August 30, 2009

San Juan Islands Weather

San Juan Island prairie (top) and lavender farm (bottom)

I just got back from giving some lectures in the San Juans and if any place has localized weather is there. The complex combination of terrain and water caused large weather variations, as does the proximity to the Olympic and Vancouver Island rainshadows.
First, there are large variations in annual precipitation. The southern portions of San Juan Island and Lopez get around 20 inches a year due to their proximity to the Olympic rainshadow, while the northern SJuan Island (say northern Orcas) gets around 30 inches, with more on Mt. Constitution and the higher terrain. Lavender likes dry conditions, so there is no surprise that a large lavender farm is found on southern San Juan Island (Pelindaba lavender farm). With dry conditions, wind, and sandy soils, southern SJ Island even has natural prairies (see image above).

Wind variations are huge there. Blockage by the terrain causes "wind shadows" in their lee. Locations (such as Mt. Constitution on the NE side of the islands get hit by the strong wintertime northeasterlies exiting the Fraser River valley. While I was kayaking one morning on the eastern side of the Orcas I was struck by the strong wind accelerations near even modest points and headlands. During the wintertime, strong southeasterlies can buffet the islands (particularly the eastern portions)...winds that are accelerated by troughing (low pressure) to the lee (north) of the Olympics. In fact, when I hiked a bit on the top of Mt. Constitution (2500ft) I could see trees that had fallen in two the SE (from the Fraser flow) and to the NW (from the strong wintertime southeasterlies).

I found lots of well-educated weather enthusiasts on the San Juans and appreciated the invitation of the San Juan Nature Institute and the San Juan County Dept of Emergency Management . And two very nice book stores--Darvill's Books (Orcas) and Griffin Bay Books (Friday Harbor) graciously attended my lecture with my NW weather books. I left signed copies at both of them.

Editorial comment: Yesterday, I went to Lime Kiln park to view the Orcas...and was not disappointed. Viewed at least a dozen of these magnificent creatures. But I was shocked that both pleasure boaters and some fishing vessels ran just offshore revving their engines and making a terrible racket as they banged repeatedly into the water. Couldn't they stay offshore to allow the poor Orcas a chance? And then a helicopter came in low and circled over them, followed by a twin-engine aircraft that came in for a look. This cacaphony can't be good for the whales, can it? The whale watching boats were out their too....shadowing the orcas...but they seems a bit more discreet than the others.


  1. Hi Cliff,
    I like the editorial comment. I was able to take a whale watch boat out of Orcas when I was there the end of July. The captain and crew were cautious to stay the correct distance away from the whales and their respect for the creatures was evident. It is very sad that not all people respect these magnificent animals.

  2. My mom attended your talk on Orcas and was delighted with your presentation. (Apparently, she bought a book for me and had you sign it.) She loves the weather in the islands, which is a welcome change from her native Alabama. I'm a little stunned to hear about how the precipitation amount differs so much between the islands. I'm glad you enjoyed your time up in San Juans.

  3. Hi Cliff, Thanks for your San Juan observations. I also liked the map that you included. Would you please cite its reference? I'd like to get a copy for myself.

  4. You mention strong wind accelerations and variations around Orcas. If you have a day with a nice SW wind blowing up Rosairo Strait, the wind will generally die as you round Pt. Lawrence and get in the lee of the island (no surprise).

    But, as you get NE of Mt. Constitution, you can get Sudden and very strong wind gusts. I would guess that the wind can go from basically zero to over 25 knots in just a couple of seconds. Fortunately you can usually get a little warning by watching for the gust developing on the water.

    I'm assuming these gusts are due to turbulence from Mt. Constitution. I'm guessing they may be a rotational wind, rotating about a horizontal axis. Can anyone shed more light on the physics of these?

  5. Hey, Cliff!

    You can buy those maps (the one you used in your post) at:

    They're great - I especially like the one of the gulf Islands with the San Juan s in the middle!

    Thanks for coming to visit us!


  6. is the moisture from jimena going to help the wildfire in southern california?

  7. Hi Cliff-

    Thanks for the talk! Discussions at the pub and in the grocery store after lecture were great with all in agreement that we'd like to see you return for a more in depth and meaty discussion some time.

    As for your comments about the whales and boat traffic, it is a difficult, complex, and multifaceted issue. Something some have been working on for close to two decades:

    The resident orcas were listed as endangered in 2005. The four areas identified were prey abundance (salmon and more specifically resident chinook), toxins, oil spill potential, and vessel traffic. As you noted vessel traffic includes everything on the water: shipping, whale watching, sport fishing, commercial fishing, dive boats, kayakers, air mattresses (don't ask), pleasure boaters, ferries, military craft, researchers, airplanes, etc etc etc....

    NOAA just released proposed rules for the vessel impact portion of the listing (the other areas are addressed in separate portions of the recovery plan) which is causing a lot of discussion in these parts. Here are the proposed regs (backtrack on the link for more if you dare):

    Also google for news stories if you want more opinions and explorations on the issue....

    Finally, one question on weather in the islands. While we all acknowledged that meteorology has come a long way in improving regional forecasts, the more glaring issue in the PNW seems to be the smaller regional anomalies. How much work and what type is going into improving forecasts for smaller areas? I suppose an analogy would be that forecasts seem awfully pixelated right now...when and how will the details become clearer?




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