Climate Viticulture Newsletter - 2023 September

Monthly average temperatures were 1 to 4 °F above the 1991-2020 normal for much of southern, central, and eastern Arizona (light orange, orange, and dark orange areas on map), including all of the Sonoita and Willcox AVAs and much of the Verde Valley AVA. Much of the rest of the state recorded near-normal temperatures (white areas on map), except for extreme northwestern Arizona, where temperatures were 1 to 3 °F below normal for several locations (light blue and blue areas on map). For reference, temperatures in August last year were within 2 °F of normal for much of the state except for part of southeastern Arizona, where temperatures were 2 to 4 °F below normal.

Area-average maximum and minimum temperatures during August 2023 were 90.5 and 63.5 °F for the Sonoita AVA, 96.5 and 67.0 °F for the Verde Valley AVA, and 94.2 and 64.6 °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.

Monthly precipitation totals were less than 75 % of normal for many parts of southern and central Arizona (light yellow, yellow, and dark yellow areas on map), including the Sonoita and Willcox AVAs. Much of the rest of the state measured a mix of either near-normal (white areas on map) or between 125 and 200 % above-normal amounts (light aqua, aqua, and dark aqua areas on map), including the Verde Valley AVA. Parts of extreme western Arizona received over 250 % of normal rainfall last month due to tropical storm Hilary (dark blue areas on map). Many locations across the state measured amounts more than 150 % of normal during August 2022 due to an active monsoon.

Area-average total precipitation in August 2023 was 2.27 inches for the Sonoita AVA, 2.65 inches for the Verde Valley AVA, and 1.62 inches for the Willcox AVA. Respective August normals are 3.95, 2.40, and 2.80 inches.

Conditions last month drove total reference evapotranspiration values for August to 7.2 inches for both the AZMet Bonita and Willcox Bench stations in the Willcox AVA. Those values bring respective growing season totals to 38.4 and 40.8 inches. Total precipitation from April through August, in contrast, is 0.78 at Bonita and 1.01 inches at Willcox Bench.

Dig into daily weather summaries from 2023 for the AZMet Bonita and Willcox Bench stations in the Willcox AVA

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

Learn more about PRISM climate data

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Temperatures over the course of this month have a slight increase in chances for being above the 1991-2020 normal across all of Arizona (light orange and orange areas on map). Monthly temperatures in September last year were within 2 °F of normal for much of the southern one-third of the state and 2 to 4 °F above normal for much of the northern two-thirds.

Area-average maximum and minimum temperatures during September 2022 were 83.3 and 57.8 °F for the Sonoita AVA, 93.5 and 61.2 °F for the Verde Valley AVA, and 87.0 and 59.0 °F for the Willcox AVA. Respective September normals are 83.9 and 57.0 °F, 90.4 and 58.4 °F, and 87.8 and 58.3 °F.

Precipitation totals for this month have a slight increase in chances for being above normal across the northwestern part of the state (light aqua area on map). Otherwise for Arizona, there are equal chances for below-, near-, or above-normal totals (white area on map). Precipitation during September 2022 varied widely across the state, with amounts generally ranging from near to 150 % of normal.

Area-average precipitation totals in September 2022 were 2.05 inches for the Sonoita AVA, 2.05 inches for the Verde Valley AVA, and 2.13 inches for the Willcox AVA. Respective September normals are 1.95, 1.53, and 1.40 inches.

We might be giving away a take-away point from the following section by writing this, but with the 2023 monsoon all but wrapped up and with the above outlook in mind, ripening conditions this year – and possible impacts to vineyards and fruit – seem to be more like those from 2020 than from other recent years. In terms of monsoon hedging, or having a variety lineup and adaptability in the winery to limit losses from unfavorable conditions at this time of the growing season, we wonder if hedges for hot-and-dry vintages help this year. Please let us know if varietal characteristics like maintaining pH under high temperatures or decisions between early and late picks are making a positive difference.

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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Based on data from the AZMet Willcox Bench station in the Willcox AVA, conditions in August led to three days with average hourly temperatures over 100 °F, for a total of 13 hours (dark red bars on top graph). This was, perhaps, a welcome change from the record heat in July, when there were 19 such days and 66 such hours.

The highest average hourly temperature last month was 104.9 °F on August 5. Temperatures between 95 and 100 °F occurred on 13 days, for a total of 45 hours (light red bars on top graph). There were 11 days with temperatures below 65 °F, with measurements below 60 °F on only two days. During these, 38 hours were between 60 and 65 °F (light blue bars on top graph), and two hours were below 60 °F (dark blue bars on top graph). The lowest average hourly temperature was 58.6 °F on August 13. Short of testing fruit composition during the ripening season, exposure to these temperature ranges may provide insight to how conditions at this time of the growing season might affect fruit quality through both early morning minimums and afternoon maximums.

Although data collection at the AZMet Willcox Bench station starts in June 2016, we only compare measurements from this year to the previous three as, before this year anyhow, temperature and precipitation from those three growing seasons represented the overall range, or endpoints, of conditions experienced in recent decades. Exposure to high maximum daily temperatures so far in 2023 is greater than that in 2022 and 2021 (middle two graphs), and more like what we measured in 2020 (bottom graph). All else equal, we would expect ripening and measures of fruit quality affected by heat to vary in a similar fashion.

Of course, we’re reporting temperatures measured by a shaded thermometer at an AZMet station near several vineyards. Actual canopy temperatures and shaded clusters likely are lower due to evaporative cooling from vine photosynthesis, while those of unshaded clusters higher due to direct mid-day and afternoon sunlight, in particular. Nonetheless, recent temperatures bring canopy management to mind as a tool to help address heat exposure like we’ve had this summer.

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