I took your lead and my tub is filling up. It's 37º F, cloudy and windy. The other day I turned on the tub, turned on the air and the whole surface had a light foam on it - it was time IMO.
It wasn't too bad as my sump pump hose didn't lose prime (I hate when it loses prime) and drained as much water as I could using my cover pump.
Depending where the water temp starts, I may be soaking tonight ... I doubt it though.
This means that my Spa Frog test is going to be officialy on!