If the weather is bad during flowering it can effect pollination and fruit set. This can result in bunches with small and large grapes, a condition known as Millerandage which is often called "Hen and Chicken". The big berries have been fertilised and have pips and sugar but the little berries are sour and seedless. Poor pollination can also result in Coulure, where large sections or even complete sets of flowers fail to produce fruit.
This year our Seyval have suffered and as a result yields may be lower than expected. However as we were planning to take off some of the bunches anyway, so that the young vines don't get to stressed, this might not be too bad news.
Interestingly the Pinot Noir and the Pinot Meunier have much better formed bunches, probably because they flowered slightly earlier when the weather was a bit better.
As flowering generally occurs during Wimbledon fortnight, the weather during the tennis is often a good indicator of potential yields for English vineyards. If the weather is good it is normally a forecast of bumper yields. If it rains throughout I probably won't be in the best of moods!