Friday looks nice - not much else to say about that. But during the day on Friday, a low pressure system will be dropping into the upper Midwest region. As that system tracks south and east through Saturday, it will bring showers and thunderstorms to the St. Louis area late in the day and the evening. As of today, I expect that the most likely time for these showers to arrive at WWTR is between 6 pm and 9 pm local time - during the race. An earlier or later arrival time, give or take a couple hours, is certainly possible, and keep in mind that a lot can change between now and Saturday. But that is my best estimate based on today's data. Hopefully the chance of rain will hold off long enough until the race is over.
I'll have another update posted before the race to give a more exact timing estimate, and I'll be watching the radar and posting live updates on Twitter (@RacecastWx) during the race on Saturday.