January 24, 2020

Super RainShadow

Sometimes the precipitation contrasts in our area can be mind blowing, and today was such a day.

During the past 24-48 h, we have observed extremely heavy rain on the southwest side of the Olympics, with a super-rain shadow to the northeast.

Below is total precipitation over the past 48 hours (ending 7 PM Thursday).

Remarkable. On the southwest side of the Olympics, totals are as high as 9.45 inches, while just downstream of the barrier on the lee side (the Port Townsend area) one site only received .01 inch.

Let me put it another way.  There was nearly one thousand times more precipitation on one side of the Olympics than over the other.   With huge differences in rain over very small differences.

You will notice that precipitation was relatively low from Seattle northward to the San Juans and then picks up substantially on the western side of the Cascades.

This pattern was evident in regional weather radar imagery, with the view at 7 PM Wednesday night providing a good illustration.

A high-resolution visible satellite image earlier today strongly suggests a lessening of the clouds northwest of the Olympics....in fact, some luck folks undoubtedly caught a glimpse of the sun, while the rest of us were immersed in the darkness, gloom, and rain.  You can see why retirees like Sequim, Port Townsend and northern Whidbey Island.

The explanation for this contrast?  That's easy.  Strong, moist southwesterly flow was approaching the Olympics and was forced to rise by the barrier.  Such lift causes the air to cool and release the bountiful water vapor of the approaching flow.   In contrast, on the northeast side of the barrier air descends and warms by compression.   This causes the relative humidity to drop and precipitation to evaporate.

Finally, I should note that our computer models nailed this pattern, as illustrated by the forecast for the three-hour precipitation ending 7 PM tonight.    Very nice rain shadow, with lots of rain on the upstream side of the Olympics.


  1. "In contrast, on the northeast side of the barrier air descends and warms by compression. This causes the relative humidity to drop and precipitation to evaporate."

    Cliff, is this what is referred to as "föhn (or foehn) wind"? Very common in Switzerland north of the Alps.

  2. I remember one early Fall day during a stay at Port Angeles there was no rain until I got around the peninsula on the way to the Hoh rainforest. Suddenly the skies opened and torrential sheets of rain came down. It kept up for hours. I love that contrast.

  3. Approximately 2 mi. N of Pt. Partridge on west side Whidbey Island 15' eleven. 48.266180 -122.745950 precip wk. .62 day .11 1500hr.

  4. Amazing, Cliff! I can better understand why you chose to pursue your career in the PNW.

  5. The 8.00 and . 18 right near Bellingham is pretty wild if not a typo. Interested to hear an explanation if there is one.


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