Between now and early September it's not uncommon for it to be 95-100 degrees with a shitload of humidity. I don't mind the heat too much if I can stay hydrated, but when it's that hot out it's hard to keep your fluid and electrolyte intake high enough to keep the cramps away.
I did a ride a couple weeks ago with a lot of short, steep hills and some mean heat. The top of one particularly nasty hill at around the 45 mile mark had a group of about 15 people, myself included, barely able to walk because of leg cramps. A couple of folks quite the ride right there and the rest of us eventually got back on our bike while swearing profusely and going about half the speed we were going in the first half of the ride.