1988 Hurricane Gilbert Tornadoes in Texas

An outbreak of forty tornadoes occurred across Texas on September 16-17, 1988 as the remains of Hurricane Gilbert tracked west-northwest across the Mexican states of Nuevo Leon and Coahuila. Fifteen of these occurred across South-Central Texas. While the majority of the tornadoes were weak, two F1 and two F2 tornadoes would touch down prior to the event’s conclusion, causing considerable damage across portions of the Del Rio and San Antonio metropolitan areas. One fatality tragically occurred in Bexar County, with nine additional injuries reported in Bexar and Val Verde Counties. The outbreak left a significant financial mark, incurring over 55 million dollars (1988 USD; approximately 142 million dollars when adjusted for inflation in 2023) in damages across the region. This story map recaps the timeline and meteorology of the costly Hurricane Gilbert tornado outbreak, in addition to sharing photographs of the event’s aftermath.

Research has shown that the majority of tropical cyclone tornadoes occur in the right front quadrant of systems (image A). This is speculated to be a result of an overlap of wind shear and instability—ingredients necessary for tornado formation in all settings—that is typically found in this area of tropical cyclones. The Hurricane Gilbert tornado outbreak conforms well to these observations, as all of the aforementioned ingredients were in place throughout the duration of the event. We walk you through said ingredients below.

1.) Sufficient Instability For Thunderstorms

Reanalysis data shows that abundant amounts of instability, or convective available potential energy (CAPE), were present across the state of Texas throughout the day on September 17. This instability undoubtedly helped to fuel bands of showers and thunderstorms—some of which were responsible for the development of tornadoes across South-Central Texas—within the outer reaches of Gilbert from early morning through the evening hours.

While there were likely several contributing factors to the instability, the distribution of CAPE appeared to be linked to a tropical, high equivalent potential temperature air mass advancing inland from the Gulf of Mexico. While complicated in name, equivalent potential temperature (theta-e) serves as a proxy for how moist a particular air mass is. Greater equivalent potential temperature implies that higher amounts of moisture are present, which tells us as meteorologists that an air mass may be trending more unstable since moist air has a tendency to rise. This concept is reflected nicely in side-by-side comparisons of the CAPE and theta-e fields, which show progressively higher amounts of instability pushing inland in tandem with a high theta-e air mass throughout the day of the tornado outbreak. These values of CAPE and theta-e likely helped to fuel development of the showers and thunderstorms responsible for the tornadoes occurring over the Coastal Plains, I-35 Corridor, and Rio Grande Plains throughout the day on September 17:

2.) Wind Shear To Keep Storms Organized

Wind shear, or changing wind speeds and/or direction with height, helps thunderstorms organize once they’ve formed. The presence of shear increases the longevity of a storm, as it helps to reduce the detrimental effects of melting and evaporation. Melting and evaporation act to cool the air within an ongoing thunderstorm, promoting downward vertical motion. The downward motion offsets the instability fueling the storm’s updraft. In the absence of shear, the sinking motion eventually wins out, leading to dissipation of the thunderstorm. If shear is present, however, cooling effects are pushed away from the storm’s updraft, allowing for upward and downward movement to coexist without compromising storm health. Sufficient amounts of shear were present across South-Central Texas on September 17, as indicated by reanalysis data:

Sufficient amounts of shear were in place thanks to strong low level winds linked to Gilbert’s wind field:

In addition to contributions from the mid-levels of the atmosphere as well. Wedged between Gilbert’s circulation over Coahuila and subtropical high pressure centered over the Gulf of Mexico, a jet of enhanced mid-level winds oriented over the Rio Grande and Edwards Plateau likely helped contribute to the favorable environment in addition to the low level wind field.

3.) Turning Winds To Help Storms Rotate & Produce Tornadoes

The final piece needed for thunderstorms to produce tornadoes is turning winds with height. Differing wind speeds with height allow storms to organize, but their ability to rotate and potentially produce tornadoes depends on vertical changes in wind direction. This vertical turning is quantified through a parameter called storm-relative helicity (SRH). SRH is commonly used by meteorologists to quickly assess the potential for tornado production in an environment. As SRH increases, so does the rotation and tornado potential in thunderstorms. While no single threshold exists to distinguish a non-tornadic environment from a tornadic one, it is generally accepted from research that tornado potential increases as SRH values climb above 200-250 m2/s2. These values were met and exceeded through the duration of the tornado outbreak, with reanalysis showing values at or above 200 m2/s2 from predawn through the early evening hours on September 17: 

SRH was well-sampled in the 00Z September 18 DRT (7:00 PM September 17) sounding, which showed easterly low level winds turning to nearly due southerly in the mid-portions of the atmosphere. It is thus unsurprising that storms were able to readily produce tornadoes across the region on September 17: