Flash floods are a primary driver of flood mortality in semi-arid regions where compound risk factors amplify convective extremes. The 4 July 2025 Kerr County flash flood in the Texas Hill Country-a region colloquially designated "Flash Flood Alley"-represents a benchmark hydrometeorological event. Triggered by a quasi-stationary mesoscale convective system derived from Tropical Storm Barry remnants, the event delivered more than 508 mm of rainfall over 72 hours, with peak 3-hour intensities exceeding the local 1,000-year recurrence threshold (National Weather Service, 2025). Using NOAA Stage IV rainfall estimates, USGS streamflow records, and mass-balance modeling, we reconstruct the hydrometeorological response of the South Fork and mainstem Guadalupe River. Estimated peak discharges at the South Fork ranged from 7,221 to 7,505 m³ s^−1 (255,000–265,000 ft³ s^−1), approaching or exceeding the regional discharge envelope curve and surpassing the historical record set in July 1932. Antecedent drought may have contributed to runoff through hydrophobic soil behavior, while overnight timing and critical communication gaps compounded human impacts (137 fatalities). Numerous affected structures lay outside FEMA-designated Special Flood Hazard Areas, exposing the limitations of historically calibrated static hazard maps. The findings support a transition toward impact-based warning protocols, expanded monitoring networks, and climate-aware hazard mapping for topographically complex, data-sparse regions.