Skies to clear by weekend as grape harvest continues

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Summer segued damply into fall this weekend as a couple of relatively weak fronts passed through the Walla Walla Valley, bringing some light rain and cooler temperatures to the area. Precipitation overnight Saturday and again Sunday evening amounted to a rather paltry 0.011 inches, but it was the fall-like temperatures that got most peoples’ attention, as the mercury dipped from a high of 82 degrees on Friday afternoon to the upper 50s in the rain on Sunday evening.

Our local wineries are receiving harvested grapes at a good clip now — particularly from warmer growing regions like Red Mountain and the Wahluke Slope. The extraordinarily warm summer has accelerated the ripening of most varieties, and some wineries are reporting that certain blocks of grapes are maturing as much as a full month in advance of their normal harvest dates.

The number of growing degree days (GDDs) accumulated here in the Walla Walla AVA this year is indicative of the exceptionally warm growing season. GDDs are calculated between Apr. 1 and Oct. 31 by taking the average of the daily maximum and minimum temperatures compared to a base temperature of 50 degrees. This is a seasonal summation, and each day’s number is added to the running total. For example, a high of 80 and a low of 50 equates to 15 degree-days (80+50=130, 130/2=65, 65-50=15). Through Sunday, Walla Walla had amassed 2,945 GDD, which is equivalent to the average for an entire season here — with more than a month left to go. Compared to recent years, grape growers are enjoying a heat luxury that has not been experienced for a decade, and the comparison to a year like 2011 (aka “The Year With No Summer”) is stunning: 2,945 through Sept. 22 vs. 2,304 through the same date in 2011. It is no wonder that local viticulturists are all smiles this year. All their fruit will almost certainly ripen to its maximum maturity, and the threat of extended periods of rain and/or an untimely frost becomes less of a concern with the earlier picking dates.

Brief bouts of rain like those we had this past weekend are not particularly troubling — especially for most red varieties, whose skins are generally thicker and tougher than many white varieties. It is when soaking rains are followed by cloudy, windless and mild days that disease issues arise in the vineyard, with bunch rots the primary culprit. The combination of free water sitting on the ripe clusters for 12-14 hours in temperatures of 70 degrees or so is a recipe for potential trouble, and an active botrytis bunch rot infection in a vineyard is about as welcome as Miley Cyrus at a meeting of the Traditional Values Coalition.

A cold and wet area of low pressure will track southeastward over Washington beginning late today, and occasional showers should carry over into tomorrow and tomorrow night as the storm exits the state. Temperatures on Wednesday may struggle to climb into the low 60s, owing to the importation of a chilly air mass with the low that originated over the north Pacific. A northerly flow on the backside of the low will lead to clearing skies on Thursday along with gradually warming temperatures, thanks to a weak ridge of high pressure moving into Eastern Washington. Afternoon readings should rise into the upper 60s by Friday under mostly sunny skies, and the low to mid-70s for the first full weekend of fall 2013. The models are having some difficulty resolving their differences over a front that lurks just off the coast on Sunday, which may or may not move ashore in time to bring a chance of showers by late in the day. Your suddenly gun-shy weatherperson is rather reluctant to opine on that particular subject, given his recent losing streak with regard to his 49ers and Orioles — both of whom, in stock trading parlance, have recently underperformed, contrary to his bold pronouncements concerning their ineluctable success.

A lifelong fan of both the weather and the Baltimore Orioles, Jeff Popick is an instructor at the Center for Enology and Viticulture at Walla Walla Community College and manages the school’s teaching vineyard. Send your questions and comments to him at jeffrey.popick@wwcc.edu.

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