Can't comment about JMR Quarry but, American Quarry has warmed up nicely. A month ago, American got pretty cool below 35 ft. However, we were at almost 40 feet there on Friday and never felt like we hit the thermocline. The lowest temp on my dive profile was 83°F.
Lake Norman Quarry is an entirely different story. The thermocline remains at 25 ft. and it dropped to 72°F at 38 feet. It actually felt cooler than that. However, there are a lot of instructors at Lake Norman quarry every weekend doing cert dives. I have no direct experience but, I'm told that the PDRA quarries remain diveable in a wetsuit through October (unless there's an early cold snap). Despite the cooler temperatures, Lake Norman Quarry is actually a very good location to do cert dives. I was out there Sunday and was a dive buddy for one of my instructor's students who was doing their cert dives.
I would imagine that Belews Lake would stay pretty warm in the winter due to the size of the Belews Creek Power Station. In the winter, the warmest water is going to be on the discharge side of the plant. If I recall, the discharge is on the southwest side of the plant. However, I don't know if there are any accessible locations near there. If you find a good location to dive there in the winter let us know.