When it said temporarily unavailable, what did the road look like in the instrument panel display?
Foggy/rainy weather can create issues in seeing the lane lines in some lighting conditions.
In that case, you'll see no lane lines or just black lines rather than the grey over black lines it shows when AP is tracking the lane correctly but not engaged, and if you try to engage you'll get the temporarily unavailable message.
(There's also no steering wheel in the top right of the speed then.)
I did have only the black road lines and not the gray. But, I'm almost sure that when I successfully engaged it yesterday it also only had black lines. And the steering wheel never appeared yesterday too. It just kicked it when I double tapped the lever.