Mountains are a staple of the Pacific Northwest, but they are more than just a beautiful sight to see. They actually have a significant role to play in the annual average precipitation totals of our region. This is thanks to a phenomenon called the rain shadow. Before diving into this further, a couple definitions are necessary.
The windward side refers to the side of the mountain which air is forced up the slope.
The leeward side refers to the side of the mountain where air descends after being forced upward.
Figure 2 above shows an example of a rain shadow on radar imagery. The colors (including gray) show areas where precipitation was falling, with yellows and reds being heavier rainfall. You can notice a “hole” of precipitation, where the region is mostly dry. Note that this is on the leeward side of the Olympics (with winds from the west and southwest). This area of reduced precipitation on the leeward side of a mountain is what we call a rain shadow. In this case, it is an example of the Olympic Rain Shadow.
The rain shadow is clearly defined when looking at an image like above, which shows how greatly reduced the rainfall amounts are in this rain shadow.
How Does the Rain Shadow Form?
As air rises up the slopes of a mountain, it cools and condenses, thus creating clouds–which leads to enhanced precipitation. We can see this in Figure 3 in where some of the higher totals are located (the red colors), which also happens to be where rain forests are located. Much of the moisture is squeezed out by the time air reaches the top of the mountain barrier.
But as the air descends down the mountainside, it starts to warm and dries out even more. This inhibits cloud formation and precipitation.
Where does the Rain Shadow happen?
There are two main locations in Washington State that are impacted by rain shadows. The first is associated with the Olympic mountains, shown in the circled region. This area includes Sequim, which sees about 16 inches of rain annually. For contrast, Seattle receives about 38 inches per year, while some areas on the SW side of the Olympics see around 200 inches annually.
A predominant wind direction as storms approach the coast is from the south or southwest. As shown in Figure 5 below, air will rise up the Olympics as shown with the blue arrows. After, it will descend as shown with the red arrow. This is what leads to the drier conditions in this region.
However, if the winds are more westerly (i.e. from the west), the Seattle/Everett area can be in the rain shadow. An example of this is shown in Figure 2. The location of the rain shadow is not always exact; it depends on the direction of the wind.
The second main location of the rain shadow in Washington is associated with the Cascade mountains. A predominantly westerly wind direction puts Eastern Washington as the leeward side of the Cascades, and thus significantly drier. This is apparent in Figure 4, where there is much less rain annually in Eastern Washington than Western Washington.
If Seattle and Everett are oftentimes in the rain shadow, why do they not rival Sequim in total rainfall?
Great question. I’m glad you asked. The answer lies mostly in four words: Puget Sound Convergence Zone. This region that oftentimes falls in the Olympic rain shadow also is a prime region for PSCZ formation. Heavy rainfall is common in convergence zones, so overall, it makes up for the precipitation “lost” in the rain shadow.