Technically, verasion is a term that applies to one of the periods of the ripening process when the fruit changes color. However, during this period the grower must be alert and sensitive to what is going on in the vineyard.
During the months of June and July the grape bunches and individual grapes gradually grow and develop. During this period the sugar level is about 6%. Along about the first week in August the grapes suddenly begin to accumulate sugar at a rapid rate. This period is called verasion. The individual grapes begin to turn purple for red wines or from opaque emerald green to translucent golden green for whites. And during all this the sugar level is steadily increasing moving in a matter of three to six weeks from 6% to 22 -24% sugar. This is the magic of photosynthesis. The grapes will generate about a ton of sugar per acre.
This is a critical and very stressful time for vintners. Weather wise, the grapes need warm days with temperatures between 85 and 95 degrees and no rain. Nighttime temperatures should be 65 - 75 degrees. And the irrigation regime has to be carefully controlled. Too much water can pump up the grape bunches with excess moisture and maybe even cause the vines to start growing new leaves and shots. Too little water can cause defoliation, wilting causing the vine to stop producing sugar. Sudden hot spells or cold foggy days can cause serious problems for the ripening bunches.
This is the current stage for the grapes that are making up the Napa Vayniac Cabernet Sauvignon. There has been a cool, wet spell that has increased the "hang time" which could result in great flavor concentration. It will also allow the grape seed to ripen from green to brown. For winemakers pursuing longer hang time has it pros and cons. The longer the grape is on the vine, the riper the tannins (astringent components in the skin, seeds and stems) become and the wines are softer and more lush and smooth. However, the grapes lose moisture (weight) as they become concentrated and this can mean a loss of profit as the growers are paid by tonnage. Also, overripe fruit loses the acidity it needs to bottle age and the wine will taste "flabby".
What constitutes full development/potential? For most of us, the results are worth the wait.