Saturday will likely be dry, and while I can't rule out an isolated shower or thunderstorm, I expect that practice and qualifying will be held on time with no weather interruptions. With mostly sunny skies, highs will reach the lower 80s.
As we've been seeing over the past several days, a front and upper level trough will be approaching Pennsylvania from the west on Sunday. By midday, the front will be located across the central part of the state, and showers and thunderstorms will be increasing ahead of the front in eastern PA. There are some timing differences in the models, but I think that the most likely time period for rain at the track appears to be from 2 pm to 6 pm. This doesn't look like a light drizzle either - there could be some brief heavy downpours at times, and total rain amounts are expected to be a quarter to a half inch. I think the chances of a rain-out on Sunday are good, around 70%.
With Sunday looking wet, I've added a forecast for Monday. Fortunately, if the race is delayed to Monday, the weather looks absolutely perfect. The front will be well east of the area, and drier and cooler air will be building in behind it. Highs will be around 70 degrees, with mostly sunny skies.