Orographic Precipitation Studies
of the
Cascade Mountains of Washington
by Don Burrows

Introduction

My interest in orographic precipitation over the Cascades began at a very early age. When I was in the first grade we moved from Seattle to Coulee Dam and I was impressed by how completely different things were on the other side of the mountains. Later we moved south to Ephrata where I completed high school. Over the years we made many trips back and forth across the mountains and I never ceased to be amazed and fascinated by the abrupt change in climate and vegetation between Ellensburg and Snoqualmie Pass. In the summer it could be perfectly clear and sunny on the east side but on crossing the pass one could descend into clouds and even below the clouds to find that it was a very gray day in western Washington. In the winter we would often go from cloudy to snowy to rainy in crossing from east to west. After graduating from high school I studied Atmospheric Sciences and Cloud Physics at the University of Washington getting my B.S. in 1964 and my Ph.D. in 1968. For the first two years after graduating I got an opportunity to directly study the details of precipitation formation over the Cascades. Unfortunately that opportunity didn't last long and my career moved on to other places and problems. Now that I am retired I plan to devote some of my time to further investigation of the effects of cold air pooling in central Washington on precipitation over the eastern slopes of the Cascades.

The Cascades Atmospheric Water Resources Program 1968-70

The ideas for this study grew out of observations and insights gained during the winters of 1968-69 and 1969-70 while I was engaged in snowfall studies at Snoqualmie Pass as part of the Cascade Atmospheric Water Resources Program (EG&G ESO, 1970). The main conclusions I reached were:

1) The precipitation rate at Snoqualmie Pass was largely orographic in nature and strongly correlated to the strength of the westerly component of the wind between 5000 and 6000 ft at Enumclaw and the mean dew point between 900 and 750 mb at Cle Elum. (see Figure 1)

Graph
Figure 1. Preciptation Rate at Snoqualmie Pass as function of westerly component of wind and mean dewpoint.

2) Easterly flow through the pass during prefrontal conditions (see Figure 2) often made it possible for snow to fall at the pass rather than rain. Such as on Dec. 23, 1968 when the Enumclaw radiosonde showed the freezing level at 6000 ft all day while at Snoqualmie Pass the temperature remained at -4.5 C and snow fell. In another case on Dec. 3, 1969 the temperature at Snoqualmie Pass was -7 C while at the top of the chairlift on Denny Mtn, 2400 ft higher, the temperature was +1 C.

Figure 2. Cold air stratus east of Snoqualmie Pass - Don Burrows

3) The low stratus clouds in the eastern valleys contributed to the amount of snow falling in the valleys. A study of snow crystal types on Dec. 23, 1968 indicated that they originated at two different levels. All showed evidence of riming growth. The Cle Elum radiosonde also indicated two levels at which snow crystals would be expected to grow by vapor growth (see Fighre 3), an upper level between 9000 and 12000 ft, and a lower level between 2000 and 5000 ft below the ridgetops.

Graph
Figure 3. Excess vapor density over ice saturation and air temperature
from sounding at Cle Elum, WA at 1600 PST 23 Dec. 1968

In March 1970, I left the program and never had a chance to further pursue these ideas in any depth for a number of years. The full report is contained in

EG&G Environmental Services Operation, 1970: Supplemental Final Report for Cascades Atmospheric Water Resources Program, submitted to Department of Water Resources, State of Washington, 15 June 1970. pp 71-100.

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