II. HOW WINE IS MADE
Weather is a major factor is determining whether a year is going to be a "good vintage" (or "year"). For example, was there enough heat during the growing season to lead to enough sugar? At harvest time, the short term effects of weather are quite important. To produce great wine, the fruit should be ripe (but not overripe), and have a high (but not overly high) sugar content ("brix"; typically about a 22 brix for table wine). Think of raisins. As the fruit dries, the water evaporates. What is left is the sugary fruit. If it rains just at the point the wine grapes are ready, and before the grapes can be harvested, the additional water will cause the water level to increase, and the brix will go down. Not good. (You might ask, why not just add some sugar in the wine-making process? Some do. Also considered "not good.")
Every year the wine grape grower plays a game of chance and must decide when to harvest. Simplistically, if you knew it wasn't going to rain, you would just test the brix until it was just right, then harvest. If you harvest too soon, you will probably end up getting a wine too low in alcohol content (there won't have been enough sugar to convert to alcohol). These wines will be "thin." If you delay harvest, there may be too much sugar, which leads to too low acid content. This also affects the taste (and the aging possibilities) of the wine.
During the harvest of 1989 I was in the Napa/Sonoma areas of California, where there was scattered rain. Winemakers in the area were not a happy bunch. As it turned out, this turned out to not be a great year "overall." But, it depends. In some areas not 20 miles away, rain was not a factor, in others it was. So you can't make a blanket statement that for all wines it was a poor year.