Climate Viticulture Newsletter - 2025 January

Monthly average temperatures were more than 5 °F above the 1991-2020 normal for much of the southeastern, central, and northwestern parts of the state (red-orange areas on map), including the Sonoita AVA and parts of the Verde Valley AVA. Elsewhere in Arizona, temperatures mostly were 3 to 5 °F above normal (light orange and orange areas on map), including the Willcox AVA. For reference, monthly temperatures in December 2023 were 1 to 4 °F above normal for much of the state. Several areas in central and northwestern Arizona recorded temperatures more than 4 °F above normal.

Area-average maximum and minimum temperatures during December 2024 were 67.3 and 37.6 °F for the Sonoita AVA, 66.3 and 33.2 °F for the Verde Valley AVA, and 66.2 and 30.4 °F for the Willcox AVA. Respective December normals are 58.3 and 31.5 °F, 57.2 and 30.0 °F, and 59.0 and 30.2 °F.

Temperature last month ranged between 75.7 and 23.0 °F at the AZMet Bonita station and between 78.1 and 21.4 °F at the AZMet Willcox Bench station.

Monthly precipitation totals were less than 25 % of normal for all of the state (dark yellow areas on map). Precipitation during December 2023 was less than 75 % of normal for much of northwestern Arizona. Otherwise, totals were near normal, with scattered areas along the southern tier of the state measuring more than 125 % of normal.

Area-average total precipitation in December 2024 was 0.00 inches for the Sonoita AVA, 0.00 inches for the Verde Valley AVA, and 0.00 inches for the Willcox AVA. Respective December normals are 1.40, 1.37, and 1.08 inches.

Total precipitation last month was 0.00 and 0.00 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 a moderate increase in chances for being above the 1991-2020 normal across all of Arizona (red-orange area on map). Monthly average temperatures in January last year were 1 to 3 °F below normal for much of southern, central, and western Arizona. Much of the rest of the state measured near-normal temperatures.

Area-average maximum and minimum temperatures during January 2024 were 56.5 and 30.7 °F for the Sonoita AVA, 54.8 and 29.8 °F for the Verde Valley AVA, and 57.0 and 30.1 °F for the Willcox AVA. Respective January normals are 58.7 and 31.2 °F, 58.1 and 30.6 °F, and 59.6 and 30.2 °F.

Temperature in January last year ranged between 69.1 and 18.7 °F at the AZMet Bonita station and between 69.8 and 17.8 °F at the AZMet Willcox Bench station.

Precipitation totals for this month have a moderate increase in chances for being below normal across the south-central and southeastern parts of the state (brown area on map). For the rest of Arizona, there is a slight increase in chances for below-normal totals (tan and dark tan areas on map). Precipitation during January 2024 was more than 150 % of normal for the southern tier of the state, and less than 75 % of normal for much of central and west-central Arizona. Otherwise across the state, amounts were near normal.

Area-average precipitation totals in January 2024 were 1.91 inches for the Sonoita AVA, 0.70 inches for the Verde Valley AVA, and 1.38 inches for the Willcox AVA. Respective January normals are 1.24, 1.45, and 1.07 inches.

Total precipitation in January last year was 0.78 and 1.08 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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For the first part of our review of this past growing season, we focus on maximum and minimum temperatures in the three Arizona AVAs. We organize the data by different periods. For simplicity, pre-veraison includes the months of April, May, and June (top two graphs), ripening and harvest the months of July, August, and September (middle two graphs), and post-harvest the months of October and November (bottom two graphs).

Pre-veraison temperatures in 2024 were within 1 to 2 °F of the 1991-2020 normal for all three AVAs and for both maximum and minimum temperatures (values to the right of the graphs). This contrasts with several recent growing seasons (variable, colored lines with points on graphs), when temperatures averaged over these months were warmer and above respective normals (horizontal, colored lines on graphs). One of the effects of year-to-year variations in temperature during April, May, and June is on heat accumulation and the timing of growth stages between bud break and veraison. Generally, cooler conditions lengthen the time it takes for vines to go from the start of the growing season to the point when grapes start to ripen. As you can imagine, this has implications for when grapes ripen and the conditions under which they do so.

Like many of the most recent growing seasons, maximum temperatures during ripening and harvest in 2024 were above normal for all three AVAs, with values 2 to 4 °F warmer in the Sonoita and Willcox AVAs and 4 to 5 °F warmer in the Verde Valley AVA. July-through-September daytime temperatures thus ranked among the hottest for each of these grape-growing areas. Minimum temperatures during these months also were 1 to 3 °F above normal for the Sonoita and Verde Valley AVAs, but within 1 °F of normal for the Willcox AVA. There’s little doubt that another lackluster monsoon this year had some say on these values, as less cloud cover and rain allow for more heat. There’s also little doubt that this part of the growing season is when climatic differences between the three Arizona AVAs show. Such differences might be a reason to suspect differences in fruit composition between the AVAs, to the extent that exposure to temperatures below 65 °F and above 95 °F affect aspects like balance between sugar levels and acidity as well as degradation of anthocyanins.

Also like many of the most recent growing seasons, post-harvest maximum temperatures in 2024 were above normal for all three AVAs, with values 2 to 5 °F warmer. Plainly, the near-record to record heat in October, and not a cooler-than-normal November, drove these statistics. Minimum temperatures averaged over October and November were within 1 to 2 °F of normal. Year-to-year variations in post-harvest temperatures – in combination with dates of the first fall freeze and dormancy – influence irrigation at this time of year as well as carbon assimilation and storage that is important for the following growing season.

We’ll have a similar review of the 2024 growing season with a focus on precipitation in the issue next month.

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