(Reuters) - Unpredictable 2011 weather has divided French vintners into cautious early harvesters keen to avoid early autumn storms and nail-biting gamblers willing to risk catastrophe for more mature grapes in a year that has been flagged as exceptional for wine. A dry spring, a wet start to summer, which has eased into more clement seasonal conditions and warm temperatures has persuaded some vintners to start harvesting in late August, while others have bided their time, measuring grape maturity with modern science and old-fashioned chewing and tasting.