What a difference a year can make. Rebecca's birth day, May 15 1997, was cold and rainy and windy, 45°F (7°C), and the trees were still not fully in leaf. Her birthday this year was sunny, sultry hot, a near-record 87°F (30°C). The trees have been in leaf since late April. Our roses that usually come in the second half of June are starting to open now!
The "El Niño" weather pattern made a big difference all winter, since it pushed the jet stream higher into Canada, and is forecast to continue into next winter.
Which is to say: heat is a possibility. Buffalousually escapes high humidity owing to the cooling breezes from Lake Erie (the same ones that bring lake-effect snow in winter). We typically get one sultry week-or-two in a summer, usually in July, but once in 1990 it fell in mid-June. Today (5/18) has been gorgeous: 75°F (24°C), sunny, with crisp air. But don't bank on this---weather in the 50°-60° degree range can happen in June as well. Be prepared for anything. The site and all hotels have air conditioning, but the dorms do not. The site has an outdoor patio that is covered by a large awning.
Update (6/4): Well, as I said, "be prepared for anything". Yesterday and today (Thursday 6/4) have seen highs in the 50's (58°F = 15°C), and a chilly 40°F overnight low. I will post the 5-day forecast late next week.