air aloft characterized by steep lapse rates—the change of temperature with height—near the dry adiabatic lapse rate. This contributed to high convective available potential energy (CAPE) exceeding 5,500 J/kg across most of Central and East Texas,[4] denoting a highly unstable atmosphere.[11][lower-alpha 6] In some cases CAPE was as high as 6,500 J/kg.[13] The lack of fronts moving across Texas in the 36 hours before the tornado outbreak allowed this instability to remain and spread across much of the state.[14]

Although these conditions were regionally conducive to the development of thunderstorms capable of producing large hail and strong winds, the potential for supercell and tornado development was not initially clear given the lack of wind shear.[4][15]^: 4–5 Other metrics and indices used to diagnose tornado potential, such as the Energy Helicity Index, storm-relative helicity, supercell composite parameter, and the significant tornado parameter, were also lower than historically observed environments featuring tornadoes.[16] For instance, the amount of storm-relative helicity within the lowest 3 km (1.9 mi) of the atmosphere—a common measure quantifying the amount of wind shear—was measured as 70 m²/s² at Corpus Christi and reached 117 m²/s² according to data from the Rapid Update Cycle computer model; values below 300 m²/s² are correlated with weak tornadoes.[12] A study published in Monthly Weather Review in 2007 described the environment as being "marginally favorable for supercells and unfavorable for significant, supercellular tornadoes."[17]

Instead, the factors that enabled the eventual tornado outbreak may have transpired at a mesoscale involving several features becoming juxtaposed over Texas. The dry line delineating the boundary between the moist maritime air mass over East Texas and the drier continental air mass farther west remained nearly stationary in the region through the morning of May 27. On the preceding evening, a mesoscale convective system developing along this dry line over eastern Oklahoma and Arkansas generated a well-defined gravity wave that traveled southwest across East Texas on the morning of May 27.[4] Its progression was evidenced on satellite imagery as an advancing band of mid-level cumuli.[4][10] A weak low-pressure area also developed along the dry line near Dallas and tracked southwest along the boundary. In the two days before May 27, a cold front associated with the upper-level low over Nebraska had swept slowly southeast across the central U.S. Prior to dawn on May 27, the front progressed across West Texas; by 7:00 a.m. on May 27, the front spanned from southeastern Oklahoma through the northwestern portion of the Dallas–Fort Worth metroplex southwestwards to near the Big Bend of Texas. This placed the cold front parallel and just to the west of the dry line along a northeast to southwest orientation. A swath of clear skies emerged between the two boundaries, maximizing surface heating during the daytime and contributing to the weak low-pressure area's southwestward movement through Central Texas.[4]

Animation of storms developing over Texas on May 27. The animation alternates between progressing forwards and backwards in time.

At around 12:30 p.m., the gravity wave, cold front, dry line, and low-pressure area overlapped near Waco, Texas, with the faster cold front overtaking and merging with the nearly stationary dry line.[4] The cold front and dry line were brought together in a northeast to southwest fashion resembling a zipper.[18] Thunderstorms formed near Waco in tandem with this juxtaposition of features and within a local maximum of surface heating and convergence of moist air.[4] While some storms had spawned along the cold front, the gravity wave's passage in the early afternoon hours coincided with a rapid intensification of the first thunderstorms.[10] The arrival of the gravity wave and its perpendicular alignment relative to the orientation of the cold front and dry line may have enhanced wind shear locally despite weak wind shear at a broader, synoptic scale. This allowed the thunderstorms to rotate and become supercells.[4]

However, the degree to which the wave contributed to the formation of supercells and eventually tornadoes remains a subject of disagreement among meteorologists.[19] The development of new discrete supercells followed the advancing intersection of the cold front and dry line and the gravity wave as they continued southwest,[4][20] though each individual storm was nearly stationary.[21] Due to the westerly winds in the mid-levels of the troposphere, the outflow from each developing storm was directed east, allowing each new thunderstorm to develop and rotate without interference from the initial storms.[4] The direction of the storms' expansion towards the southwest deviated over 100 degrees away from these westerly winds, representing an extreme and highly unusual motion.[22] The southwestward generation of storms brought them towards increasingly unstable air with higher CAPE, allowing the storms to acquire strong inflow and the requisite rotation to produce tornadoes.[23]

There were at least three families of thunderstorms that developed as part of the initial southwestward-moving complex of storms,[24] composed of at least 16 distinct supercells; six of the cells formed along the cold front, five formed along the dry line, and five formed along the gust front imparted by previous storms.[25] The gust fronts pushed outwards by the downdrafts of the thunderstorms served as foci for tornadogenesis; boundaries like gust fronts can provide low-level vorticity and rising air necessary for the formation of tornadoes.[26] Some of the day's tornadoes were spun up by the gust front itself while others spawned from thunderstorms with preexisting mesocyclones aloft enhanced by the gust front underneath.[27]

The day's first thunderstorm near Waco in McLennan County strengthened quickly in response to the highly unstable atmosphere. A severe thunderstorm warning—the first of the day—was issued for the county at 12:50 p.m. Weather radars showed increasing rotation within the storm, prompting the issuance of a tornado warning at 1:21 p.m. This storm produced the outbreak's first tornado 5 mi (8.0 km) of southwest of Hewitt near Lorena. The same storm produced a second brief tornado near Bruceville followed by a third, intense F3 tornado in southwestern McLennan County that lasted for 20 minutes.[28] A new thunderstorm developed along the flanking line associated with the first storm. This second storm spawned several tornadoes as it expanded towards the southwest, including an F3 tornado near Lake Belton beginning at 2:25 p.m. and the F5 Jarrell tornado beginning at 3:25 p.m.[28][24][26] The NEXRAD radar in Granger used to monitor this storm suffered a power failure and went out of commission at 3:38 p.m.[29][30] A control switch left in an improper position on a prior maintenance visit prevented its emergency power supply from activating.[30] Thunderstorms developing to the southwest later in the afternoon and evening produced additional tornadoes, including the F3 tornado that struck Cedar Park and the F4 tornado that struck Lakeway.[15][26] The Lakeway tornado was the first instance of an F4 tornado striking Travis County on record.[5] The final tornado of the day touched down in Frio County and lifted at 7:23 p.m.[15]^: 2[2]^: 214

Farther south away from the southwestward-moving storm cluster, the cold front and dry line curved west, making them parallel to the advancing gravity wave; this triggered the development of numerous and intense storms between Austin and the Big Bend after 3:00 p.m. when the gravity wave intersected the front. The parallel orientations of the wave and the other boundaries precluded these storms' rotation.[4] They tracked east and eventually merged with the southwestward-propagating complex of tornadic storms along the Pedernales River. This coalescence produced a south- and southeastward-moving squall line and ended the strong tornado activity. Nonetheless, strong and damaging winds were produced by the squall line for several hours. Kelly Air Force Base in San Antonio documented a wind gust of 106 kn (122 mph; 196 km/h), marking a record high wind velocity for the site.[31]

Jarrell tornado

F5 tornado
View of the Jarrell tornado as it moved near the Double Creek Estates area
Duration	13 minutes
Max. rating1	F5 tornado
Fatalities	27 fatalities, 12 injuries
Damage	$40.1 million
1Most severe tornado damage; see Fujita scale

Parts of Jarrell, Texas, were struck by an extremely powerful F5 tornado on the afternoon of May 27.[5][29] The tornado destroyed approximately 10 percent of the homes in Jarrell;[62]^: A1 at the time, the city had a population of about 450 people and had been previously struck by tornadoes in 1987 and 1989.[63]^: A1[64]^: A10[65] Hardest-hit was the Double Creek Estates subdivision west of downtown Jarrell.[15]^: 6[66]^: A19 The 1997 Jarrell tornado was the first and only known occurrence of an F5 tornado in Williamson County.[5] It was also the deadliest tornado in Texas since the 1987 Saragosa tornado.[67] The thunderstorm that spawned the Jarrell tornado began west of Temple along the flanking line of another thunderstorm earlier in the afternoon of May 27. The storm produced several tornadoes in Bell County, including the F3 tornado that impacted communities along Lake Belton.[28] Weather radar observed a strengthening mesocyclone within the thunderstorm, with the speed of rotation rising above 40 kn (46 mph; 74 km/h).[29] As the storm moved into Williamson County, it produced two short-lived F2 tornadoes north of Jarrell at 3:25 p.m. and 3:35 p.m.; the latter of the two was a multiple-vortex tornado and lifted at 3:39 p.m.[45][46][15]^: 5 The Austin/San Antonio National Weather Service forecast office issued a tornado warning for Williamson County at 3:30 p.m. in response to the storm's approach; the warning was put into effect for one hour. This was the first tornado warning of the day issued for the office's warning area and warned that "the city of Jarrell is in the path of this storm."[68] Local warning sirens went off about 10–12 minutes before the tornado struck.[69]

The precise start of the Jarrell tornado was difficult to pinpoint.[29] The most prominent and destructive part of the tornado's evolution was preceded by the apparition of short-lived, small, and rope-like funnel clouds. These may have been separate tornadoes or simply an earlier part of the Jarrell tornado's evolution.[40]^: C3 An aerial survey conducted by the Birmingham, Alabama, office of the National Weather Service included the damage caused by the earlier F2 tornadoes—mostly to trees and roads—as part of the overall Jarrell tornado path.[40]^: C3 Some reports also include the F1 tornado near Prairie Dell as an earlier continuation of the Jarrell tornado.[26] The final, unambiguous apparition of the Jarrell tornado began as a narrow and rope-shaped funnel when it touched down at 3:40 p.m. just within the Williamson County line 3 miles (4.8 km) north of Jarrell.[48][15]^: 6 Like the two F3 tornadoes earlier in the day, it developed along the gust front produced by its parent thunderstorm.[26] This mechanism is typical of tornadogenesis not associated with supercell thunderstorms.[70] Traffic along Interstate 35 came to a stop as the tornado descended nearby.[62]^: A1 The Texas Highway Patrol also stopped traffic on both sides of the interstate under the expectation that the tornado would cross the highway; it ultimately moved parallel to Interstate 35.[71] Tracking south-southwest,[29] the tornado quickly intensified and grew to a 1⁄2 mile (0.80 km) in width, changing from its initial thin and white appearance to a blue and black color.[15]^: 6[62]^: A1 F5 tornado damage was identified early in the tornado's path.[40]^: C3 Its intense winds scoured the ground and stripped pavement from roads.[15]^: 3 The tornado tore 525 feet (160 m) of asphalt as it crossed County Roads 308, 305, and 307;[15]^: 6 the thickness of the asphalt pavement was roughly 0.8 inches (20 mm).[72] A culvert plant at the corner of Country Roads 305 and 307 collapsed. Nearby, a similar plant and a mobile home sustained some damage, with the latter struck by a 2×4 piece of lumber. The occupants of a mobile home 500 feet (150 m) north-northwest of the culvert plant fled to a frame house that the tornado later struck; the evacuees were killed while the mobile home sustained only minor damage.[73][63]^: A21 Some of the most extreme damage at this location was inflicted to a small metal-framed recycling plant that was obliterated, with little left of the structure besides a few twisted structural beams.[74]

The Double Creek Estates subdivision was pulverized by the tornado, with many homes swept off their foundations and disintegrated. This photograph shows one remaining foundation with plumbing pulled out of the concrete.

The tornado then slowly entered the Double Creek Estates subdivision where it exacted its most catastrophic impacts. Concurrently, the tornado expanded further to its maximum width of 3⁄4 mile (1.2 km).[15]^: 6 Eyewitnesses indicated that the tornado's movement slowed to around 5–10 mph (8.0–16.1 km/h) as it entered the neighborhood; this may have contributed to the resulting extreme destruction.[15]^: 7 The tornado destroyed the first home it encountered at the northwestern corner of the subdivision; a clock recovered from the remaining debris was stopped at 3:48 p.m., presumably marking the time the tornado entered the community.[15]^: 6 Much of the neighborhood was completely swept away with little debris remaining, with what was left being reducing to small and unrecognizable fragments that were dispersed over a wide area. The lack of large items that were recovered, and the granularity of the debris was indicative of the sheer strength of the tornado.[15]^: 7 Due to the tornado's slow movement, homes near the center of its path experienced tornadic winds for approximately three minutes.[1] The mostly wooden-framed residences, some well-built and anchored, were completely obliterated and swept away, leaving behind concrete slab foundations swept clean of all debris.[73][2]^: 6 In some cases, parts of outbuilding and house foundations themselves in the subdivision were scoured away, and several were found missing all of their sill plates that connected the wood-frame homes to the foundations.[75][76] Pieces of debris were found deposited in fields miles away from the subdivision, and extreme ground scouring occurred, reducing grassy fields into wide expanses of mud in the most severely affected areas.[2]^: 6[64]^: A10[77] In some cases the ground was scoured out to a depth of 18 inches (46 cm).[19] Vehicles in the neighborhood were tossed and mangled beyond recognition; at least six were found flattened in open areas and coated with mud and grass.[15]^: 7[2]^: 5 Some were thrown as far as a 1⁄2 mile (0.80 km) away, and others were torn into multiple pieces of unrecognizable metal.[78] Trees in the neighborhood were completely denuded and stripped entirely clean of all bark as well, including one that was found with an electrical cord impaled through the trunk.[15]^: 7[2]^: 5[79]

All 27 fatalities associated with the Jarrell tornado occurred at Double Creek Estates,[15]^: 7 which at the time consisted of 131 residents living in 38 single-family homes and several mobile homes.[29][80]^: A20 Entire families were killed at Double Creek, including all five members of the Igo Family, all four members of the Moehring family, and all three members of the Smith family.[81] Bodily remains were later found at 30 locations, and the physical trauma inflicted to some of the tornado victims was so extreme, that first responders reportedly had difficulty distinguishing human remains from the remains of animals at the site.[82][83] Most of the deaths were attributed in the Morbidity and Mortality Weekly Report to multiple trauma, though one death was attributed to asphyxia.[3] The high intensity of the Jarrell tornado left those in its path with little recourse;[84] most homes in Double Creek Estates were built on cement slab foundations and few had a basement or any form of storm shelter;[2]^: 6 nineteen people sought refuge in a single storm cellar.[63]^: A21 Some residents who followed prescribed safety measures nonetheless perished.[85] One survivor holed up in a bathtub and was flung several hundred feet from her house onto a road.[63]^: A21 The walls of some homes along the periphery of the tornado path remained intact, protecting some of those who survived the tornado.[84] Others chose to evacuate ahead of the approaching tornado.[85] Forty structures were obliterated in Double Creek Estates.[15]^: 7 Three businesses adjacent to Double Creek Estates were also destroyed.[29] In total, the tornado dealt $10–20 million in damage to the neighborhood.[80]^: A20 Around 300 cattle grazing in a nearby pasture were killed and some were found 0.25 miles (0.40 km) away.[15]^: 7[2]^: 5 Hundreds of cattle were also dismembered and a few cows were also skinned by the tornado.[78][66]^: A19

The tornado turned slightly towards the south-southwest after traversing Double Creek Estates.[15]^: 7 The damage in these outlying areas was somewhat scattershot; in one case, a mobile home suffered only minor damage while an adjacent house lost half of its roof.[86] Metal buildings were unroofed along County Road 305 south of Jarrell. The road's guardrail was impaled by wooden planks thrown by the strong winds.[62]^: A12 The tornado then again crossed County Road 305 and entered a forest of cedar trees.[87] Some of the damage to the trees suggested that the tornado may have been a multiple-vortex tornado, which was documented by Scott Beckwith, receiving a nickname as a “dead man walking”.[88][89] The path of damage left behind by the tornado ended abruptly shortly after entering this forested area, with the National Centers for Environmental Information indicating that it lifted at 3:53 p.m. after remaining on the ground for 13 minutes and 5.1 miles (8.2 km).[48][15]^: 7 Other government accounts of the tornado list a total path length of 7.6 miles (12.2 km) after incorporating the preceding tornadoes north of Jarrell and near Prairie Dell.[29][40]^: C3 Between May 29 and June 1, the Office of the Federal Coordinator for Meteorological Services and Supporting Research carried out aerial and ground surveys of the tornadic damage in Texas in coordination with the Texas Wing Civil Air Patrol.[90] The Jarrell tornado damage was classified as F5 severity throughout most of the tornado's path.[40]^: C3 However, a critique of the Fujita scale published by the National Institute of Standards and Technology suggested that winds between 158–206 mph (254–332 km/h), corresponding to an F3 rating on the scale, were sufficient to explain the damage wrought by the Jarrell tornado.[lower-alpha 8][92] The critique noted that some of the homes at Double Creek Estates, although built within the preceding 15 years, exhibited structural weaknesses in their design such as the lack of anchor bolts and steel straps in their foundations.[76] Approximately $40 million in damage was inflicted upon property with another $100,000 inflicted upon crops. Twelve people were injured by the storm in addition to the twenty-seven killed.[48]

Due to the unusual southwestward motion of the thunderstorm that caused the tornado, the sequence of weather events experienced by those affected was in the opposite order of typical tornadic events: the tornado arrived first, followed by the hail, wind, and rain of the parent thunderstorm.[15]^: 7 Despite the violence of the tornado and the presence of its associated mesocyclone aloft, the thunderstorm did not exhibit a distinct hook echo on weather radar typically associated with such tornadoes. This may have also been caused by the unusual southwestward motion of the thunderstorm, resulting in the tornado's placement in an atypical position relative to the thunderstorm's motion.[29]