The weather still looks great for all on track activities Thursday through Saturday. I don't expect that rain on Thursday morning will be an issue, as it will be very light and likely be long gone by the time cars take to the track. The Indy Lights Freedom 100 will have perfect conditions on Friday, as will the Pro Mazda and USF2000 races at Lucas Oil Raceway on Saturday afternoon. If you're on the fence about attending these events, the weather certainly won't be a reason not to go.
Clouds will be increasing Saturday night as a low pressure system develops over the Plains and tracks east. Its associated warm front will lift northward across central Indiana Saturday night, and the resulting southerly flow will bring more humid air and more clouds for Sunday. The big question is when the rain will develop. It appears that some showers will be developing across central Indiana Sunday afternoon, but the bulk of the rain with this system will come Sunday night and Monday. I am confident that rain chances will be increasing through the day - I'm just not certain yet that the rain will come before the end of the race. Coverage of showers may be sparse enough to avoid interfering with the race, but it's still too far out to know that with any confidence. This timing uncertainty is why I'm keeping the rain chance at 50% for this update. I'd say the chances are 50/50 right now that there will be rain during the race.