The French weather service (Météo France) shows a 50% chance of rain in Rambouillet for Sunday between 14-20h with west winds of 15km/h and gusts of 45! They're also calling for a chance of rain in Loudéac on Monday so who knows. If their climate is anything like ours there's a lot of variability in the rain forecasts so I am not too concerned either way... still bringing all the rain gear I'd usually bring on a long brevet.