Climate Viticulture Newsletter - 2024 August

Monthly average temperatures were 1 to 4 °F above the 1991-2020 normal for much of the southeastern, northeastern, and north-central parts of Arizona (light orange and orange areas on map), including the three Arizona AVAs. Much of the rest of the state measured record-level temperatures of more than 4 °F above normal (dark orange areas on map). For reference, monthly average temperatures in July last year also set heat records and were 3 to 5 °F above normal for much of the northern half of the state, and more than 5 °F above normal for much of the southern half.

Area-average maximum and minimum temperatures during July 2024 were 92.2 and 64.8 °F for the Sonoita AVA, 103.0 and 68.4 °F for the Verde Valley AVA, and 95.4 and 66.4 °F for the Willcox AVA. Respective July normals are 88.5 and 62.8 °F, 98.1 and 66.0 °F, and 93.6 and 64.9 °F.

Temperature last month ranged between 103.6 and 61.3 °F at the AZMet Bonita station and between 104.5 and 60.1 °F at the AZMet Willcox Bench station.

Monthly precipitation totals were near normal for much of the southeastern and north-central parts of the state (white areas on map), including the three Arizona AVAs. Much of the rest of Arizona recorded totals less than 75 % of normal (light yellow, yellow, and dark yellow areas on map). Precipitation during July 2023 was between 0 and 75 % of normal for many locations in north-central, extreme south-central, and southeastern Arizona. Otherwise across the state, most locations measured less than 25 % of normal rainfall.

Area-average total precipitation in July 2024 was 3.66 inches for the Sonoita AVA, 1.94 inches for the Verde Valley AVA, and 2.77 inches for the Willcox AVA. Respective July normals are 4.06, 1.75, and 2.67 inches.

Total precipitation last month was 3.22 and 1.35 inches at the AZMet Bonita and Willcox Bench stations, respectively.

Dig further into daily weather summaries for the AZMet Bonita and Willcox Bench stations in the Willcox AVA

View seasonal summaries of monsoon rainfall through regional maps and weather station statistics

Learn more about PRISM climate data

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Temperatures over the course of this month have a moderate increase in chances for being above the 1991-2020 normal across all but the extreme northern part of Arizona (dark orange and red areas on map), where there is a strong increase in chances for above-normal temperatures (dark red area on map). Monthly average temperatures in August last year were 1 to 4 °F above normal for much of southern, central, and eastern Arizona. Much of the rest of the state recorded near-normal temperatures except for extreme northwestern Arizona, where temperatures were 1 to 3 °F below normal for several locations.

Area-average maximum and minimum temperatures during August 2023 were 90.3 and 63.5 °F for the Sonoita AVA, 97.2 and 66.8 °F for the Verde Valley AVA, and 93.7 and 64.5 °F for the Willcox AVA. Respective August normals are 86.3 and 61.7 °F, 95.7 and 65.2 °F, and 91.2 and 63.8 °F.

Temperature in August last year ranged between 106.3 and 59.0 °F at the AZMet Bonita station and between 104.9 and 58.6 °F at the AZMet Willcox Bench station.

Precipitation totals for this month have a slight increase in chances for being above normal across the southeastern and east-central parts of the state (light aqua area on map). For the rest of Arizona, there are equal chances for below-, near-, or above-normal totals (white area on map). Precipitation during August 2023 was less than 75 % of normal for many parts of southern and central Arizona. Much of the rest of the state measured a mix of near-normal and between 125 and 200 % above-normal amounts.

Area-average precipitation totals in August 2023 were 2.44 inches for the Sonoita AVA, 2.60 inches for the Verde Valley AVA, and 1.53 inches for the Willcox AVA. Respective August normals are 3.95, 2.40, and 2.80 inches.

Total precipitation in August last year was 0.65 and 0.41 inches at the AZMet Bonita and Willcox Bench stations, respectively.

To stay informed of long-range temperature and precipitation possibilities beyond the coverage of a standard weather forecast, check in, too, with the six-to-ten-day outlook and eight-to-fourteen-day outlook issued daily by NOAA’s Climate Prediction Center.

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Heat accumulation from April through July this year at the AZMet Bonita and Willcox Bench stations (dark gray bars on graphs) is more than what it was during this time last year, albeit not by much. This suggests that the time between bud break and veraison, or the start of berry ripening, may be slightly less this growing season than what it was in the previous one. It might also suggest slightly earlier veraison dates, assuming similar bud break dates in 2024 and 2023. Also, heat accumulation continues to be greater at the AZMet Willcox Bench than at the AZMet Bonita, suggesting that a given variety in the south-central part of the Willcox AVA is at a more advanced growth stage than in the northern part, all else equal. Over the past four years (dark gray and gray bars on graphs), the greatest amount of heat accumulation for both locations at this point in the growing season happened in 2022, and the least in 2023.

Unlike previous years, we’re using heat units with an upper threshold of 94 °F and a lower threshold of 55 °F to measure heat accumulation. With heat units, in theory, plants slowly start to progress through growth stages under temperatures near the lower threshold, reach an optimal development rate at temperatures between the lower and upper thresholds, and progressively slow development under temperatures near the upper threshold. Outside of the range between the lower and upper thresholds, as what happened during some afternoons last month at both the Bonita and Willcox Bench stations and respective to the latter, plant development does not occur. Heat Units 94-55 °F is one of the heat unit variables provided by AZMet and the one with lower and upper temperature thresholds closest to what we’ve read are those for wine grapes.

Is this best measure of heat accumulation for modeling growth stages of wine grapes in Arizona? We don’t know. Nonetheless, it provides a relative characterization of conditions thus far this growing season. We’re looking to work on answering this question in the coming months.

More information on the calculation of heat units is in Extension bulletin AZ1602 'Heat Units'. An online data tool to calculate cumulative heat units by station and date range is available on the AZMet website.

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Now that the ripening period has started, we return to tracking hours of temperatures below 65 °F and above 95 °F. The reason why is that temperatures affect fruit quality through both early morning minimums and afternoon maximums as fruit ripens. With the former, for example, morning minimums above 65 °F can increase malic acid metabolism and lead to berries with a higher pH. With the latter, afternoon maximums above 95 °F can degrade anthocyanins, particularly in red-berry varieties.

With data from the AZMet Willcox Bench station in the Willcox AVA, above-normal temperatures last month led to three days with average hourly temperatures over 100 °F, for a total of nine hours (dark red bars on top graph). The highest average hourly temperature was 104.5 °F on July 8. Temperatures between 95 and 100 °F occurred on 18 days, for a total of 56 hours (light red bars on top graph). This is less heat exposure than in July 2023, when record-level heat led to 19 days and 66 hours with average hourly temperatures above 100 °F, and 30 days and 123 hours between 95 and 100 °F.

There were two days last month with average hourly temperatures below 65 °F (light blue bars on top graph), for a total of five hours. No values were below 60 °F (dark blue bars on graphs). The lowest average hourly temperature was 60.1 °F on July 13. Plainly, the increase in humidity due to the monsoon during July is a typical feature that keeps daily minimum temperatures higher, as less heat radiates away from the surface and into the upper levels of the atmosphere during the night.

Based on experimental measurements in 2022 near the Willcox Bench station, most canopy temperatures were within 3 to 5 °F of station temperatures, with a tendency for canopy temperatures to be warmer than those at the station between 7 AM and 4 PM, most notably around noon. During other hours, canopy temperatures tended to be cooler than those at the station. This pattern makes biophysical sense and may be due in part to the absorption of sunlight by the canopy and subsequent dissipation of excess heat to the surrounding air. This pattern, and the temperatures measured last month, also bring canopy management to mind as a tool to shade clusters from mid-day and afternoon sunlight, as exposed berries are hotter than the surrounding air.

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