Climate Viticulture Newsletter - 2020 November

Average temperatures last month were 2°F to 6°F warmer than the 1981-2010 normal for much of Arizona (light and dark orange areas on map). Such warmer-than-normal temperatures also were widespread in northwestern and southwestern New Mexico, while near-normal temperatures occurred over much of the rest of the state (yellow and light green areas on map). Despite the mostly warm conditions, a few days of noticeably colder temperatures appeared late in the month, resulting in some records as well as the first fall freeze that marked the end of the 2020 growing season for many locations. For comparison, temperatures in October 2019 were within 3°F of normal for much of the region, with parts of northern and eastern New Mexico 3°F to 6°F below normal.

Even though the cold, late-October storm brought rain and snow, monthly precipitation totals still were less than 50% of the 1981-2010 normal for almost all of Arizona (dark orange and red areas on map) and between 25% and 75% of normal for most of New Mexico (dark and light orange areas on map). A few locations in the latter state did luck out, however, and measure above-normal totals (green, blue, and purple areas on map). October precipitation last year was similarly dry except for southeastern New Mexico, where tropical-storm remnants arrived and above-normal amounts occurred.

For some areas of the region like southeastern Arizona, fall 2020 so far ranks as the warmest and driest on record. After a record-setting hot and dry summer, such conditions likely did no favors during the post-harvest period, unless vineyard irrigation was adequate. Further affecting this part of the growing season for the second year in a row was a late-October hard freeze. The time after harvest for vines to photosynthesize and store carbohydrates thus may have been less effective and shorter relative to some previous years, particularly for warmer growing areas. Hopefully, these events do not impact carbohydrate reserves that plants will use for canopy and fruit growth in 2021.

There is a moderate increase in chances for above-normal temperatures across all of Arizona and New Mexico (dark orange area on map). As a comparison, temperatures in November last year were within 2°F of normal for much of the region, except for southern Arizona where temperatures were 2°F to 4°F above normal.

A moderate increase in chances for below-normal precipitation exists for eastern New Mexico (brown area on map), while a slight increase in chances for below-normal precipitation exists for western New Mexico and all but southwestern Arizona (light and dark tan areas on map). Equal chances for below-, near-, or above-normal precipitation exist for this corner of the Grand Canyon state (white area on map). Monthly precipitation totals in November 2019 were above 200% of normal for many areas, as several storms passed through the region.

With an elevated potential for warm and dry conditions to continue, due in part to the ongoing La Niña event, soil moisture in the vineyard may need closer attention this winter relative to the past two. Adequate levels will help guard against delayed spring growth, which manifests as uneven bud break, stunted growth, reduced flower clusters, and other issues.

The average temperature between April 1 and October 31 is a simple measure used to compare different winegrape growing regions and as a first guess of which grape varieties might do well in a new area. Looking at the Sonoita, Willcox, and proposed Verde Valley AVAs as an example for our region, 2020 growing season temperatures – unsurprisingly – are the warmest of the past 40 years (orange lines and shading in graphs¹). They also are the most recent in a string of mostly above-normal years since 2000.

As noted in our look at this topic last year, however, the devil is in the details. It matters when below-, near-, or above-normal temperatures occur during a growing season, because temperature influences so many aspects of vine growth and fruit quality. We’ve touched upon several of these in this and previous newsletter issues this year, often in the context of heat stress. Please let us know how much of a mark you think temperature will make on the 2020 vintage.

¹ Orange lines represent the average growing season temperature across an individual AVA, and orange shading represents the range of growing season temperatures across an individual AVA, except for the coldest and hottest locations.

One way in which we’ve tracked the influence of temperature this year is by the number of hours below 65°F and above 95°F during the ripening period. The reason why is that temperatures affect fruit quality through both afternoon maximums and early morning minimums.

At the AZMET Willcox Bench station in the Willcox AVA and ZiaMet Fabian Garcia Science Center station in the Mesilla Valley AVA, hours below 65°F were almost non-existent (blue and purple bars on graph) during July, August, and the first week of September in 2020, whereas hours above 95°F were almost a daily occurrence (yellow, orange, and red bars on graph). Only during the second, third, and fourth weeks of September did hours above 95°F relent and hours below 65°F return.

How similar is this amount of heat to other growing regions? Using California as an example, the hours per day below 65°F and above 95°F at these two stations look similar to those at the CIMIS Fresno State station in Fresno, CA. The pattern of daily temperature ranges at the Atascadero station in the Paso Robles AVA and Oakville station in the Napa Valley AVA, however, is different. Most days throughout the ripening period show several hours below 65°F, whereas very few days – many of which are during mid-August and early-September heat waves – show hours above 95°F. Comparisons like this can help with evaluating vineyard management options, such as trellis design and canopy characteristics, that are used in other growing regions and determining whether they are optimal for mitigating heat impacts in warmer growing areas of the Southwest.