October 15, 2014

The Western Puget Sound Rain Shadow

In Seattle, it was quite a wet, blustery day, but if you had taken a short ferry ride to Bainbridge Island you would have experienced hardly a drop.  To illustrate this, here is the 24hr precipitation ending 8 PM Wednesday over Puget Sound and nearby environs.

.01 inch in Bainbridge...barely enough to wet the pavement and .02 inches in Belfair on the Kitsap Peninsula.  Cross the Sound and there was over .40 inches.  In general, it was much drier on the western side of the Sound.  Yes, Sequim to Port Townsend were fairly dry too.

As you might imagine, this contrast was due to rain shadowing in the lee of the Olympics and upslope enhancement on the western side of the Cascades.   We could watch it happening in today's weather radars (showing you the radar-based 1-hr precipitation)

11 AM.  Rain shadow NW of the Olympics, consistent with SW flow.

 At 12:59 PM, the flow was in transition (becoming more westerly) and you notice an increase in precipitation on the western side of the Cascades)

This transition was even more obvious at 3:46 PM,  A broad rain shadow downstream of the Olympics was obvious, as was the substantial enhancement of rain on the western side of the Cascades.

Even more so at 5:02 PM

We did have radiosonde launch at Qullayute at 4 PM and here is the sounding of winds (barbs), temperature and dew point (the dark lines slanting to the left).  Height is in pressure units (hPa), with 700 being 10,000 ft, 850 about 5500 ft., etc.).  Winds approaching the Olympics were from the west-southwest at that time, which fits the radar pattern.

My point is that Olympic rain shadows don't only hit Sequim and environs, they can extend down the eastern slopes of the Olympics under the right conditions.  And it also illustrates that if you are weather-wise and are ready to travel an hour or so, you can often find much better weather.  Or stormier weather if you prefer.


  1. I haven't taken notes, but regularly observe what sort of rain shadows we get down here in Chehalis. The southern coastal range even though much lower than the Olympics does have some effect. On rarer occasions with winds out of the NW we even get some Olympics shadowing.

  2. The West Sound Rain Shadow was one the three microclimate things I noticed after moving to Bainbridge Island. The other two were how much windier it tends to be here when a front passes (more exposed location, thanks to being surrounded by water), and cooler summer high temperatures (again, due to being surrounded by water).

  3. The changeable rain shadow of the Olympics turned out to be a very big deal on June 5, 2012. Venus transited the Sun on this day, and there were no second chances as the next transit occurs in 2117.

    I got into my car from Seattle and started driving south toward Long Beach, which is where I think Cliff said to go the night before. With real time updates midday on the 5th, there was a small patch of sinking air right at Nisqually. After backtracking from around Chehalis, I and many others ended up at the Nisqually Bird Sanctuary which has a nice expansive parking lot. We all pinched ourselves at how lucky we had gotten -- gray skies all around, but in this one little hole, clear skies and full Sun. I had shielded binoculars, while others had Cassegrains, H-alpha solar telescopes, and other equipment. It was a FANTASTIC showing!

    Thanks, Olympic Mountains. You saved the day!


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