A big reason for my low confidence is that the weather pattern across the area is not well defined. In other words, the flow through the mid and upper levels of the atmosphere is weak, so there is not a significant trough or front that will be moving through the area to provide lift and help to focus thunderstorm development. In a weak pattern like this, thunderstorm development will depend mainly on weak surface boundaries and afternoon heating that creates instability. That is very difficult to predict this far out. It may be a situation where we will just have to monitor the radar each day and see how things develop.
I still think Friday is going to be a dry day as there is reasonably good model agreement that there will be dry and stable conditions in place, A southwest flow will bring warm temperatures in the upper 80s in the afternoon.
In my previous forecast I mentioned a low pressure system that was expected to move across the Plains and Great Lakes over the weekend. That system now looks weaker and farther north than before. However, there will still be increasing low level moisture in the southwest flow, and a little greater instability that may allow for afternoon showers and storms to develop on Saturday. The pattern doesn't change much for Sunday, so both days should be pretty similar to each other. I only have a 20% chance of rain each day, so there's an 80% chance of dry conditions and no interruptions to the race. Again, we'll just have to wait and see how things develop, but I can't rule out an isolated storm at the Speedway each afternoon.