He's never won it in October, when the temperature is really cold and the ball isn't bouncing nearly as high.
For the first few days of the tournament, the temperatures are supposed to be fairly warm (but not hot). Of course, weather forecasts can be pretty unreliable this far out. And since they usually change the forecasts about 10 times a day, I wouldn't be surprised if the forecast looks completely different tomorrow.
But one thing's certain—the average temperatures for Paris aren't that high right now. So, the odds definitely favor cooler weather for the tournament.
Edit: Please ignore the first paragraph. I thought the main draw started on Sept. 21.