With the hot air, low wind, the strongest sun of the year and the high dew point rapid warming of the top 3 to 5 feet of water is to be expected. All the ambient conditions favored this.
When the front passes later today, the warm water near the surface will mix down some. The water at the very surface will lose heat due to cooler air and much lower dew point. However, the lake has gained a lot of heat the last few days and will retain most of it.
Enjoy it while it is here!
R2B