Climate Viticulture Newsletter - 2024 May

Monthly average temperatures were 1 to 4 °F below the 1991-2020 normal for much of the southern half of Arizona (light blue, blue, and dark blue areas on map), including all three Arizona AVAs. Much of the rest of the state saw near-normal temperatures (white areas on map). For reference, monthly average temperatures in April last year were within 2 °F of normal for almost all of the state.

Area-average maximum and minimum temperatures during April 2024 were 71.3 and 40.3 °F for the Sonoita AVA, 73.6 and 42.3 °F for the Verde Valley AVA, and 73.7 and 40.2 °F for the Willcox AVA. Respective April normals are 73.5 and 41.9 °F, 75.5 and 42.6 °F, and 76.7 and 42.0 °F.

Temperature last month ranged between 87.3 and 28.0 °F at the AZMet Bonita station and between 87.8 and 29.7 °F at the AZMet Willcox Bench station.

Monthly precipitation totals were more than 200 % of normal for much of the southern half of the state (dark aqua, blue, and dark blue areas on map), including the Willcox AVA and part of the Sonoita AVA. Of course, April is a relatively dry month, and it doesn’t take much precipitation to make such high percentages. These areas of above-normal amounts transition to mostly near-normal ones in much of the northern half of Arizona (white areas on map), including the Verde Valley AVA, which transition to totals below 75 % of normal (light yellow and yellow areas on map) across the northern tier. Precipitation during April 2023 was less than 25 % of normal for almost all of Arizona.

Area-average total precipitation in April 2024 was 0.66 inches for the Sonoita AVA, 0.57 inches for the Verde Valley AVA, and 0.70 inches for the Willcox AVA. Respective April normals are 0.38, 0.55, and 0.25 inches.

Total precipitation last month was 0.18 and 0.42 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

Learn more about PRISM climate data

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Temperatures over the course of this month have equal chances for being below, near, or above the 1991-2020 normal across all of Arizona, except for the extreme western part of the state (white area on map), where there is a slight increase in chances for near-normal temperatures (light gray area on map). Monthly average temperatures in May last year were within 2 °F of normal for almost all of the state.

Area-average maximum and minimum temperatures during May 2023 were 80.7 and 46.8 °F for the Sonoita AVA, 84.6 and 49.8 °F for the Verde Valley AVA, and 83.4 and 48.0 °F for the Willcox AVA. Respective May normals are 81.7 and 49.6 °F, 84.5 and 50.0 °F, and 84.8 and 48.9 °F.

Temperature in May last year ranged between 89.8 and 34.7 °F at the AZMet Bonita station and between 90.9 and 34.2 °F at the AZMet Willcox Bench station.

Precipitation totals for this month have equal chances for being below, near, or above normal across all of Arizona (white area on map), except for the extreme eastern part of the state, where there is a slight increase in chances for below-normal amounts (light tan area on map). Precipitation during May 2023 was more than 150 % of normal for many areas in the west-central and southeastern parts of the state. Otherwise, and apart from much of the southwestern and south-central parts of the state that measured amounts below 50 % of normal, monthly totals mostly were near normal.

Area-average precipitation totals in May 2023 were 0.22 inches for the Sonoita AVA, 0.71 inches for the Verde Valley AVA, and 0.43 inches for the Willcox AVA. Respective May normals are 0.22, 0.44, and 0.29 inches.

Total precipitation in May last year was 0.06 and 0.17 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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With bud break likely done in perhaps all but the latest varieties and coolest growing areas in the state, we’ll take one last look at chill and heat accumulations influencing the start of the growing season this year. As we only use temperature data here from the AZMet Willcox Bench station, our assessment is most applicable to the south-central part of the Willcox AVA.

Temperatures in April maintained what we observed in the newsletter issue last month, which is similar chill and heat accumulations in 2024 and 2020 (dark blue and blue dots on graph, respectively). But, as temperature variability in the weeks leading up to bud break influence bud break dates, conditions this year also are like those last year at this point on the calendar. Using the lens of deacclimation kinetics and past ‘analog’ years, this suggests a start date for the 2024 growing season in the range of those from 2023 and 2020. Granted, something that might upend this estimate for this year and for some varieties is the overnight low temperatures of 29.7 and 31.3 °F on April 7 and April 16, respectively.

Chill portion accumulation through May 1 this year is above the station average (dark blue dot and gray dotted line, respectively, in left graph), with higher values only in 2019 and 2023. Heat accumulation is below average (dark blue dot and gray dotted line, respectively, in middle graph), and the lowest value of the past eight years. As soil temperature sometimes is part of the discussion about the start of the growing season, we show those data, as well. The daily average on May 1 this year is near average (dark blue dot and gray dotted line, respectively, in right graph).

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With the start of the 2024 growing season, it’s time to review this past dormant season. Much like the recent reviews of the 2023 growing season in the January and February newsletter issues, we focus on average maximum and minimum temperatures (top and middle graphs, respectively) as well as total precipitation (bottom graph) in the three Arizona AVAs. We also add some statistics from the AZMet Bonita and Willcox Bench stations in the Willcox AVA at the end of this section.

Average maximum temperatures during the period from November through March were above the 1991-2020 normal (dashed horizontal lines on graphs) for all three Arizona AVAs. For the Sonoita and Willcox AVAs (green and purple lines on graphs, respectively), these differences were about 2 to 3 °F, whereas it was about 3 to 4 °F for the Verde Valley AVA (orange lines on graphs). One need not go back far in the data to find similar anomalies, only to the dormant seasons in 2020-2021 and 2021-2022.

Average minimum temperatures during this past dormant season also were above the 1991-2020 normal for all three Arizona AVAs. For the Verde Valley and Willcox AVAs these differences were about 1 to 2 °F, whereas it was about 2 to 3 °F for the Sonoita AVA. Once again, similar anomalies occurred in the 2020-2021 and 2021-2022 dormant seasons.

Precipitation totals from November through March were between 110 and 120 % of normal for the Sonoita and Willcox AVAs, with amounts between 5 and 7 inches. Similar past dormant seasons include 2018-2019 and last year. For the Verde Valley AVA, total precipitation during the past five months was about 90 % of normal, between 5 and 6 inches and less than last year. Similar past dormant seasons include 2018-2019.

Average daily maximum and minimum temperatures during this past dormant season were 63.3 and 34.0 °F at the AZMet Bonita station, with the highest temperature of 85.5 °F on November 5, 2023 and the lowest of 18.7 °F on January 9, 2024. Total precipitation and evapotranspiration was 4.30 and 14.89 inches, respectively. Respective values for the AZMet Willcox Bench station were 64.7 and 33.8 °F, 86.9 °F on November 5, 2023 and 17.8 °F on January 9, 2024, and 4.21 and 15.28 inches.

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