Is San Juan Island the sunniest location in western Washington?

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While waiting for the ferry in Friday Harbor, I passed some time at the window of a local real-estate agent, entertained by ads for houses I will never be able to afford (unless someone buys a few hundred thousand of my books!). In some of their literature, there was the claim that the San Juan's were "sun central" for western Washington with more than 240 days of sunshine a year! And looking around on the web, I found that claim repeated on many web sites. Earlier that day I hiked around Lime Kiln Park, on the south shore of the the island and looked southward at the Olympics. It was sunny and warm, the prairie around me sandy and dry. Miles to the south, low clouds had moved through the Strait, and Port Angeles, Sequim, and Pt Townsend were in clouds. I wondered...could the sunniest place in western Washington be where I was standing?

Well, the traditional answer to the sunniest location in western Washington question has been Sequim and adjacent areas--locations downstream of the Olympics during the winter. During our wet season the winds are generally from the southwest, so air moves up the SW flanks of the Olympics (enhancing clouds and precip) and down the NE side (producing drying and less clouds). You can see this effect clearly in satellite imagery (see first image above) and the Camano Island weather radar. The San Juans, and particularly the southern San Juans, get some of this Olympics action, but they are clearly on the northern edge of the downslope drying. I suspect a quantitative analysis would show that Sequim and nearby towns are sunnier during midwinter than anywhere else in western Washington. And this explains Sequim's SunLand Condominium, Sunshine RV Park, and Sunshine Herb and Lavender Farm, among hundreds of other sun-related business names.

But hold on there! The climatological winds are only southwesterly during midwinter and during the spring and summer the incoming winds are often from the west and northwest . And there's more! There is another topographic barrier that has a rainshadow--the mountains of Vancouver Island. And with westerly flow the rain shadow from it is centered right over the western San Juans...and NOT Sequim and its fellow travelers. Take a look at the satellite picture this morning( above)...can you see the lack of clouds over San Juan Island? While Sequim is in clouds. This happens ALL the time in the late spring and summer.

So what is the bottom line here? Sequim is rainshadowed and sun endowed during the winter, but the San Juan's get a lesser piece of this action. During the warm half of the year, San Juan Island gets rainshadowed by Vancouver Island and escapes all the gunk passing through the Strait of Juan de Fuca. Thus San Juan Island could be sunnier than Sequim during the summer.

It may well be that considering the whole year that southern San Juan Island and Lopez could be as sunny or sunnier than Sequim.

This could be calculated more quantitatively using satellite imagery, but at this point I will be content to let the real estate agents battle it out!
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