I don't think the event will be as lucky on Friday. There continues to be an upper level low pressure are off the west coast of France, and this will generate a low pressure center at the surface that will track across northern France tomorrow. While there is a chance of rain all through the day, I think the late morning and early afternoon hours will have the best chances of rain. I expect the rain to be tapering off through the late afternoon and evening.
High pressure at the surface will build over the region Friday night and Saturday, which should keep any chance of rain away. However, the upper low to the west will continue to supply plenty of clouds. The start of the race will likely be dry but cloudy, with comfortable temperatures in the mid 70s.
Another surface low develops over southern France Saturday night. It appears to bring quite a bit of rainfall to southern and central France on Sunday. It will be very close as to whether this rain reaches far enough north to affect the race, and whether it arrives before the end of the race. The chance of rain will be increasing in the afternoon, and the later stages of the race will have better chances of rain. But it is certainly possible that it may stay to the south, and the entire race could be dry.
I'll have another forecast update posted before the start of the race, and hopefully the weather on Sunday will become a little more clear.