In Michigan in summer and fall, we often have frost delays in the morning.

Is there any good rule of thumb to look at the night before to predict a frost delay? Temp, humidity, etc.??

Lose the Ryder Cup and expect the ice storm in the morning!

In Florida, frost happens when the humidity is high the day before, and a cold front blows through overnight with little cloud cover. If there is lots of cloud cover and breezy, it keeps it too warm (greenhouse effect) to allow the frost to form.

I would look at a big drop in temperature over night, with really clear skies as having the potential for frost. Of course, weather is about as predictable as women - just when you think you have them figured out - the rules change.