Weather in Big Bend National Park. Weather - Big Bend National Park (U.S. National Park Service) Average Temperatures and Precipitation. Average (inches) Summers are hot; temperatures vary greatly between the desert floor, which is often above 100 degrees Fahrenheit, and the Chisos Mountains, which are ten to twenty degrees cooler. June-August are the hottest months. Visitors should be familiar with heat safety. The rainy season runs from May through September, with locally heavy thunderstorms and some flash flooding. The rainy season can be a delightful time to visit since afternoon and evening rains often cool the desert. Automated weather station readings throughout the Big Bend area. Live Weather Readings.