Orographic Precipitation Studies
of the
Cascade Mountains of Washington

Statewide Wind-Precipitation Investigation
by Don Burrows

Around 1980 I discovered that there was a very good correlation between mean monthly 850 mb winds and monthly precipitation for much of the state. The study utilized only two years and two months worth of data from January 1978 - February 1980, obtained from Washingtion Climatological Data published by the National Climatic Data Center. Monthly precipitation for 172 climatological stations was correlated with mean monthly wind averaged between Quillayute and Spokane rawindsonde values.

For each station the wind direction producing the best correlation to the monthly precipitation was determined to the nearest five degrees. The principal wind directions and max correlation coefficients are plotted for each station in Figures 4 and 5. Figure 4 shows the contours of the maximum correlation coefficients and Figure 5 shows the contours of the principal wind directions.
Correlation Map
Figure 4. Contours of the correlation between mean monthly wind components of 850 mb wind and monthly precipitation for stations in Washington. For each station the upper number is the component of the wind used in degrees and the lower number is the correlation coefficient. The mean of Quillayute and Spokane 850 mb winds were used for the entire state.
Red: r >= 0.80, Orange: 0.80 > r >= 0.70, Yellow: 0.70 > r >= 0.60, Green: 0.60 > r >= 0.50, Blue: r < 0.50.

In general the best correlations are found along a line connecting Quillayute with Spokane. The highest correlations are found on the windward side of the Olympic Peninsula, The windward side of the NW Cascades and a small local maximum over the Waterville Plateau. The correlation breaks down over the far eastern and southern parts of the state. Some of those areas may correlate better with a different rawindsonde location. Local minima are also found in the rain shadow area of the Olympic Mtns and in the immediate lee of the Cascades. The most easterly values were in south central Washington.
Critical direction map
Figure 5. Same as Fig. 4 except contours are the for the principal wind components of 850 mb wind for best corrleation with the monthly precipitation for stations in Washington.
Wind component by color - Purple: >= 240°, Red: 240° to 210°, Orange: 210° to 180°, Yellow: 180° to 150°, Green: 150° to 120°, Blue: < 120°

The principal wind directions are generally south to southwest for the area from the Cascades westward. Central Washinton and easternmost Cascade stations had principal directions from south to southeast. The most westerly principal directions were found in the lee of the Olympic Mountains and near the crest of the Cascade Mountains.

A quick test of the idea that cold air pooling contributes to precipitation over the eastern slopes was made. The idea was to compare precipitation over the western slopes of the Cascades with that over the eastern slopes and see if there is a difference in the relative amounts of precipitation between the months when cold air pooling is present and the months when it is not.
For this test climatological normals for 1931-60 and 1941-70 were used for Washington State Climatological Divisions, Cascade Mountains West and East Slopes Cascades. The ratio of West/East is plotted in Figure 6.

Graph
Figure 6. The ratio precipitation in the Cascade Mountains West climate division to that in the East Slope of Cascades division by month of year. The blue line represents the 1931-60 normals and the red line the 1941-70 normals.

From the plot it is clear that the ratio has a distinct minimum during the period from November through February. The minimum indicates that during the winter there is more precipitation over the eastern slopes relative to the western slopes than during the warmer parts of the year. These months are precisely the time when cold air pooling is most prevalent in Central Washington.
This test of course says nothing about mechanisms or even directly measures cold air pooling. It is, however, consistent with the idea that cold air pooling in Central Washington can increase precipitation over the eastern slopes of the Cascades and suggests that further investigation might be worthwhile.

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© 2011 by Don Burrows, All Rights Reserved