but being in a 5mm for any length of time in 99 degree August temps increases your risk of falling-out in the parking lot
What's not always apparent, is that you may be "comfortable" in your suit, but how do you feel at the end of the day? If you're exhausted beyond what you should be for the physical activity of the day, you may be suffering from the thermal stress your body experiences trying to keep your core temp within operating range.
I usually wear a 3mm w/hooded vest in the deep summer (because the air temp is so toasty) and a 5mm w/hooded vest the rest of the time, or if I plan on being in the water for over an hour.
Hah true, it can get real hot. Good reason to hydrate, shower off and pull your suit down quick once out of the water. Also consider a 5mm compressess to less with depth. Morrison isnt too bad though.