Scattered showers and thunderstorms will be possible Saturday afternoon, getting more numerous later in the afternoon. I'd put the chance of rain during the race at 40%. I don't think it will be heavy enough to stop the race, but it may necessitate a change to rain tires or perhaps a caution flag.
The majority of rain along a cold front will pass through on Saturday night and Sunday morning. The question with this forecast all week has been when the front will pass through and the rain chance will end. Today that timing looks a little later than it did a couple days ago, with the front expected to move through between 8 am and 11 am. The session that may be at risk is the morning qualifying session. It will be very close to being impacted, but I'm going to take the optimistic side that the rain will exit before 10 am, with the qualifying scheduled for 10:15 am. It is possible that wetness on the track could cause a short delay. The rest of the day looks great, with increasing sunshine and highs in the mid 70s.