The Olympic Mountain rain shadow is a very well-known phenomenon in
northwestern Washington, one that's led to Sequim, WA becoming a small
retirement mecca due to the drier, sunnier weather.
The rain shadow is so localized that the northwest side of San Juan
Island receives 50% more rain than the opposite corner does -- only 10
miles away. During the "Pig War" between the U.S. and Great Britain
in the late 1850's it was significant because George Pickett's forces
were at American Camp in the dry spot, while the British enjoyed the
lush end of the island. But that's a different question.
This KOMO TV graphic shows graphically how the prevailing pattern for
northwestern Washington brings moisture from the southwest "What is
the Olympic Rain Shadow? Short Answer: Sequim's Best Friend"
By contrast, when weather comes unblocked from the northwest, it tends
to be cold, dryer air.
Sequim, WA receives an average rainfall of about 17" each year,
contrasted to Seattle's 37.5" rainfall. As you'll see from weather
measurements below, the lowest rainfalls are in the immediate Sequim
area, making it the "epicenter."
It happens because the Olympic Mountains wring out the moisture from
the prevailing winds, providing 200" of average rainfall in the Hoh
Forest near Forks, WA. The rain shadow extends towards the northeast
over the lower tip of San Juan Island, where rainfall only averages
20" at Cattle Pass.
This Average Annual Precipitation chart shows the Sequim area in tan
at the northern tip of the Olympic peninsula:
As you'd indicated, Port Townsend (to the east of Sequim) and Whidbey
Island, further to the east across Admiralty Inlet, receive more rain
-- but still average less than 30" per year. The San Juan Island
chain is to the north of Sequim.
A more-detailed chart shows the persistence of the rain shadow to the
north, over San Juan Island. At its southeast corner where Cattle
Pass is located, rainfall is still only 20" per year, while parts of
the northern stretch of this 10-mile long island receive 30" or more.
"San Juan County Characterization Report" (June, 2000) has a
precipitation chart showing local effects on that island:
The Western Regional Climate Center publishes these detailed reports
on rainfall at each area measurement point. It includes rainfall for
about 15 places in the area you've inquired about:
Here are some relevant ones:
Port Angeles: 26"
Sequim (2 miles east): 16.24"
Port Townsend: 19.34"
Olga (Orcas Island): 29"
Coupeville (Whidbey Island): 20.7"
Sunshine data is not as readily available, as there are NOAA weather
reporting stations only at wider intervals: in Port Angeles, Oak
Harbor (Whidbey Naval Air Station) and Friday Harbor.
It's a fascinating micro-climate that even leads to types of cactus
appearing in the Sequim area and near Cattle Pass on San Juan Island.
If there's any information that you need to help understand it, please
let us know before rating this question.
Google search strategy:
"Olympic Mountains" + "rain shadow" + diagram
"Sequim" + "rain shadow" + diagram
"San Juan" + "rain shadow" + diagram
Who lives in the Snohomish County convergence zone -- but that's
Request for Answer Clarification by
31 Oct 2002 20:17 PST
Beautiful response, thank you. One more associated question:
Am thinking about buying a lot on Salt Springs Island, part of the
southern Gulf Islands in BC, next to Victoria. The "Gulf Islands"
make the same claim, about being in the rain shadow. Can you provide
the annual rainfall on the Gulf Islands?
Thanks again for the comprehensive response, I may opt for Sequim
Clarification of Answer by
01 Nov 2002 16:29 PST
It took me just a little bit to find the Canadian data, but one thing
that I can tell you is ignore the claims of real estate agents. One
site said the annual rainfall on Saltspring Island is 29" and another
36". According to Environment Canada, the numbers are 40.5" at
Cusheon Lake and 39.4" at St. Mary's Lake.
Basically, the driest locations are on a line from Sequim to Friday
Harbor. As you get further west -- or east -- you'll get wetter.
Victoria, BC is due west of Friday Harbor and gets 34.8". Also, as
you get further north of Sequim, it gets wetter -- 16.7" at Sequim;
20" at Cattle Point, 30" at the top end of San Juan Island.
Environment Canada has detailed information on this page:
"Canadian Climate Normals, 1971-2000"
Select "Canadiam Normal Climate" and province (British Columbia), the
Search. You'll get all of the weather reporting stations in the
Saltspring Island is stunning. So too is Sequim. I have a close
friend who lives in the hills above Sequim, looking down across the
Straits of San Juan de Fuca towards Victoria, BC.