I am keeping the chance of rain at 40% today because I do not believe that the entire day will be a washout. There should still be plenty of on-track activity, so I have the Impact Forecast at Low. The most likely time period for rain at the track appears to be in the late morning to early afternoon hours, mainly between 10 am and 3 pm. Looking at the event schedule, the rain may affect IndyCar practice, PWC practice, USF2000 Race 1, and Indy Lights qualifying. But it won't be raining through all those activities - I expect scattered coverage of on/off showers. Thunderstorms are possible, but they should be few and far between. Rain amounts are most likely to be around a tenth of an inch, but up to a quarter inch may be possible if a thunderstorm happens to directly cross over the track.
Behind the exiting trough, high pressure will be building across the Great Lakes region on Saturday. A brisk northerly flow will keep temperatures relatively cool all day, with highs in the mid 70s. A north wind will be blowing between 10 and 20 mph, with gusts to near 25 mph at times, so pin down your tents and lawn chairs.
Sunday looks absolutely perfect for racing - highs in the upper 70s with plenty of sunshine. North winds will be a little lighter on Sunday, between 10 and 15 mph.