An end-of-season nip as summer gets tucked away

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The final weekend of summer 2012 has come and gone in much the same fashion that many preceding it have, with abundant sunshine and warm temperatures.

Though afternoon maximums have been well into the 80s recently, the heat has lost a great deal of the edge it once possessed owing to the ever-decreasing angle of the sun, which has lengthened afternoon shadows and softened the harsh glare of mid-day. The increasing hours of darkness are also permitting the accumulated heat of the day to dissipate overnight, making for generally cool, fresh mornings.

Following the passage of a fairly strong cold front last week, a combination of clear skies, very dry air and calm winds along with the longer nighttime hours brought the Valley its first brush with freezing temperatures.

Early Tuesday morning as minimum readings fell into the low 30s in notable cold spots south and west of Walla Walla. There were reports of light frost from some of the vineyards, and wind machines were used in a few areas to mix the cold air near the ground with warmer air aloft.

Whitman Mission, where weather records have been kept for 50 years, set a new low temperature record that morning of 31 degrees.

In low-lying backyard gardens, some of our more sensitive backyard garden plants — like tomatoes — also fell victim to the chilly air, although this was not a widespread frost event by any means. It was, however, a gentle reminder that fall officially begins on Saturday and that sometime next month we can expect a general frost that will end the growing season for many plants as autumn slides colorfully, but inexorably into winter.

The only thing to mar the otherwise near-perfect weather we can expect over the next several days is the poor air quality due to the smoke from large wildfires burning in Washington and Oregon.

With an impressive mound of high pressure ensconced over Eastern Washington, winds will be very light and there will be little, if any, mixing of the air. Areas of high pressure tend to inhibit vertical motion in the atmosphere as the air beneath sinks due to the added weight of the “mountain” of air above.

In addition, with cool air pooling near the surface and warmer air above, the resulting inversion conditions do not allow for much upward atmospheric motion. Normally, the air is cooler with increasing elevation, but in an inversion these conditions are reversed and colder air is trapped near the ground.

The condition eliminates the normally buoyant state of the atmosphere. The opposite is true when the surrounding air is cooler, and thus pollutants like smoke become trapped in the lower reaches of the atmosphere until something changes — usually an influx of cooler maritime air aloft from the west that wipes out the inversion and restores the atmosphere to its normal buoyant state.

These warmer-than-normal days are just what the viticulturist ordered — particularly if he or she is hanging a plus-size crop on the vine. The heavier yield will demand extra time to accumulate the sugar and flavor compounds that distinguish truly ripe fruit, so this dry and warm interlude is much appreciated — especially by yours truly, a college grape grower whose California experience may have spurred him to leave a few more clusters on the vine than he should!

The near-term outlook favors this enterprise, as the aforementioned high pressure system is forecast to strengthen over the next couple of days, leading to afternoon temperatures a good 10 degrees above normal. The dry regime in place should continue for several days, though some moisture may creep into southern and eastern portions of Oregon this weekend from a low pressure system coming ashore in northern California.

There has been no measurable precipitation here since July 17. If the 16-day Global Forecast System outlook is to be believed, it will remain rainless here until the end of the month, when the latest runs of that model are beginning to hint at a turn to the colder and wetter between Sept. 29 and the first of October.

In the meanwhile, your somewhat anxious viticulturist/prognosticator will be lighting candles and sacrificing small animals to the weather gods in an effort to ensure that his vineyard will ripen the sizable crop that currently hangs expectantly on his vines.

A lifelong fan of both the weather and the Baltimore Orioles, Jeff Popick is an instructor at the Enology and Viticulture Center 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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