|George R. Kasica
San Antonio Climatology
The area around San Antonio ranges in elevation from about 550 to nearly 1000 feet above sea level, not a dramatic difference in altitude by standards that would affect temperatures, however given that the dry adiabatic lapse rate (click for a definition and details) tells us that temperatures for unsaturated air parcels will drop about 5.3F degrees per 1000' feet of altitude increase we could conceivably see a change in temperatures of about 2F degrees from the lowest to the highest point of the San Antonio area simply due to the difference in elevation. The area itself only sees about 20 days per year that are below freezing, and only once has it hit zero degrees, on January 31, 1949.
Another item to consider in terms of temperature extremes is the fact that the temperatures in the San Antonio area can vary by as much as 40 to 50 degrees from low to high on a daily basis under clear skies given the strong heating effects of the sun and the equally rapid cooling under clear, calm conditions at night. These mild days are caused by the modified maritime air which has its source in the Gulf of Mexico and is brought into the region on south to southeasterly winds. Conversely, San Antonio can experience much colder days and nights due to the influence of more continental air flow brought to the ares on north to west winds behind strong cold fronts. You can see the obvious prevalence of wind directions in the wind roses for January and February which clearly show the prevailing northerly and southeasterly winds (click for full size images). In the cooler January month almost 14% of the days had a wind from the NE, yet in the milder month of February that was almost equally split with 12% from the NE and about 8% coming from a milder SE direction.
Overall temperatures for San Antonio range in the low 60's for highs during the winter months on an average basis, though they have seen days as warm as 89 back on January 30, 1971. For a detailed look a the daily average and extreme temperatures click here to open a new window to daily normal high and low temperature and extreme readings provided by National Weather Service Forecast Office in San Antonio Texas.
In terms of precipitation, measurable snow is rare, though not unheard of, in San Antonio and does occur every three to four years with the city receiving amounts as large as 2-4" about once every 10 years. The driest months of the year are from December through March and again in July. (All of the above climatic statistical data was provided courtesy of the San Antonio National Weather Service Forecast Office climatic narrative site).
As you can see, the climate of San Antonio, while not as extreme as some other locations in the continental United States, does present some challenges in terms of forecasting, especially in terms of it's temperatures extremes and the occasional possibilities of snow, though forecasting an 18" blizzard followed by -20F temperatures would certainly be stretching the boundaries of extreme, so we need to also keep in mind what is reasonable for the location in terms of it's historical climatology while we make our forecasts.