Harvest in monsoon season: how Arizona picks between storms

Arizona picks its wine grapes in the wettest part of its year. Since 2008 the National Weather Service has treated June 15 to September 30 as the monsoon season, and the storms that matter to a vineyard come mostly in July and August, while the fruit ripens. At Page Springs Cellars in Cornville the harvest tour runs only from late August to mid October, a fair guide to when the picking crews are out.

How much rain, and where

June is dry almost everywhere. Then the pattern turns. These are the area-average rainfall normals, in inches, that the University of Arizona team behind the Climate Viticulture Newsletter uses for the three AVAs:

Sonoita normally gets about 8 inches across July and August, twice what the Verde Valley gets.

Averages hide how patchy the storms are. In July 2024 the Willcox AVA as a whole received 2.77 inches, close to its normal. The university’s two weather stations inside it recorded 3.22 inches at Bonita and 1.35 at Willcox Bench. Two vineyards in the same AVA can live through quite different summers.

The same team’s long-term records show the Verde Valley warmer and drier than Willcox through the ripening months, and Willcox warmer and drier than Sonoita. Wet summers also change the temperature. Rainy stretches bring cooler afternoons, but cloud and humidity keep the nights from cooling as much, so the gap between day and night narrows, and that gap is often tied to fruit quality in writing about wine.

What a storm does to ripe fruit

Rot is the main risk. The newsletter notes that rot in ripening berries becomes the issue in July and August, and that varieties with tight clusters dry slowly after a storm. A cluster that lost a few berries in spring, which growers call shatter and usually count as a loss, can turn out better placed, because the looser fruit dries sooner. Rain is not the only damage. A vineyard at 4,300 feet in the Willcox AVA has described storms that bring down vines with 50 mph winds and damage plants with hail, as well as rotting fruit with too much rain.

The university’s Cooperative Extension guide to grape problems advises scouting for sour rot a few weeks before harvest, from about 12 to 13 Brix. Fruit flies lay eggs in damaged berries and carry the yeasts and bacteria that turn them to vinegar.

Three summers that disagreed

The years 2020 to 2022 showed how far one vintage can sit from the next. In 2020 the ripening months were record hot and record dry across the state’s vineyards. In 2021 and 2022 many places had near-record or record rain. In July 2021 a large part of Arizona received more than twice its normal amount. The Verde Valley was the exception in 2022, with close to normal totals. The climate team called the sequence weather whiplash and suggested that growers treat it as a trial of which varieties coped with each extreme.

Hedging the vintage

No one can pick the weather, so growers spread the risk instead. The newsletter’s June 2023 issue called this monsoon hedging. In the vineyard it means planting early and late ripening varieties side by side, choosing grapes that hold their pH in heat, and choosing others that resist rot when it is wet. In the cellar it means being ready to switch between a varietal wine and a blend, or to pick some fruit early for sparkling wine instead of later for a table wine. The newsletter’s authors put it simply: they think “the monsoon makes the vintage.”