March 18, 2013

Ode to Northwest Rain Shadows

(Coming Attraction:  Windy on the coast, the Strait and NW Washington on Wednesday, as a moderately strong low crosses northern Vancouver is)


We think about our lives here in the Northwest being shaped by the rain, but one could argue that our lives are even more influenced by the rain shadows, those locations downstream of major mountain barriers that are considerably drier than much of the region.  Most Northwest folks live in rain shadows---locations with less annual precipitation than the bulk of the eastern and central U.S.

Precipitation enhancement occurs on the upstream (windward) sides of mountain barriers as terrain forces air to rise.  On the other hand, precipitation is suppressed as air descends the downstream (lee) sides of terrain.  Often the drying (the rain shadow) extends quite a distance from the barrier, with the driest conditions often stretching 50-150 miles away.   The immediate lee of the barrier still can have quite a bit of precipitation, since precipitation generated on the windward side can blow over for 5-10 miles (or even more under strong winds).

One is always struck by the annual precipitation map of the region (see below), which shows some of the greatest precipitation gradients in the U.S.   Yes, we have very heavy precipitation (over 160 inches a year) over the SW side of the Olympics, yet in the rain shadow to the NE of the barrier they only get 15-20 inches a year.  Why is the annual average heavy rainfall and rain shadow oriented that way?  Because the winds approaching the region below 8000 ft during the winter are generally from the SW when precipitation is occurring.  Although Sequim, San Juan Island, and northern
Whidbey are in the middle of the rain shadow, the Puget Sound lowlands also get a piece of the rain shadow action because we also in the lee of the Olympics and coastal mountains to some degree.  The farther north one goes over the Puget Sound lowland, the drier.  Olympia and adjacent areas are wetter because they have a fairly clear shot to the oceans...thus less rain shadowing.

So what would Seattle be like if the Olympics and coastal mountain were removed?  You don't want to know!  Seriously, I would expect it be similar to say Grayland, a station on the central WA coast. (see table below).  A place with 73 inches of rain a year, summer highs around 67F, and nearly no snow.   Grayland indeed.

We should be thankful for the Olympics!  But the most profound rain shadow is to the east of the Cascade crest, with large areas getting less than ten inches a year.   This is very special rain with an abundant water source from the Columbia River, which drains off the terrain of British Columbia and northern Washington.  The combination of the two provides ideal conditions for some of the best grape and orchard areas in the world, as well as other valuable crops like hay and hops.  Head a bit farther east, where precipitation starts to edge up a bit, and the rain shadow provides a superb zone for dry-land wheat.

Can rain shadows move?  You bet.  If the flow approaching the mountains shifts direction, the rain shadow will follow....thus when air is from the west, the Kitsap Peninsula and Seattle can be dry while those usually smug folks around Sequim and Port Townsend are getting a good soaking.  As noted in my blog last week...such a shifted rain shadow occurred last week.   Here is a nice radar image of this event (late Tuesday).

Looking farther afield, rain shadows maintain their importance, with Portland and the Willamette Valley profoundly drier than the Oregon coast.  I could write volumes about local rain shadows....and some have (see below).  And I hear that werewolves don't like rain shadows.

Rain shadows define our lives here in the Northwest.   We should never forget that.


  1. LOL - I don't think Forks qualifies as a rain shadow.

  2. Brilliant, Mr. Mass

    Also, don't forget the yummy water we get too!

    I was back in Indianapolis over the Winter Holidays, and YUCK, I forgot how bad their water tasted.

    Rain Shadows Rock!

  3. I'd like to know what conditions promote rain shadows, and what produce convergence zones, in the same topographical situations.
    You are pleased when the forecast gets these two features right.
    What's different about them?

  4. Down here in Chehalis that off shore high which has hung around much of the winter results in a lot more winds coming out of the north and northeast. We have got a lot of nice days when rain was forecast. I have assumed just a bit of rain shadow from the Olympics. My 12 foot deep pond (spring fed)is already down about 6-7 inches, half of it's normal drop.

  5. Geography is everything. How fortunate are we to have a massive rain shadow and a big river running through it? Not to mention just the right amount of heat. Places like dinky Mattawa are some of the best fruit-growing areas in the world.

  6. Hi! I live your blog and have been using your iphone app. I just noticed that the days of the week dating your iphone app posts are wrong, though. For example, your latest post says it was done on Sunday, March 19, not Tuesday, March 19. Not sure how to report this, so I hope this comment works. Cheers, Heather

  7. If you zoom in on the state rainshadow map, you see that apparently the higher you go on Mt Rainier the drier it gets. Is this true?? Can Mt Rainier be so high that the precip gets wrung out on the lower slopes?


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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