Here are the factors:
a. Weather: If a great year has a long, dry summer, then the weather for grapes is TERRIBLE this year. First, the grapes budded out about two weeks later this year than last, due to a late, cool Spring. (And, even last year, another cool year, the grapes budded out later than normal.) Second, we didn't have enough heat in this short summer to let the grapes catch up, so they stayed behind. Third, these heavy rains we're getting yesterday, today, tomorrow and Tuesday are super-early -- about a week earlier than last year (which was also very early) and three weeks earlier than the end-of-Septembe/early-October rain return date that we grapegrowers hope for. Heavy rain prevents further grape ripening and dilutes the grapes' flavors (the water content rises inside the grape). That can throw the desired sugar/acid balance out of whack. And it can even split the grape, which ruins it.
If you like numbers, look at Growing Degree Days (GDDs), which use temperature as a proxy for sunshine: Through today, my vineyard (Woodland, WA) has had 1,940 GDDs year-to-date, whereas in the warm year of 2015, we saw about 2,100 GDDs through this date -- a huge difference.
b. Grapes' defenses against predation: When it is cloudy or rainy, the birds take it as a signal to come in and eat the grapes. I saw that start up big-time a few days ago. It doesn't take many birds to eat out a good-sized vineyard in just a few days. Even nets don't totally protect the fruit. Also, wasps need to eat sugar before the winter, and they love grapes. Wasps have trouble piercing thick-skinned grapes, but thin-skinned grapes are easy prey. A grape like Riesling can hang into November wtih its tough skin, but a grape like Regent is toast after heavy rains and wasps. Pinot Noir is not very tough, either, and neither are some of my modern varieties.
c. Grapes' defenses against disease: I don't have to spray for fungus because my modern grape varieties are resistant, but a susceptible grape can succumb to fungus during a rainy spell when the farmer can't spray. Grape disease pressure rises very high, with high humidity.
d. Vineyard and grape variety factors: In an average summer, if Fall rains come early, I can have most of my grapes already picked and safely in the winery, because my vineyard has steep South-facing slope (which receives more solar radiation then a flat vineyard), and because my varieties ripen very early. But the grapes budded out so late this year and the rains came back super-early, so I was able to pick only two varieties so far, just before this deluge (half my Leon Millot and all my Labelle). I have many very-early varieties, but they are all way behind, so outside they sit, with my fingers crossed. Surely the most-common vinifera varieties (Pinot Noir, Chardonnay, Pinot Gris) are also way behind, because they ripen long after mine.
All of that sounds pretty bad, for this year.
However, if the grapes survive these rains, it looks like there are still many sunny days probably coming, after next week's two-day return of yet more rains. That is why it's so tough to say, now, whether this is a good or bad grape year here. The big question will be: Does anybody still have healthy grapes, with good flavors, after the rains, birds, wasps, and fungus have all done their worst? I am not very optimistic that most vineyards will do well this year. I know that I will lose a lot of my remaining fruit.
Many grapegrowers put such a heavy spin on late-season rain that they begin to lose credibility. They want you to think it's not so bad, because they have wines to sell. I've heard remarks like, "Oh, we needed some rain, because the grapes were very dry." Maybe, if the vine was so drought-stressed that there was danger the fruit wouldn't ripen. But usually a super-dry summer will result in great fruit, as the plant (pardon my anthropromorphism here) is worried about the drought conditions, and is in fear for its own survival, and tries extra hard to make its grapes the most-delicious-possible so the grape-eating predator will scatter the seeds for the mother plant. Irrigated vineyards might water during the summer, but by about mid-August they shut off the watering, because the fruit is best when it finishes dry. That is the plain truth.
On the hotter, dry side of the Cascades, it is probably still looking good.
This kind of summer makes me very glad that my vineyard is small and I don't need the wines from this year's fruit to be able to pay the bills. And yet, I have to say that the wines I'm already making should turn out good. But overall yield could be quite low this year, and overall quality could also be low, depending on the above factors for each specific location and variety.
(photo credit: Tanzania)