Floods and Droughts in Texas

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Episode released on November 13, 2025 
Episode recorded on September 5, 2025

John Nielsen-GammonJohn Nielsen-Gammon discusses climate extremes, including the July 4th flooding and 2011 drought, with insights into communicating risk. 

Highlights  |  Transcript

  • John Nielsen-Gammon presentation to the legislature during the Interim Hearings (Youtube link).
  • Kerr County flooding on July 4th, 2025, high intensity rain (3 – 4 in/hr, totaling 10 – 15 inches)
    • Remnants of Hurricane Flossie in the Pacific, Tropical Storm Barry in the Atlantic, plus high humidity from the Gulf of Mexico.
    • Slow-moving low-pressure area tapping high levels of atmospheric moisture.
    • Extreme rainfall watch covered much of west and central Texas, difficult to forecast location of storm.
    • Contributing factors to fatality, happened at night, holiday (July 4th), intense rain over 3 hr period.
  • Previous floods on the Guadalupe:
    • 1935 flooding, June, ≤21 inches of rain on May 31 in 3 hours, Woodward Ranch on Seco Creek. World record for precipitation intensity over 3 hr. Peak discharge ~230,000 cfs in Seco Creek. High antecedent soil moisture, tropical storm from Tampico (MX), happened during the day, no fatalities of campers.
    • 1921 storm Thrall Texas, 36 inches of rain in 18 hr, impacted Thrall and Austin, Category 1 hurricane landfall in MX Sep 7 1921.  
    • 1921 flooding in San Antonio, 51 fatalities, creation of flood control system after this flood, including the San Antonio River Walk.

Flood planning

  • Texas Water Development Board flood planning, 15 regional flood planning groups (Fig. 1), designated by TWDB in 2020, resulting in the 2024 State Flood Plan.
  • John Nielsen-Gammon, James Doss-Gollin (Rice Univ.), and TWDB staff are developing future rainfall frequency grids for Texas in a new project (Texas Rainfall Analysis Incorporating Nonstationarity, TRAIN, now called TxRAIN).
    • Increasing T increase moisture holding capacity of atmosphere (Clausius Clapeyron Equation).
    • Considering historical trends (regionally, Bayesian Spatial framework) (Lu et al., 2024) merging with future trends.
    • Maximizing data from short and long-term records and regionally to estimate extreme precipitation probabilities (intensity, duration, frequency).
    • Extreme precipitation data are critical for future planning, risk management, and infrastructure development. 

Hazard Communication:

  • Hunt, Texas near core of recent July 4th flooding has had 110 flash flood warnings over the past 23 years.
  • Hurricane Rita: deadly evacuation from Houston, 100 people died (link).
  • Need to communicate not just hazards but appropriate action (e.g., stay, go etc).
  • National Weather Service, warnings, e.g., tornado warning, go to safe place.
  • Minor flash flood, appropriate action, e.g., stay put, don’t drive.
  • Major flash flood, may need to go to higher ground, risk of lightning.
  • Future may include personalized messages to people with weather app knowing your location, elevation data, predicting height above river. Will need cell phone coverage.
  • Local monitoring: example during recent flooding, water softener beeping, designed for leak detection but detected flooding in home.
  • Norfolk, Virginia, Waze flood alert app, provides information to drivers on flooding, passable roads, with drivers providing feedback. 

Need for Increased Monitoring:

  • Monitoring of atmospheric conditions (Temperature, Pressure, Wind) varies regionally, can have widely spaced monitoring.
  • Water vapor, can have fine structure, need more detailed monitoring.
  • For example, atmospheric rivers coming from Pacific landfalling in West Coast, monitoring using ships releasing weather balloons in Pacific, then airplanes, and more recently satellite data.
  • Texas:
    • need monitoring along the Gulf of Mexico which sources about half of the incoming air during extreme events.
    • Between Brownsville and El Paso, only one upper air site (Del Rio).
    • Test the value of proposed additional monitoring stations by including them in modeling past extreme events.

Drought:

  • 2011 worst one-year drought in Texas, did not recover until 2015, based on Texas Drought Monitor data (Fig. 2).
  • Reservoirs also showed the impact of the 2011 – 2015 drought (Fig. 3).
  • Rainfall during 2012, 2013, and 2014 was fairly normal but insufficient to overcome large drought deficit during 2011. Flooding during 2015 ended the long-term drought.
  • Highland Lakes (Lakes Buchanan and Travis, water supply reservoirs for Austin, recovered to 88 – 98% of capacity after July floods) (Fig. 4).
  • 2023 drought: 2nd week of June, storms carried away from Texas: air sinking and warming, heat wave, humid, then humidity decreased, heat dome developed (presentation link). 
    Time series of Palmer Modified Drought Index showed the drought evolution in 2023.
  • Droughts begin when it stops raining, but only if it stays dry for months or more; difficult to predict when this might happen.
  • El Niño Southern Oscillation may be linked to droughts and wet cycles (Fig. 5):
    • El Niño results in a southward shift of the Pacific Jet Stream amplifying the storm track (Fig. 5a).
    • La Niña results in a northward shift of the Pacific Jet Stream, lowering precipitation in the southern third of the US (Fig. 5b).
    • Strong La Niña conditions would be expected to reduce rainfall in Texas by 20 – 25% but decreased in by 50 – 60% in 2011 (Table 1).
  • Dry soils provide positive feedback on drought, amplifying drought.
  • If drought extends into Northern Mexico, air coming into Texas aloft is dry, providing a lid on convection.
  • Drought can be self-perpetuating.
  • Different types of droughts:
    • Meteorological drought, lack of rainfall, flash drought, rapid onset.
      • Agricultural drought: soil moisture, impacts crop yields
      • Hydrological drought:  observed in streamflow, reservoir storage, and groundwater levels.
  • Southern Regional Climate Center: developing new product partitioning rainfall into soil moisture or runoff using approach developed by USDA.
  • TxH2O published by the Texas Water Resources Institute had an excellent publication documenting droughts in Texas since 1900 (Texas drought: now and then).
  • Water supply droughts often end in floods making it difficult to predict the end of a drought: e.g., 1950s drought, flooding in 1957. 

Report: Extreme Weather in Texas: 1900 – 2036 by Nielsen-Gammon and others, 2024:  for Texas2036 (Fig. 6). 

  • 2036 represents the bicentennial of Texas independence.
  • Only projecting for 10 – 15 years, can rely heavily on recent trends in data.
  • Not all extremes are getting worse: cold extremes are getting better with rising temperature.
  • Tornadoes trends? increased data in recent years with people chasing tornadoes, impacts decreasing with improved buildings, etc., but severity of tornadoes may not have changed.
  • Number of 100°F days increasing, projected to double between now and 2036, some related to local urbanization (heat out effect from air conditioning, less natural vegetation etc) and some linked to climate change.
  • Rising temperatures, increasing evaporation, drier soils, droughts getting worse even if rainfall stays the same. 
Figure 1. Regional Flood Planning Groups in Texas (TWDB link).
Figure 1. Regional Flood Planning Groups in Texas (TWDB link). 1. Canadian – Upper Red; 2. Lower Red-Sulphur-Cypress; 3. Trinity; 4. Sabine; 5. Neches; 6. San Jacinto; 7. Upper Brazos, 8. Lower Brazos; 9. Upper Colorado; 10. Lower Colorado-Lavaca; 11. Guadalupe; 12. San Antonio; 13. Nueces; 14. Upper Rio Grande, and 15. Lower Rio Grande.
Figure 2. US Drought Monitor for Texas time series.
Figure 2. US Drought Monitor for Texas time series. (legend: D0: abnormally dry; D1: moderate drought, D2: severe drought, D3: extreme drought; D4: exceptional drought). Much of the state was in exceptional drought during 2011. Drought lasted until 2014 and ended abrupty with floods in 2015 (including the Wimberley floods). 2015 and 2016 were El Nino years that often result in increased rainfall in Texas. (link)
Figure 3. Time series of conservation storage in Texas water supply reservoirs, showing impacts of 2011 – 2015 drought. (TWDB Water Data for Texas).
Figure 3. Time series of conservation storage in Texas water supply reservoirs, showing impacts of 2011 – 2015 drought. (TWDB Water Data for Texas).
Figure 4a. Lake Buchanan recovered to 98% of capacity after July floods, 2025. (TWDB Water Data for Texas)
Figure 4a. Lake Buchanan recovered to 98% of capacity after July floods, 2025. (TWDB Water Data for Texas)
Figure 4b. Lake Travis recovered to 86% of capacity after July floods, 2025. (TWDB Water Data for Texas)
Figure 4b. Lake Travis recovered to 86% of capacity after July floods, 2025. (TWDB Water Data for Texas)
Figure 5a. Schematic of impact of El Nino on Pacific Jet Stream, resulting in wetter conditions in the southern US.
Figure 5a. Schematic of impact of El Nino on Pacific Jet Stream, resulting in wetter conditions in the southern US. (link)
Figure 5b. Schematic of impact of La Nina on Pacific Jet Stream, resulting in drier conditions in the southern US.
Figure 5b. Schematic of impact of La Nina on Pacific Jet Stream, resulting in drier conditions in the southern US. (link)
Table 1. Oceanic Nino Index is a measure of the El Nino Southern Oscillation (ENSO) with red (warm, El Nino) and blue (cold, La Nina) periods based on a ±0.5°C for the ONI.
Table 1. Oceanic Nino Index is a measure of the El Nino Southern Oscillation (ENSO) with red (warm, El Nino) and blue (cold, La Nina) periods based on a ±0.5°C for the ONI. (3 month running mean of ERSST.v5 SST anomalies in the Niño 3.4 region (5oN-5oS, 120o-170oW)]). (National Weather Service Climate Prediction Center).
Cover page of Extreme Weather in Texas report by John Nielsen-Gammon and others for Texas 2036.
Figure 6. Cover page of Extreme Weather in Texas report by John Nielsen-Gammon and others for Texas 2036. (link)

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