I've been out there many times when it's quite warm, and it has never bothered me. Just bring enough fluids. That's why it's nice that the park is open in the evenings.
We went Monday, it was hot during the day but was a lot more pleasant as it got closer to 5:00 PM or so. The animals aren't real active out in the enclosures when it's hot. Saw them running some practice Segway runs. They'll be running out part of the old tram roadbed and will have time included for photo stops.
