Tuesday, November 27, 2012
A ridge of high pressure over Washington brought a brief respite from a rather prolonged period of inclement weather last week, and the Walla Walla Valley actually experienced a couple of mostly sunny and dry days over the extended holiday weekend.
Sunday turned out to be an absolutely delightful day for just about any activity as the ridge forced the storm track north into southern British Columbia, bypassing southeastern Washington and bumping any disturbances in the upper air flow well off to our east.
As the ridge strengthened and settled in over the top of our area, the combination of clear skies, long November nights, light winds and ample moisture remaining near the surface from our recent rains allowed for excellent radiational cooling. That resulted in an inversion and a dense -- but not terribly thick -- layer of fog at ground level on Monday.
Normally, air temperature decreases with increasing altitude. Under an inverted condition the opposite is true: cold air is pooled in a layer just above ground and temperatures actually increase with increasing altitude.
From a stability point of view, this sort of condition is extremely resistant to mixing, as vertical motion is strongly inhibited by the lack of buoyancy in the atmosphere. Once formed -- especially in winter -- these inversions can provide their own momentum that over the course of a few days may ensure their continuing existence over a week to two weeks, even longer.
The fog is formed when the moisture in the air closest to the ground is cooled sufficiently to its condensation point, causing the water vapor to become visible to us as a cloud of tiny droplets -- a cloud that's at ground level, unlike those familiar to us in the sky.
As the fog layer thickens, it becomes more and more difficult for sun's rays to penetrate it and heat the earth to any appreciable extent, which only serves to strengthen the inversion as the disparity in temperature between the fog-shrouded ground and the sun-warmed air above becomes more pronounced.
This phenomenon is easily observed when one makes a car trip from downtown Walla Walla, shrouded in the 33-degree fog, and finds 50-degree sunshine well up in the Blue Mountains.
It will take a serious change in the overall weather pattern to bring an end to such a well-established inversion -- usually a frontal incursion of sufficient strength to wipe out the low-level system and restore the atmosphere to its more normal non-inverted condition.
On winter nights during inversions, sub-freezing temperatures will cause the formation of "freezing fog," when super-cooled droplets (water cooled below 32-degrees Fahrenheit but still retaining its liquid form) freeze on contact with any surface that is at that temperature or below.
This can cause major travel headaches where those patches of "black ice" are difficult to see on the roadway. Therefore, your weatherperson recommends great caution with all late night and early morning driving into Wednesday morning.
Forecast charts are indicating the ridge responsible for the inversion and its patchy dense and freezing fog will be nudged off to the east by a moister, somewhat milder westerly and southwesterly flow by Wednesday afternoon. Increasing winds ahead of this new trough will help mix out the fog, and there is a fair chance rain will return to the area by late that day.
This will mark the first wave in a long progression of systems that will become increasingly wet as the week progresses. A significant blob of eastern Pacific moisture may get caught up in this flow and provide several fairly wet days to the Walla Walla Valley through the coming weekend, though the timing of each system remains a bit uncertain at this time.
In the longer term, the 16-day Global Forecast System indicates this sort of a pattern will be maintained well into the first part of December.
That means a continuation of the milder than normal temperatures coupled with above normal amounts of precipitation, combining for a snowless start to winter 2012-13.
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 email@example.com.