A cold front is expected to move north to south across the St. Louis area on Thursday night. The front ix expected to be positioned south of the area by the time on-track activities start on Friday afternoon. However, it won't be very far away, and an upper level disturbance is expected to draw moisture northward over the front on Friday afternoon. As a result, scattered showers and possibly a thunderstorm will be possible in the St. Louis area. It appears that the afternoon hours will have the best chance of rain, which could impact the qualifying sessions for IndyCar, Indy Lights, and Pro 2000. With scattered coverage, they could be hit-and-miss, but I'd put the chance of rain at the track at 40% right now.
The front to the south is not expected to move much through Sunday, but a little drier and more stable air may be able to build into the St. Louis area on Saturday as high pressure tracks east across the Great Lakes. With the front still being nearby, I don't feel confident enough to keep Saturday entirely dry, but the chance of rain is low, just 20%. If there is an isolated shower Saturday, it would probably be in the afternoon during the Pro 2000 and Indy Lights races. I expect that the IndyCar race in the evening will remain dry.
One piece of good news in this forecast is that temperatures will be very pleasant through the event. Afternoon high temperatures will be in the lower to middle 80s, and temperatures during Saturday night's race will be in the 75 to 80 range.
I expect that there will be some changes to this forecast as we get closer to the weekend and the details come into better focus, so stay tuned.