Most inland water bodies will run a temperature roughly equal to the average temperature across a full diurnal cycle *at thermodynamic equilibrium* (in essence, when the lake temp and air temp/radiative input are in balance). But a water temp 15°F warmer than the average air temp is absurd. Sure, a thin layer of surface water near the shoreline could theoretically be 74°F right now, but would be very brief and not representative of the lake as a whole.

I was wrong about that part. Lake Sammamish isn't that deep. But Lake Washington is. So I think Sammamish and some local lakes are able to warm into the right threshold.

The fact it’s deep makes it all the more unlikely. That’s a larger volume of water to heat with more cold bottom water available to mix with the warmer surface waters. It’s very likely that shallow waters near the shore are being sampled in this case.

Official data from King County Most recent sampled temperature was 74.3 °F (23.50°C) on Tue 6/16/26, 11:58 AM Water body Lake Sammamish Many lakes are well above 70 already, including Lake Meridian near me.

Lake Sammamish is deep. The surface can warm up especially by late summer, while the water beneath is much colder. So I don't think it violates the laws of thermodynamics.