'Four Corners high' to edge into Valley

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It was a near-perfect summer week in the Walla Walla Valley as a weak trough off the coast furnished just enough of a moderate westerly flow to keep daytime highs in the very tolerable 80s.

The very dry air cooled quickly after nightfall, and afternoon readings dropped into the 50s just before sunrise. This is a perfect recipe for our local grapes, which thrive on warm days and cool nights, helping to ensure that precious natural acids are not lost to respiration during the overnight hours. These acids provide a crucially crisp counterpoint to the sweetness of ripe fruit and are absolutely essential to the production of a balanced and harmonious wine. Grape-growing regions that can satisfy this requirement are thus among the most highly regarded in the world. Walla Walla’s combination of long, cloudless summer days with ample warmth and clear, generally cool nights allows it to assume its rightful place among an illustrious global group of locations that can boast of such an ideal confluence of growing conditions.

And speaking of cool nights, your weatherperson just returned from a weekend of camping at Bull Prairie Lake at just above 4,000 feet in the Umatilla National Forest of north-central Oregon. When the sun set on a 70-degree day there Friday, the bottom dropped out of the thermometer, and just before sunrise Saturday, the mercury fell to a very un-summer-like 37 degrees — cold enough to make one think twice about exiting the camper for any reason at all, including building a campfire. That goose bump-inducing low was possibly one of the coldest overnight minimum temperatures in the entire country!

With the Four Corners high (so called because it is usually centered during the summer months in the “Four Corners” region of the southwestern U.S. — where Utah, Arizona, Colorado and New Mexico converge) reasserting itself at the beginning of the current week and expanding northward, hotter weather is on tap for southeastern Washington. Desert heat will be shunted northward on the southerly flow around the high’s clockwise circulation, and maximum temperatures should top out in the mid- to upper 90s.

Some monsoonal moisture caught up in this flow may pair up today with a weak disturbance approaching our area from northern California and touch off a round of thunderstorms later in the day. These could continue well into the night and possibly Wednesday morning.

This disturbance will provide one of the key ingredients for any sort of precipitation: lift. A lifting mechanism is a vital element in getting water vapor to rise to an altitude where it can cool sufficiently to condense into clouds that can eventually produce rain. The lift associated with this little disturbance consists of air flowing into the middle of a counterclockwise-spinning pinwheel of circulation. At the center of this circulation, where inflowing air converges, air rises into the higher reaches of the atmosphere where the necessary cooling can occur. Lift can also be supplied by cold fronts forcing warmer air out ahead of them to rise as they progress across the land; by mountains (orographic lift); or by convection, whereby columns of land-heated air become less dense than the air around them, rise and take moisture contained within them high into the atmosphere above where it can cool and condense.

Following the passage of these storms, the remainder of the week appears to feature sunny and abnormally warm weather until the weekend, when some slight cooling of 3-6 degrees may occur as the high pressure dome to our south flattens and recedes a bit.

In the longer term, according to the 16-day Global Forecast System outlook, there are indications that this cooling trend will persist into next week. Renewed troughing over the Pacific Northwest should allow for the intrusion of some maritime air that will keep our daytime maxima confined to the very pleasant mid- and upper 80s, with very little chance of any precipitation.

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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