A cold front is expected to pass through the area on Wednesday night. This front will stall somewhere south of Indianapolis, likely near the Ohio River. It should be far enough south of the Speedway that I expect Friday to remain dry, but there will be a chance of showers across southern Indiana.
On Saturday, the front may start to lift northward, so that the chance of showers shifts northward as well. Where this front is positioned will affect the chance of showers in the area, but for this forecast, I expect scattered coverage of showers and thunderstorms during the afternoon.
Sunday looks similar to Saturday at this time, although some models show a weak disturbance that would bring higher rain chances. For this initial forecast, I expect scattered showers and thunderstorms around central Indiana during the afternoon. The chance of rain affecting the race is about 30% at this time.
Check back later in the week for updates, and hopefully the details will come into better focus.