Monday, August 31, 2009

Is San Juan Island the sunniest location in western Washington?

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!


Big White Ball said...

Thanks for this. I've been wondering if this is the case for years!

mainstreeter said...

Where cactus and fir grow within sight of each other.

Blake said...

Very interesting, Cliff. Somewhere in my stack of maps I have a real estate flyer from Port Townsend that actually attempts to show contour lines of precipitation in the rain shadow. I don't know where they got the info (it's not cited), but the driest part is shown out in the water between San Juan island and the peninsula.

Although San Juan island may be sunny, that doesn't mean it's warm! The winds and constant marine air mean it's often in the 60s, while in Seattle it could be pushing 80 on the same day.

WeatherNerd said...

On that second photograph the only lowland area that had no fog was where the PCSZ normally forms. Is it an "anti-PCSZ" if you will? Enough or just the right winds to keep fog out of this small area? Or just a coincidence? I live in that area and saw no fog at all yesterday morning.

Andy Wappler said...

Cliff --

Great posting, and what a job settling AND stirring up a debate all at the same time!

By the way, perfect conditions today to power up PSE's Wild Horse and Hopkins Ridge wind facilities in Central and Eastern Washington.

Cloudy west side + clear east side = plenty of wind power.

Andy Wappler
Puget Sound Energy
@PSETalk on Twitter

Cliff Mass Weather Blog said...

Andy..BUT..there is no wind in Ellensburg now..cliff

Ian said...

I'd hazard a guess that the Olympic foothills about 5 miles south of the Strait near Sequim are sunnier than either. I live about 2 miles southwest of Sequim and on the morning of Aug 31 I could see sun to the south and fog to the north for quite a long time.

I work in Dungeness and it's generally far foggier (and COLDER in summer) there than at home (no surprise really). I'd say this spring/summer for every foggy morning I've had here at home - which haven't been that many - there have been 5 foggy mornings out at Dungeness. And if it does form here, it burns off much earlier.

Less commonly, I've observed a low cloud deck to the south stacked up against the Olympics (sometimes extending just over me) and sun to the north. But this scenario seems much less frequent.

All this to say, a detailed analysis should consider multiple points throughout the north Olympic Peninsula and islands. PA, PT and even Sequim proper are all too close to the water to really represent the weather/climate of the North Olympic Peninsula. I've also noticed a temperature difference of as much as 30 degrees on hot summer days (but usually more like 10-15) between here and the Strait. Ironically most of the 'sun' business are in the fog belt.

Nick said...

What would be truly interesting is a piece of software which pull the satellite images as often as possible (allowed/refreshed), archive those after some photo analysis. Over time it could be easy data mined and proven with fact. Heck - the myriad of data mining metrics which could be performed would be quite fun. :

P.S. - Love the blog. :)

Benjamin Hargrave said...

WRONG. Do your research.

Sequim:132 sunny days
Seattle:152 sunny days
San Juan Island(Friday Harbor):157 sunny days
Clinton: 164 sunny days

I lived in Sequim for years and it's HORRIBLY cold and grey there, compared to other places in Washington. The "rain shadow" that real estate agents have been lying about for years does NOT mean more sun. It means less rain but MORE CLOUDS, as you can see the facts show above.