There will be a weak disturbance that moves by quickly on Thursday that may spark a few afternoon and evening showers. There's just a slight chance that one will affect the track, and if it does, any rainfall should be light.
Friday and Saturday should be dry as high pressure builds back over the area. A cold front will be approaching from the northwest on Sunday, but it looks like any rain with this should hold off until after the end of the race. That slight chance of rain I have in for Saturday night should mainly be late in the night, probably closer to sunrise Sunday morning. As teams and fans pack up to leave on Sunday, there is a chance that there will be some showers around.
I can say with full confidence that this race, meteorologically speaking, will be nothing like the 1965 edition of the 12 Hours of Sebring.