I've been in Yellowstone in Sept. (although it was late Sept) and it thundered, rained, hailed, and snowed.....all in the matter of about 45 minutes.
This happened at night. When I went to get in my truck the next morning, there was a nice shell of ice crusted all over it. After I chipped away enough ice to get in it and get it warmed up, I headed out to fish. It ended up being in the mid 70's that day with some fantastic fishing.