Climate Viticulture Newsletter - 2021 March

Average temperatures last month were within 3 °F of the 1981-2010 normal for almost all of the region (light green and yellow areas on map). Being on the periphery of the mid-month cold-air outbreak across the center of the country, far eastern New Mexico experienced monthly average temperatures 3 to 9 °F below normal (dark green and blue areas on map). As a comparison, temperatures in February last year were within 2 °F of the 1981-2010 normal across much of the region.

Relatively dry conditions once again plagued much of the region, with monthly totals less than 50% of the 1981-2010 normal (dark orange and red areas on map). In contrast, a few areas scattered across New Mexico received near- to above-normal amounts (yellow, green, and blue areas on map). This is different from February of last year, particularly for growing areas in the southern half of the region, when monthly precipitation totals were above normal.

One upshot from last month is that temperatures did not move the needle much in terms of heat accumulation and its influence on bud break and the start of the growing season for warm-climate growing areas in the region. We’ll look more at this below. In terms of precipitation, we are simply left shaking our heads. Just how dry is it? If a recent summary from the Tucson forecast office of the National Weather Service is any indication, it’s really dry. Total precipitation from March 2020 through February 2021 ranks as the driest 12-month period on record, with just over 3.5 inches. Many other areas throughout the region also are posting record to near-record dry conditions over this time span. Close attention to soil moisture levels and irrigation this dormant season looks necessary to help guard against delayed spring growth.

View more NOAA ACIS climate maps

There are equal chances for below-, near-, or above-normal temperatures for almost all of Arizona (white area on map). A slight increase in chances for above-normal temperatures exists for all of New Mexico (orange and light orange areas on map). Temperatures in March last year were near or up to 5 °F cooler than normal for much of Arizona, and near or up to 5 °F warmer than normal for much of New Mexico.

A slight increase in chances for below-normal precipitation exists for almost all of Arizona (light and dark tan areas on map), whereas a moderate increase in chances for below-normal precipitation exists for almost all of New Mexico (brown area on map). Monthly precipitation totals in March 2020 were near- or above-normal across much of the region, with a particular exception being northeastern New Mexico. In fact, most of Arizona and southern New Mexico recorded totals greater than twice the normal amount we would expect in March. Unfortunately, the odds do not look to be in favor of a repeat performance of such wet conditions over the next four weeks.

Without a strong signal of which way temperatures might lean for the rest of this month, it remains hard to say what their effect on heat accumulation and the start of the growing season will be from where the situation is right now, something we’ll take a look at in the next section. What is becoming easier to say for precipitation is that the current La Niña event is waning and that ENSO neutral – neither La Niña nor El Niño – conditions most likely return by late spring or early summer. Not that the need to irrigate will have much time to relax once the growing season is underway, however, as April, May, and June typically are relatively dry months for the region, regardless of what is happening in the tropical Pacific Ocean.

Read more about the March 2021 outlook

At the start of this month, values of both cumulative chill portions and cumulative growing degree days closely track those observed during the past two years, winters 2019-2020 and 2018-2019, at the AZMET Willcox Bench station in the south-central part of the Willcox AVA (green, pink, and blue lines on graphs). This suggests that chilling requirements – which can be different between varieties – were met and the switch from endodormancy to ecodormancy occurred on similar dates during these three years. Although the cumulative amount of chill portions corresponding to this switch for individual varieties is unknown, chilling requirements in general are low for grapevines and easily met in growing areas in Arizona and New Mexico. Vines remain in ecodormancy until environmental conditions become optimal for root activity and bud growth, a point commonly estimated by heat accumulation. Based on degree-day calculations from the National Phenology Network, heat accumulations for other warm-climate growing areas in the region also appear similar to those from this time last year.

Although it’s still early in the season for heat accumulation, things start to pick up during March, as shown by the lower graph. We’ll check back into this topic with a special newsletter issue in the middle of the month.