A few scattered light showers are observed on radar across southern Ontario today, moving toward the east. Light showers may move in and out of Toronto through tonight, but the main area of showers and thunderstorms associated with a cold front is expected to arrive late Saturday morning. One model, the NAM, shows quite a bit of rain falling from late morning through the early afternoon, up to one inch. I think that's probably overdone, with around a quarter inch being closer to what most models are showing. That would still be enough to cause some short delays, particularly with a chance of lightning as well. The races most likely to be impacted are the MRTI races, which run from 10:50 am to 1:40 pm, and IndyCar qualifying from 1:55 pm to 3:10 pm.
The front should be south and east of Toronto by Saturday evening, with high pressure building in for Sunday. Race day continues to look mostly sunny and warm, with high temperatures in the upper 80s (lower 30s C).