April 26, 2010

Rainshadow Land

Living here in the Northwest we often think about how much it rains, but to truly understand our weather, you have to appreciate our rainshadows. We have world-class rain, but we also have world-class rainshadows. Take a look at the radar image above (from the Camano Island radar). Precipitation (indicated by the colors) all around the place, but there is a rain-free zone downstream of the Olympics...stretching from Sequim to the San Juans. Or take a look at the precipitation over the last six hours from the SPU Rainwatch System (see graphic). You can see that the dry-zone ground zero is just offshore from Sequim. That is why so many people retire there and cacti are native species. As air approaches a barrier it rises, producing precipitation, but on the opposite side there is sinking, drying, and rain shadow creation. You can look at the rainshadow above and immediately know the wind direction near the Olympics--from the south-southwest. To prove this, here is the latest radiosonde sounding at Forks, on the Olympic Peninsula. The general crest level of the Olympics is roughly 5000 ft--approximately 850 mb, with higher terrain above that to roughly 8000 ft. So look at 850-800 on the sounding chart. If you can read the wind barbs, you will see the southerly flow.

An important thing to know about rainshadows is that they can move since their position is controlled by large scale wind direction interaction with terrain barriers. So Sequim being dry is not religion. It is just the wind here in the winter is typically from the south to southwest. If the large scale wind shifts direction, so will the rainshadow. So if the wind switches to westerly or northwesterly, Sequim is no longer effectively shadowed, but Seattle and central Puget Sound is! That will give the retirees and golfers in Sequim something to think about! Such Puget Sound rainshadowing is really quite frequent and helps explain why Seattle is really quite dry (only 37 inches a year), receiving far less than the east coast of the U.S., and WAY less than the Washington Coast. That is why vampires live in Forks but keep away from the Puget Sound lowlands.

But there is one ironic complication to our rainshadow under westerly flow....something that literally rains on our parade: the Puget Sound convergence zone, which produces a band of rain IN THE MIDDLE OF A PROFOUND RAINSHADOW. That is why north and south of a convergence zone it can be profoundly clear.

Finally, not only can rainshadows move, but we have lots of them due to our complex mountains. Big rainshadows in the lee of the mountains of Vancouver Island, the Cascades, the Blue Mountains. Smaller ones in the lee of Mt. Rainier and the other volcanoes. Even rainshadows int the lee of Queen Anne Hill and Tiger Mountains. Yes, this is really rainshadow land here.


  1. I can tell here in Bellingham when there will be a strong rainshawdow. Yesterday was classic. Warm south/southeast wind ahead of the front. Then the wind slacks and the moistures begins to fill in. The previous front had little wind and more moisture in the beginning. From observation it seems like some of the islands like Lummi Island can disrupt falling moisture oover Bellingham proper. I have seen on the radar large cells of post frontal storms in direct path for Bellingham get wiped out after passing through Lummi.

    Thanks Cliff

  2. I think I've experienced the Queen Anne Hill rain shadow in the last few weeks. I frequently bike commute via Dexter, and on several occasions it's been dry going up the hill north bound and wet right after I crested the hill and headed down toward the Fremont Bridge. Wasn't sure if this was really a rain shadow or if it just coincidentally decided to rain then.

  3. It rained from about 3:30 to 5:30 yesterday here in Sequim, but it probably didn't amount to more than a couple hundredths of an inch. We need the rain - it sure rained enough here last October and November.

    Speaking of cactus, the large Opuntia engelmannii pictured in your book was recently destroyed. I salvaged hundreds of pieces of it for propagation.

  4. I was just thinking this morning about the rain shadow and convergence zone. Why is it that sometimes Seattle gets rainshadowed with no convergence zone, and sometimes it gets rainshadowed and a convergence zone develops? Is it the relative strengths and directions of the winds at different levels of the atmosphere? Does it just depend on each unique situation? Is this covered in the book????

  5. Can you explain a sort of anomaly in the local weather Monday afternoon? From around 2:30 to 3:30 PDT the wind picked up sharply, with gusts of around 15 m/s in both Seattle and Tacoma. (Students between classes at the U were getting wet from the fountain spray.) At the same time, the dewpoint fell sharply, only to rise later as the cold front came in in the evening. As far as I can tell, the wind event caught the NWS by surprise.

    What was going on?

  6. LVDLM ~~~ Out here in Eatonville at around the same time our winds really picked up too. They had been around 15-25 all day, gusting in the 30's. Then around 2:30 pm they really picked up and were gusting in the 40's. Our top gust was 53.

  7. Hi Cliff,
    I've always enjoy reading your posts. Very informative.
    I was wondering maybe in a future topic, if you could explain the difference between a "Meteorologist" and an "Atmospheric Scientist".
    When I grew up in the 1970's I never heard of the term Atmospheric Science. Is this relatively new?
    Thanks and I apologize for derailing your current topic.
    Best Regards,

  8. Vindication! I'm always telling folks about that rainshadow effect in Victoria and Gulf Islands due to the Olympics and prevailing weather from SSW. But for most their eyes glaze over and they nod condescendingly, saying, 'it rains a lot in Victoria'.

    Apart from the obvious ecological differences between dew laden Sooke a few kms west and the Garry Oak meadows in the south Sannich area there just isn't the rain in Victoria that there is in very close adjacent locations.

    Your description nicely explains why there can be such a drastic difference in only a short distance.

    Which also clears up why as a kid I recall my relatives calling White Rock, the 'Banana Belt' compared to sopping wet Vancouver.

  9. I always wondered why the UW 4km WRF-GFS had such a persistent lack of precip there when I used to forecast for McChord/Ft Lewis. I never considered a mesoscale rainshadowed area existed to the west of the Cascades. My time forecasting there was brief (6 mos) but I wish I would have known of this then. But precip wasn't ever the enemy, just low ceiling/vis...


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