Mission, Texas, is in the sub-tropics zone. In September, this will likely mean HIGH humidity and temperatures, even at night; with the possibility of some rainfall. 

Fortunately, the Rio Grande Valley is often windy and it's this wind that keeps us from spontaneously combusting.  We recommend dressing for the desert with lightweight, loose-fitting, cotton clothing that will provide some protection from the sun and allow for natural cooling by wicking perspiration. 

Remember to also bring a windbreaker or sweatshirt, just in case, and a hat, handkerchiefs or bandanas, mosquito repellant, sunscreen and reusable water bottle.