The Pacific Northwest is home to some of the best rain shadows in the world: areas with dry conditions downstream of topographic barriers.
And on Sunday, we were shown one of the best examples of those features, in the lee (northeast) region of the Olympic Mountains.
As many of you know, when air approaches a barrier, it is forced to rise, creating clouds and rain (see image below), and as it sinks downstream, the air descends, warming due to compress and dry.
This is where the rain shadow is found.
On Sunday, strong south/southwest winds are approaching the Olympics (see wind map at around 11am on Sunday). As a result, the air sank to the northeast of the Olympics.
The satellite image visible at this time clearly shows an area of almost no clouds in the rain shadow areas. This is sometimes referred to as the “Blue Hole.”
Radar images at the time showed this as a rain-free area:
Do you want to be impressed?
Here are the total rainfall for Sunday. About 3 inches downwind, southwest of the Olympics, but only 0.01 inches over parts of Whidbey Island. Mama Mia! It’s a rain ball.
Air passing over the Olympics not only creates rain shadows, but also affects pressure and wind.
Upstream, the air cools as it rises, thus increasing the pressure at the low end (high upwind) since cold air is denser/heavier than warm air.
Downstream (rainball), the air sinks and warms, causing low pressure, because warm air is less dense than cold air.
Here is the Sunday morning sea level forecast (sea level isobars). You can see both the upwind elevation and the low leeway. Sometimes these pressure features cause strong winds, as the air accelerates from high pressure to low pressure.
And, of course, the Olympic features aren’t the only rain in the Northwest. We have dozens of them downstream of the local mountains. We live in an area rich in rain shadows.
