Whatever season, the rule in Jackson Hole is that the weather can change quickly. This is even more true if you are planning to spend some time in the mountain heights or visiting the back country. Plan to pack a number of layers so you can add or remove them as you need. The weather can be a bit unstable during this time of the year. Snow storms give way to spring showers and brilliant sunny days as winter makes its slow retreat.
Rivers swell with runoff rushing off the mountains.
Temperatures can make wild swings as storms swing in and out of the valley. Yes, it can sometime be hot in Jackson Hole. In the last several years, we've had dozens of days in July and August where the temperature has reached the upper 80's and lower 90's. Evenings can be chilly, dropping down into the 50's or colder, especially in June or September. Afternoon thundershowers can be intense as they travel quickly across the valley.