October 1998 Texas Flood

Multiple rounds of showers and thunderstorms resulted in historic rainfall accumulations across South-Central Texas between October 17 and 20, 1998. Widespread 16-20 inch totals were reported along the I-35 Corridor between San Antonio, New Braunfels, and San Marcos, and along the I-10 Corridor between San Antonio, Seguin, and Gonzales. Rainfall totals approaching and exceeding two feet were reported from New Braunfels northeast into far southern Hays County and far northern Guadalupe County. A post-event survey would confirm a storm-total accumulation of at least 30 inches at one observing station in far southern Hays County.

While several rounds of showers and thunderstorms occurred over the multiple day period, the majority of these impressive totals were observed from pre-dawn through the afternoon of October 17, when showers and thunderstorms trained repeatedly over portions of the I-35 Corridor and Coastal Plains.

The heavy rainfall resulted in catastrophic flash and river flooding over the impacted areas. An estimated $750 million in property damages were reported as a result of the flooding, which would total to around $1.4 billion—with inflation adjustments in 2023. A total of 4,000 structures were determined to be total losses in the wake of the flood waters, with an additional 12,000 reporting damage. Twenty-nine deaths tragically occurred, with 24 four the result of drowning. This story map documents the timeline, meteorology, as well as photos & videos of this truly historic and tragic South-Central Texas weather event.

Early Morning - Late Afternoon, October 17: The most impactful round of heavy rainfall unfolds as showers and thunderstorms form along the Balcones Escarpment from San Antonio to points northeast along the I-35 Corridor. Rain rapidly begins to accumulate as storms repeatedly train over the same locations. The situation starts to become dire in some locations by the afternoon, as swift water rescues are reported and evacuation orders are issued. Numerous Flash Flood Warnings are disseminated by NWS Austin/San Antonio, with increasingly urgent wording used to communicate the rapidly-deteriorating situation to the public.

Late Evening, October 17 - Early Morning, October 18: Additional showers and thunderstorms develop over South-Central Texas as a cold front approaches and passes through the region. While rainfall totals are substantially lighter relative to the daytime hours on October 17, the added rainfall begins to runoff immediately as soils are saturated across the area. This continues to exacerbate the river and flash flooding ongoing in the wake of the first round of heavy rainfall. Additional FFWs are issued by NWS Austin/San Antonio.

Afternoon, October 18 - Early Morning, October 20: Additional periods of moderate to locally heavy rainfall continue behind the now-departed cold front. Rainfall totals remain light relative to the beginning of the event, though runoff is quick to occur given continued saturated soils. More FFWs are issued by NWS Austin/San Antonio, with messaging gradually transitioning from the threat of heavy rainfall to major river flooding and continuing inundation at low water crossings, which are expected to last several days beyond October 20.

Water Vapor Satellite Imagery

Infrared Satellite Imagery

Guadalupe River Basin

San Antonio River Basin

InFRM Inundation Map for Guadalupe River above Comal River at New Braunfels

InFRM Inundation Map for Guadalupe River at New Braunfels

InFRM Inundation Map for Guadalupe River at Seguin

InFRM Inundation Map for Guadalupe River at Gonzales

InFRM Inundation Map for Guadalupe River at Cuero

NOAA AHPS Inundation Map for Guadalupe River at Victoria

How high were some of the river crests during the October 1998 flood? Check out these examples below. The yellow arrow at the top of the pole in each example is how high the water was during the October 1998 flood.

Courtesy KSAT TV

Courtesy KENS TV

The flow in the middle and upper portions of the atmosphere was conducive to upward vertical motion through the duration of the heavy rainfall event. Rounding the base of an upper level trough progressing from the Rocky Mountains, a split flow — or dual jet stream — pattern was evident from the evening of October 16 through the evening of October 18, 1998. South-Central Texas was placed favorably beneath the right entrance region of the southern speed max throughout the duration of the event (figure 1). Right entrance regions of upper jets are known locations of diverging, or separating, pockets of air, which induces upward vertical motion through the lower and middle portions of the atmosphere. These rising motions undoubtedly aided thunderstorm formation during the October 1998 flooding event.

Beneath the favorably-placed upper level jet streams, several mid-level features played noteworthy roles in the October 1998 flash flood. In the mid-latitudes, a trough was apparent over the Four Corners region during the evening hours of October 16. A subtropical high pressure center was evident further east over the southeastern United States. The Four Corners trough axis would progress northeast toward the Hudson Bay by the evening of October 18, helping to shove the subtropical high pressure center off the Carolina coastline. South-Central Texas was wedged between these two features through the duration of the event, resulting in persistent southwest flow in the middle portions of the atmosphere. Several shortwaves - or miniature trough axes - were evident in this southwesterly flow, crossing the region between the morning of October 17 and the evening of October 18 (figure 2). Rising air tends to be found along the leading edges of these shortwave troughs, suggesting that each impulse likely contributed further to an already-favorable mid to upper environment for shower and thunderstorm development.

Further south in the tropics, two tropical cyclones were evident over the Pacific Ocean. Initially located off the southern coast of Baja California, Hurricane Madeline would gradually weaken as it moved inland from the Mexican Pacific coast through the evening of October 18. Further southeast off the coast of Acapulco, Hurricane Lester would persist through the period as it gradually drifted west over the open waters of the Pacific Ocean (figure 2). While further removed from South-Central Texas, each tropical cyclone would play a pivotal role in priming the local environment for heavy rainfall during the October 1998 flash flood. This is discussed further in section b below.

High amounts of moisture were present throughout the entirety of the atmosphere leading into and during the October 1998 flash flood. South-southeast low level winds helped to transport Gulf of Mexico moisture north into the area during the evening of October 16, with a nearly uninterrupted influx continuing through the evening of October 18. Contributing further were Hurricanes Madeline and Lester, which helped to transport upper-level moisture originating from the Pacific Ocean into the region throughout the event as well. Integrated vapor transport — a variable used by meteorologists to track the movement of water vapor through the depth of the atmosphere — confirms the presence of the three moisture fetches from the evening of October 16 through the evening of October 18 (figure 3). With the atmosphere continuing to moisten locally, precipitable water values unsurprisingly increased to near and above two inches by the early morning hours of October 17, with most locations along and east of Interstate 35 remaining above two inches through the evening of October 18 (figure 4). Soundings taken at Corpus Christi on the evening of October 16 and morning of October 17 captured the steady increases in precipitable water occurring regionally (figure 5). The nearly continuous supply of moisture certainly helped to fuel heavy rainfall in thunderstorms forming from the early morning hours of October 17 through the evening of October 18.

Surface observations taken from the evening of October 16 through October 18 showed the approach and passage of a cold front across the state. Progressing southeast from the Panhandle southeast into Hill Country and the Coastal Bend, the boundary passed through South-Central Texas from pre-dawn through the afternoon hours on October 18 (figure 6). It’s noteworthy that the boundary was still well to the northwest of the region during the morning and afternoon of October 17, when the heaviest and most persistent round of rainfall brought catastrophic impacts to the Interstate 35 Corridor between San Antonio and San Marcos. While it remains difficult to diagnose a precise cause of this activity, the terrain of the Balcones Escarpment and embedded disturbances within the flow aloft likely helped to focus shower and thunderstorm development during this phase of the event.

The National Weather Service's West Gulf River Forecast Center (WGRFC) had the primary forecast responsibility for river forecasts during this evening. WGRFC uses a suite of river and soil moisture models to assist with generating forecasts. SACSMA is a soil moisture accounting model that treats the soil in a river catchment as if it is a collection of 5 buckets which each behave differently and each represent different soil processes in different parts of the soil. 

In the examples above, representing the Guadalupe River at Gonzales and Cuero catchment areas reconstructed from the 1998 flood, the top section shows precipitation and runoff. The light blue shows what percentage of the rainfall went to total runoff. The remainder stayed in the various soil buckets. 

The middle section shows the 5 buckets, particularly what fraction of the bucket was simulated to be full. The UZTW and LZTW are buckets which represent water which can only be removed from the soil by evapotranspiration, from the upper and lower soils. The UZFW is free flowing water in the upper soil. The LZFS and LZFP represent faster and slower responding baseflow from the lower soils. Finally, ADIMP is a portion of the soil which represents parts of the basin that fill up more quickly than others and begin producing faster responding runoff more quickly than the rest of the catchment.  

The bottom section shows how various parts of the SACSMA model combined to produce the total runoff seen in the upper plot (light blue).  IMP_RO is runoff from heavily urbanized areas that are lacking soils called "impervious areas". SUR_RO is Surface Runoff which generates large amounts of flow when heavy rain falls on soils that cannot take in the water fast enough to keep up. This will be the type of runoff that drives the most significant floods. The DIR_RO is the fast response runoff produced by the ADIMP portion of the model. The INT_RO is a slower surface runoff called interflow which can peak hours after the rainfall. In some locations, minor floods can be driven by interflow.  The SUP_RO and PRI_RO are the two baseflows which peak days to weeks after a rain and do not contribute to flooding. 

The October 1998 heavy rainfall event quickly generated large amounts of surface runoff and eventual mainstem river flooding primarily due to the rainfall intensity and volume of rainfall that occurred October 17-18 in upstream portions of the Colorado, Guadalupe, and San Antonio River Basins. In addition, a precursor rainfall event that occurred on October 6 primed downstream soils and had a smaller role in the degree of flooding.