The most significant rain of the week is expected on Thursday as teams load into the track but before any on-track sessions begin. A fairly strong trough and cold front will pass through on Thursday, with some potential for heavy rainfall. This system will be clear of Toronto by the time on-track sessions start on Friday morning. Friday will be a nice day with mostly sunny skies and highs in the upper 70s F, or mid 20s C.
A weaker trough is expected to cross the area on Saturday. This far out in the forecast, timing is uncertain, but the best chance appears to be in the afternoon and evening as daytime heating creates some instability while the upper trough approaches from the west. Some thunderstorms will be possible as well, which may result in a stoppage of some sessions.
With the uncertainty of timing of the trough, it is possible that some showers could linger into Sunday morning. From what I'm seeing today, it is more likely that the chance of rain ends before sunrise on Sunday, but I can't rule out a morning shower completely. In either case, the majority of the day should be quite nice, with decreasing clouds and temperatures peaking in the lower 80s F or upper 20s C.