San Antonio, Texas, is situated in South Central Texas, a location that significantly influences the types of natural hazards the city faces. The city lies at the southern edge of the Texas Hill Country, where the flatter coastal plain begins its ascent onto the Edwards Plateau. This area is defined by the Balcones Escarpment, a major fault line that creates a sharp elevation change and acts as a barrier for weather systems moving inland. This unique geography and proximity to the Gulf of Mexico create a regional risk profile that differs greatly from other major Texas cities.
The Primary Threat: Flash Flooding
San Antonio is located directly within a region known as “Flash Flood Alley,” considered the most flash-flood-prone area in the United States. This designation results from a unique convergence of geological and meteorological factors that lead to rapid, dangerous water accumulation. The Balcones Escarpment plays a central role by forcing warm, moist air originating from the Gulf of Mexico to rapidly rise, cool, and condense. This orographic lifting effect enhances the intensity and duration of rainfall, often producing torrential downpours with little advance notice.
The geology of the underlying terrain further accelerates the flooding process. The region is characterized by karst topography, consisting of porous limestone with shallow, clay-rich soil on top. The shallow soil cannot absorb heavy rainfall quickly, causing the vast majority of water to immediately become surface runoff. This quick-moving sheet flow is then funneled rapidly down the steep slopes and narrow stream channels of the Hill Country toward the city.
Urban development has compounded this natural vulnerability by increasing the amount of impervious cover. Replacing natural landscape with concrete and rooftops prevents rainwater from soaking into the ground, sending it instantly into storm drains and local waterways. This dramatically increases the volume and speed of runoff, causing rivers and creeks to swell to dangerous levels in minutes. Flash flooding is the leading cause of weather-related fatalities in Texas, with water levels sometimes rising 20 to 30 feet in surrounding river basins.
High-Impact Severe Storms
San Antonio regularly experiences severe thunderstorms that pose threats from wind, hail, and, less frequently, tornadoes. The city is positioned in a transition zone where dry, continental air masses collide with the warm, humid air streaming in from the Gulf of Mexico, creating an unstable atmosphere conducive to storm development. These severe events typically peak during the spring and fall months.
Large hail is one of the most frequent and costly hazards in the region, often resulting in widespread property and vehicle damage. Storms in the area have produced hailstones several inches in diameter. These powerful updrafts, fueled by the collision of air masses, suspend large amounts of water high enough in the atmosphere to freeze into destructive projectiles. San Antonio averages a significant number of severe weather warnings annually, confirming the high frequency of these damaging hail events.
Severe thunderstorms are also capable of generating damaging straight-line winds that can reach speeds of 70 miles per hour or more. These powerful gusts can topple trees, down power lines, and cause structural damage to homes and businesses. While the threat of tornadoes is lower than in parts of North Texas, isolated tornadoes do occur.
Extended Periods of Extreme Temperature
The city’s climate profile includes two major risks related to temperature extremes: prolonged periods of high heat and drought, and hard freezes. Summer in San Antonio is characterized by extended stretches of intense heat, with the number of days exceeding 100 degrees Fahrenheit projected to increase. This persistent heat creates dangerous conditions for human health and places significant strain on the energy grid as air conditioning demand spikes.
High temperatures are often coupled with extended drought conditions, which trigger a series of water restrictions. These restrictions are dictated by the water level of the Edwards Aquifer, the city’s primary water source, with rules progressing through stages that limit outdoor watering. Prolonged drought conditions also increase the risk of wildfires in the surrounding drier landscapes.
On the opposite end of the spectrum, severe winter weather, though rare, can cause widespread disruption because the infrastructure is not designed for sustained freezing temperatures. Hard freezes, such as the one experienced during Winter Storm Uri in February 2021, can lead to catastrophic consequences. The extreme cold paralyzed natural gas production and other power generation sources, resulting in mass, prolonged power outages for millions of Texans, including those in San Antonio.
The associated infrastructure failure extends beyond the power grid, as sub-freezing temperatures cause pipes to freeze and burst in uninsulated homes and businesses. This damage often leads to widespread water service interruptions and boil water notices. These low-frequency, high-impact cold events demonstrate the vulnerability of the region to temperature extremes.