Showers and thunderstorms are likely at the track on Thursday. While showers are possible at any time of the day, I think that there will be two main periods of rain - one early in the morning just before and around sunrise, and a second in the afternoon and evening as the surface cold front moves through. I expect that the second round will be stronger than the first, and there could be some strong thunderstorms with it. If you're at the track, you will definitely want to stay weather-aware on Thursday. Frequent lightning and gusty winds will likely accompany the storms. The IndyCar and Indy Lights practice sessions at 3 pm and 4:30 pm will be threatened by these storms. Total rainfall amounts on Thursday are expected to be around a half inch.
Friday is still expected to be dry during the day when all the on-track action is taking place. Clouds will be increasing in the afternoon as the next front approaches, but any rain associated with it will hold off until after midnight. The second round of showers is expected to move through early Saturday morning, and it should be gone in time for the start of the Pro Mazda Race #2 at 10:20 am. Clouds will be decreasing through the day, so by the time the Angie's List Grand Prix starts at 3:50 pm, there should be a good amount of sunshine. Temperatures will be noticeably cooler on Saturday, topping out near 60 degrees, and a northwest wind at 10 to 20 mph will make it feel even cooler. You'll want to bring a jacket to the track on Saturday.