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| Winter
weather usually leads to widespread low clouds (stratus) and fog across
much of the lower elevations of eastern Washington and north Idaho. This
veil of low level moisture can be viewed as a positive, as it acts as a
blanket keeping the lower elevations from getting too cold during the long
winter nights especially at higher latitudes. Although the gray, gloomy
skies can be viewed a negative as they block the incoming solar radiation
and allows pollutants to build up. This is the type of weather regime that
was experience during the week of January 25-29, 2007 and visualized by
the images above spanning the Spokane area and westward across the Columbia
Basin. |
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To understand the winter time fog and stratus pattern, one has to understand the topography of the region. The Columbia Basin of eastern Washington behaves like a "bowl" surrounded by mountains. The Cascades, Selkirks, Bitteroots, and the Blue mountains, with elevations of 3500 to over 6000 feet in elevation, act as a wall or lip of the bowl. This is be seen in the topographical map above, where the dark browns are lower elevations and the dark greens are the mountains. During the winter, cold dense air can settle to the surface and remain trapped in the bottom of the "bowl". This is especially true after a clear night with light winds, a fresh snow pack, and strong radiational cooling. Meawhile, transitient Pacific weather systems can usher in relatively mild air into the region. This warming usually does little to scour our the cold air in the "bowl" and remains aloft, being found several thousand feet high in the mountains. It takes instability and mixing from a vigorous storm system or increased wind and pressure gradient from a building ridge to break out of the low overcast. |
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| In this case, the upper air sounding taken in Spokane (OTX) at 12z on January 26, 2007 showed the cold, stable, and saturated layer up to 900 mb with warming a drying above to 800 mb. The warm layer or inversion acted like a "lid" and prevented mixing between the layers in the vertical. The cold and saturated layer near surface layer was the stratus deck with surface temperatures in the 20s, meanwhile drier and warmer layer had temperatures in the mid 30s at around 6000 feet. |  |
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| Above are images taken on January 26, 2007 from the mountains across the Inland Northwest showing the mostly sunny skies at the mountain peaks with stratus below in the valleys. The image on the left is from Schweitzer Ski Resort in North Idaho, while the one on the right is taken from Mission Ridge Ski Resort in the Washington Cascades looking east toward the Columbia Basin. | |
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| Above is a pair of satellite images taken from 9am on January 26. The one on the left is a fog product with stratus and fog noted in black, while the one on the right is a visible image with the stratus and fog noted in white, along with snow cover. Each image is overlaid with mean sea level pressure and surface observations. By comparing the images, you can see much of the lower elevations from the northern valleys of eastern Washington to north Idaho and south to the Columbia Basin were shrouded in the low cloud deck. |
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| Above is a pair of satellite images taken from 3pm on January 26. Again, the one on the left is a fog product while the one on the right is a visible image. Comparing the images, you can see most of the low level cloud deck remains entrenched across the lower elevations of the eastern Columbia Basin and the fringes of north Idaho. Meanwhile clearing has taken place south of the Canadian border down the Okanogan Valley and adjacent valleys in the Okanogan Highlands and south to the Waterville Plateau and southern Grant county. This was due to increased winds and mixing as high pressure nosed south from British Columbia. |
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Here is a map of surface conditions from 3 pm on January 26. Due to the clearing and increasing winds in the Okanogan Valley, the temperature warmed up to 38 degrees in Omak (KOMK) with a north wind of 15 knots. This would be the similar to temperatures depicted in the mountains, as seen in the morning upper air sounding. The northerly winds blowing across the Waterville Plateau were strong enough to erode the stratus from the top and generate clearing into Grant County. Meanwhile the stratus continued to blanket locations from Spokane, Coeur d'Alene to Pullman, and it remained cold in the upper 20s to lower 30s despite the northerly winds. In this case, the stratus was beginning to lift in the Coeur d'Alene area, although the pressure gradient was not strong enough (winds in Coeur d'Alene were only 5 knots) to totally break up the inversion and allow the low clouds to clear. Under a persistant inversion, the same low level airmass can remain over the Inland Northwest for days and even weeks, and just get swirled around the region. For instance, if the surface winds switched to the south or southwest, the stratus will fill back into the northern valleys while some clearing may be possible in the southern Columbia Basin. This persistant low overcast can trap pollutants and lead to stagnant conditions within the boundary layer of the atmosphere. In closing, stratus
and fog is a common occurrence across the Inland Northwest due in part
to the unique topography. A low overcast can keep
the region protected from bitter cold air, but also limits the incoming
solar radiation and provides gloomy conditions with little diurnal temperature
variation. Due the surface wind patterns and passing weather systems aloft,
the stratus and fog can swirl around the region while remaining trapped
beneath the low level inversion. This can lead to stagnant air conditions
and a buildup of pollutants within -Robin Fox |
Webmaster
US Dept of Commerce
National Oceanic and Atmospheric Administration
National Weather Service
Spokane Weather Forecast Office
2601 N. Rambo Rd.
Spokane, Washington 99224
Tel: (509) 244-0110