The graphic that I have created below is meant to be representative of the general winter rainfall patterns around Western Washington. To this end, I happened to choose December of 1990. It was a December marked by a severe cold wave during the last week of the month. Rainfall-wise, it was somewhat sub-normal, but there was variability along a generally north-south pattern, with Bellingham in the far north actually slightly above normal, while the Southern Sound around Olympia had the most severe moisture deficit. Further discussion is found beneath the graphic.

Although many tend to think of all of Western Washington as a damp green rainforest, the truth is rather different. There are great variations. Most are traceable to the huge mass of the Olympic Mountain range, a massif with peaks up to nearly 8,000 feet, lying directly in the path of the normal storm track, like a giant wall. Since the majority of rain-bearing storms, and especially the wettest ones, tend to attack the region from the SW, one notes a strong Rain Shadow effect on the NE side of the Olympics. On the graphic, one notes an eight-fold decrease in rainfall as one takes a direct SW line across from Sequim (2.4 inches) to Clearwater (16.6 inches), an airline distance of only 80 miles.

Areas in the most extreme part of this rain shadow receive only about 16 to 20 inches of rainfall per year, only half of what the Seattle/Tacoma area receives! This driest area centers around Sequim, which appears to be the driest town in all of Western Washington, with a bare 16 inches of yearly rainfall. For year, pilots for years have called the Sequim area "The Blue Hole" due to its tendency to have relatively clearer skies than areas to the east and west of it.... The rain shadow extends out onto the nearby islands somewhat, eg. to Coupeville with just under 21 inches, and to Anacortes with just over 26 inches. On the eastern shores of Puget Sound the rainfall increases, the driest site I could locate being Mt. Vernon at 32.3 inches. The main Seattle area has several recording sites, and they vary between about 35 and 37 inches per year (remember, the Sound's wettest large city is a lot wetter than Seattle- it's Olympia, with about 52 inches)

..Why doesn't the Rain Shadow area look like a desert? To the casual traveler's eyes, these deeply rain-shadowed areas do not appear as dry as they actually are. There are at least three major reasons: 1. Since the entire region is at a relatively high latitude (about 48 degrees), the intensity of solar radiation is low enough that evaporation and other desertifying processes are relatively weak.. .2 Despite the big reduction of rainfall in the rain-shadowed zones, there is only a modest increase in clear skies and sunshine, which remain at the low annual levels which afflict all of Western Washington... 3. Although yearly rainfall decreases to only 16-20 inches in the driest zones, it is still not dry enough to change the basic vegetation very much, especially once one factors in the protective effects of the damp and fairly sunless climate; thus the main trees of Western Washington continue to be seen, even though the area's climate is near their limits in terms of dryness...This having been said, the discerning visitor can note some significant vegetation changes; chiefly, dry-area plants such as Garry oaks and grasslands that really cannot survive in the wetter areas of Western Washington begin to be seen, the relative dryness allowing them to get by despite the lack of the heat and dry air that they prefer. One correspondent even tells me that a few cacti can be found in the Sequim area (and she tells me that these were apparently native to the area, being reported by the first white settlers! Since the average daily high in even the warmest month is only 72 degrees, these must be some pretty cold-adapted cacti).

The Southern Puget Sound area is routinely wetter than the mid-Sound area around Tacoma and Seattle. Apparently what is occurring in places such as Olympia is that the typically SW track of the major storms has a direct line to Olympia, basically unimpeded by Olympic rain-shadow effects. In my graphic above, Decemer 1990 was a significantly subnormal month in the Olympia area. The average for December is actually well over 8 inches, not the 5.1 inches that fell in December 1990. Shelton, about 15 miles NW of Olympia, is even wetter, averaging about 66 inches of rain per year.

Yakima Valley Rain Shadow-- A final note: as one ascends up the Cascades onto Mt. Rainier, rainfall increases at Paradise Lodge (5,500 ft) to levels as high as the low-lying areas on the western slopes of the Olympics, but then drops off rapidly into an extreme rain shadow on the east side of the Cascades, particularly extreme behind the massive bulks of Mt. Rainier and Mt. Adams. This is where the Yakima Valley lies. Descending off the wet and heavily-forested crest of the Cascades, even the casual traveler will note the very obvious and radical alterations in the vegetation as they drive toward the Yakima Valley-- in this, quite unlike the much more subtle rain shadow around Sequim (see above).... In December 1990 the city of Yakima saw a meagar 0.2 inches of rainfall, less than a tenth as much as Sequim, the driest location in Western Washington (see graphic at page top) ). And unlike areas like Sequim, the Yakima area is not only dry, but also experiences a great deal of sunshine and clear skies, creating a harsh climate where even oak trees have trouble surviving without additional water. The actual long-term precipitation averages give the Yakima-Sunnyside-Wapato area a scanty 7 to 8 inches of rain per year. That is really dry!

Copyright 2007-2008 by Bruce B. Johnson
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