Average maximum summer temperatures are usually in the 70s (25C) and occasionally in the 80s (30C) in the lower elevations. Nights are cool and temperatures may drop into the 30s and 40s (0-10C). Depending on the elevation, temperatures may even fall into the 20s (-5C) with a light freeze. June can be cool and rainy; July and August tend to be somewhat drier, although afternoon thundershowers and lightning storms are common. During lightning storms get off water or beaches and stay away from ridges, exposed places, and isolated trees. At any time of year, be prepared for sudden changes; unpredictability, more than anything else, characterizes Yellowstone's weather.
We recommend that you bring a warm jacket and rain gear even in the summer. If you plan to visit Yellowstone during spring or fall, call ahead for current weather and road information since sudden storms may result in cold temperatures, snow and/or temporary road closures. In summer, stop at visitor centers or ranger stations for weather forecasts. Always carry extra clothing when hiking.