Clear autumn skies herald end of 2013 wine vintage

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A mixed bag of weather this past weekend paved the way for the first frost of the season in many Valley locations early Monday morning as fall reared its hoary head and emphatically proclaimed its 2013 regnum.

A weak system had brought a measly 0.02 inches of rain to the city (as recorded at the airport) on Saturday — a nonevent that had been very accurately predicted in this very space last Tuesday — but did nothing to put a damper on the Entwine event at Walla Walla Community College that evening, where all involved had a jolly good time.

Sunday dawned mostly cloudy as residual moisture associated with the moribund front was slow to exit the area, but it soon gave way to gorgeously sunny skies and mild temperatures in the 60s, which invited many citizens outdoors to feast upon the panoply of fall colors that annually makes this season such a pleasure for the eye.

Your weatherperson is not immune himself to such a rich display. However, given the fact that both of his favorite NFL teams (Ravens and 49ers) were engaged in televised action, his appreciation of the visual charms offered outdoors was limited to glancing out the window occasionally during the supernumerary commercial breaks that annoyingly interrupt the action every few minutes or so.

A rather impressive ridge of high pressure has set up shop in the wake of Saturday’s system, out around 130 degrees west longitude, and the clockwise flow around it has brought a cool, dry northerly flow to Southeastern Washington. Under this regime, clear skies, light winds and low relative humidity are the perfect recipe for some very chilly nights — particularly given the lengthening hours of darkness as we proceed through the month. As a result, Monday morning saw the coldest temperatures of the season so far, with several spots in the Valley flirting dangerously close to the freezing mark — and some well below.

Frost can be a serious viticultural issue here in mid-October. Many local vineyards still have grapes on the vine, and an untimely frost can be damaging in a couple of ways. Leaf cells that contain water will freeze, expand and rupture their walls, leading to a rapid browning and shedding of those leaves. That loss, of course, will render the vine incapable of any further photosynthesis, making it impossible for the plant to ripen the fruit with additional sugar that is produced by that process. In addition, at temperatures below about 29 degrees, the pedicels (stems) that attach the grapes to the cluster framework will freeze, and the fruit will quickly (in a matter of days) lose its ability to remain on the vine, leading to a wholesale loss of grapes, which fall to the ground.

As explained here last week, elevation is the key to both the timing and intensity of spring and fall frosts. Yesterday at 6 a.m., the 40 degree reading at Walla Walla Regional Airport (1,204 feet) looked positively balmy compared to the 26 degree reading at Touchet (518 feet), the 32 degrees reported at Whitman Mission (617 feet) and the 34 degrees recorded at Peppers Bridge Road and the Walla Walla River (760 feet). With the pooling of cold air in lower spots a well-known phenomenon, it is no wonder that climatologically the Mission’s first occurrence of a 28.5 degree night/early morning is Sept. 15, while the airport’s earliest date for this same event is Oct. 13. At the Mission, there is a 70 percent chance of such an occurrence by Oct. 14, while the airport’s 70 percent probability of reaching the same temperature does not occur until Nov. 20. This helps explain why site selection is absolutely the most crucial decision that can be made in the state of Washington when contemplating vineyard establishment here.

The current pattern is forecast to be with us for quite some time. Sunny, mild days with afternoon temperatures in the mid-60s, and clear, cold nights in the 30s to near 40 should be the rule — perhaps well into next week. Monday morning’s 16-day Global Forecast System outlook offered a very benign picture for the next two weeks, with zero precipitation. However, a disturbing final day or two of the forecast period stands out with chilling clarity: A 19 degree reading around the 29th or 30th of the month should serve as a powerful incentive to wrap up vintage 2013 before that occurs.

Forewarned is forearmed!

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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