The pre-race festivities and practice sessions on Wednesday will see a chance of rain arriving in the afternoon. It is only a 30% chance, so most of the day should be dry with some morning sunshine, but clouds will be increasing through the day. A low pressure system will be taking shape somewhere over northern Spain or southwest France. This low will track slowly north or northeast, making rain likely Thursday and Friday.
One model holds this low pressure system over France through the weekend, which would potentially make for a very wet race. Another model tracks the low toward Belgium, and brings drier air into northern France behind it on Saturday. I'm leaning my forecast more toward the second model, but due to this uncertainty, I have to keep a chance of rain mentioned on Saturday If the second model is right, it could be dry for the start of the race. On Sunday, another low pressure system over Ireland will track southeast, and this could bring an increase in rain chances for Sunday. Again, timing is very uncertain, so it is too early to say with confidence whether it will bring rain during the race or hold off until after the race ends.
Hopefully the details will become more clear in the next few days, and I'll have updates here on the website through the week. You can follow me for updates on Twitter at @Race4caster.