It is more than the UHI.
The increased moisture content in the air slows the temp swings. Dry air heats and cools faster.
Throw in the UHI and you get morning temps a few degrees either side of 80 in NYC.
The big story of this summer is not the heat, but the relentless dewpoints. It leads to higher low temps, steamier day time temps and increased convective activity.
If, and I say if, this is what NY summers look like in the future...we are in for a world of hurt.
You can imagine entire summers without getting below 70, with many evenings staying around 80. Scary stuff