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)

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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 shadow....one 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.