Texas is the second largest state in the United States, and is home to a wide array of Climates and Geography, that all make for diverse weather, and extremely challenging weather forecasts. The City of San Antonio, can be exceptionally hard to forecast for, as it lies in a transition zone between two climates, and also happens to be in a location where the topography of Texas begins to change as well. Hence, Geography plays a critical role in Weather Forecasts here in South Texas.
San Antonio lies in South Texas, about 140 miles from the Gulf of Mexico. Throughout the year, it is dominated by an Easterly, Southeasterly flow off the Gulf of Mexico, which keeps the area rather humid. This flow brings in steady moisture and humidity, which gives this area, a Humid Sub-Tropical Climate.
In West Texas, the area is dominated by a Dry Westerly Flow from the Desert Southwest, the Deserts of Mexico, and is mostly dominated by a Continental Tropical Air mass. Low humidity, and low dew points are very common in this area of Texas, which extends from El Paso, eastward, to just West of the San Antonio area. In the Fall, Winter and early Spring, this is the typical atmospheric setup that is in place. You get Cold Fronts coming in from the West, or from the North, and they meet up with the warmer, moisture laden air from the Gulf, right here in South Texas, many times here in San Antonio. The clashing of air masses usually happens directly over this area.
The San Antonio area lies in a very unique location here in South Texas. Once you begin heading West from San Antonio, the climate begins to dry out, and quickly switches over to a Semi-Arid Climate, the desert climate of West Texas. It's a dramatic change. You drive west from San Antonio, and within 100 miles, you are in Desert areas. San Antonio also happens to lie along the Balcones Escarpment. The Balcones Escarpment is where the flat coastal plains of South Texas, begin to change in elevation, from gradual rolling hills to the higher altitude of West Texas. The sea level throughout the Escarpment changes anywhere between one, to five hundred feet, but that change in elevation is sometimes enough to impact the weather.
Geography can throw a complicating variable into the forecast at almost any time of the year. During the late Spring, Summer, and early Fall, warm, moist air from the Gulf of Mexico, moves North and West across the Coastal Plains of South Texas. As this moist air mass travels West, it is forced to rise when it hits the Balcones Escarpment, which has a slightly higher altitude. When that happens, flooding rainfall often will occur because of the height change and Orographic lifting. You never know if the slight change in elevation, in some cases, only a couple of hundred feet, will be enough to get the air to rise even further, contributing to the development of convective storms. If there is enough lift, you get heavy rainfall since you have such a juicy air mass, that is already rising because of the orographic processes over the Balcones Escarpment. Because you never know how the atmosphere is going to react, it is very hard to forecast where and when storms will develop. Flash Flooding in a lot of cases results over communities that lie in the Balcones Escarpment, and the amount of precipitation, and duration, is always difficult to forecast. A classic example of this was the South Texas Flood Event of 2002.
The Balcones Escarpment adds complicating factors to a forecast during the Fall, Winter, and early Spring, which is typically when we see the clashing of air masses, in the form of fronts, or storm systems that move through our area. The Escarpment not only marks a change in the topography, but it also marks a transition zone of climates, between the Humid Subtropical Climate to the East, and the Semi-Arid Climate to the West. Storm systems that are usually associated with Cold fronts, and many times Arctic Cold fronts, move through our area quite often between the Winter and early Spring. A lot of time, prior to the passage of the Cold Fronts, San Antonio is the Warm Sector, dominated by Warm Air Advection, and in a humid moist Southerly flow off the Gulf of Mexico. That humid moist air, usually does not make it too far West from San Antonio. And it is in this area, where the Cold Unstable Air, clashes with the Warm Moist air from the Gulf. The result is usually severe weather, and severe thunderstorms. Again here, the reaction that will take place when the air masses collide, is hard to forecast and determine, until it happens. That reaction, many times, happens very close to San Antonio, if not directly over the forecast area, which is where the Escarpment is located, and where two climate zones converge. There is very little lead-time, in terms of identifying what's already occurring, and what will be happening. Areas to the East like Houston, don't have this problem. The storms will fire up over San Antonio, or just to the East of San Antonio, giving Houston the lead-time it needs to see what's coming to their city. They already know what's taken place with the clashing of air masses, and have a better handle on what will be coming their way since the squall lines have already developed. Quite often, a weather forecast may miss the possibility of Severe Weather, Heavy Rains, and Flash Flooding, resulting from the explosive collision of air masses.
Some of the expert opinions on this topic have come from the Chief Meteorologist at my station, Steve Browne, at KSAT-TV. Steve Browne has been forecasting weather in the San Antonio market, and here in South Texas for over a decade, and has recognized this problem when putting his forecasts together. Recognizing the roles that Geography and Climate play in putting forecasts together for our area, have made him the leading Weather Authority here in South Texas.
There are also many case studies that have looked at the role that the Balcones Escarpment plays in Flash Flooding situations, which involve moist humid air advecting towards the West.
One of the things that I think can be done to get a better grasp of the local conditions would involve Weather Balloon Soundings, here in San Antonio. Right now, the sounding information we get for our area comes from two separate locations. They are Del Rio, and Corpus Christi, Texas. Del Rio is about 140 miles Southeast of San Antonio, and sits in the Drier Semi-Arid Climate of South Texas. Corpus Christi is about 140 miles Southeast of San Antonio, and sits in a more Humid Tropical Climate. These locations are relatively close by, but the climate differences are big enough that we don't get a good grasp of the atmospheric profile over South Texas, from those two readings. Having weather balloon soundings here in San Antonio, would give us a better grasp of the "true" atmospheric conditions over our area, and would allow us to see more accurately the conditions that are in place, which would only help improve our forecasts, especially when it involves the clashing of air masses with Winter Storm Systems, or with Moisture Advection from the Gulf. In both situations, it would help us get a better grasp of the Flash Flooding Potential, Severe Weather Threats, and would give us some concrete information that could be used as Climatological tools in the future.
1) "FLOODING ALONG THE BALCONES ESCARPMENT, CENTRAL TEXAS"
S. Christopher Caran and Victor R. Baker (pages 1-14): Walter Geology Library - University of Texas, Austin