As with all Randonneuring events, Paris-Brest-Paris is a rain or shine proposition. Past PBPs have experienced a wide range of weather. The last PBP in 2007 was probably one of the coldest and wettest on record with near constant rain and night time temperatures in the 40s. The 2003 event on the other hand was warm and dry with temperatures in the 60s to 80s.
Since the weather can be such a huge factor in randonneuring, I've started keeping an eye on what it's doing along the route. Of course, when it comes to weather you never really know what you're going to get until you're in the middle of it, but if Brittany is experiencing a particularly warmer/dryer or cooler/wetter weather pattern than usual over the next month, it may be a somewhat reliable predictor of what we'll encounter during PBP.
To that end, thanks to Weather Underground (www.wunderground.com), below you'll find "Weather Stickers" for each of the PBP contrôle towns. Click on one to go to a more detailed weather report and forecast. But don't blame me if you get rained on in France.