The general pattern later this week will be a high pressure area over New England, drifting east off the coast through Friday. It will provide a really nice day for racing on Friday, with lots of sunshine and temperatures in the mid 80s. Winds will be from the south at 5 to 10 mph, but the humidity won't be uncomfortably high with dewpoints in the upper 50s to around 60.
The high pressure center moves farther east on Saturday, while a low pressure system tracks into the Great Lakes region. This pattern will spread more moisture into the atmosphere from the south, but I don't expect it will be enough to produce rain on Saturday - just a little more cloud cover. The chance of rain should hold off until Saturday night and Sunday.
With my last forecast for IndyCar in Toronto, the models started off too fast to bring in rain, and tended to get slower and slower with each run. I think this trend will continue for this forecast as well, and hopefully it won't go the other way. This is one weekend to be thankful that there's no racing at Lime Rock on Sunday.