Thursday, April 26, 2012

Rain Shadow Secrets

We moan about the rain around here, but rain shadows define our meteorology in as profound a way.  For Northwesteners looking for relief from the rain, there are few more important topics to master that the details of our local rain shadows. Just a glance at our annual precipitation map screams out with their importance (see graphic).

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There is the rain shadow to the northeast of the Olympics, of course, that drops the rainfall to 15-18 inches around Sequim and downwind.   And the Cascade rain shadow creates the sagebrush desert of eastern Washington, with the driest conditions (less than ten inches) stretching from Ellensburg and Yakima to the Tri-Cities.  The last two weekends I basked in the sun and warmth of the Cascade rain shadow and loved it.  But that is rain shadow 101 you already know.  Lets explore the topic some more.

The first thing to keep in mind is that rain shadows are not stuck in place...they can move, strengthen and weaken, and morph in shape.  For example, when the coastal winds approaching the Olympics are from the southwest or south, the rain shadow is over the area from Port Angeles to Port Townsend and extends to N. Whidbey and the southern San Juans (image below).

But then if the winds shift to the west, the rain shadow moves to the eastern side of the Olympics and Kitsap is sunny and dry (image).  Even Seattle can get a piece of the rain shadow action then!

Since the midwinter winds are generally from the  south or southwest, the Sequim area is rain shadow central during the heavy precipitation time of the year.   But in spring the coastal winds are more frequently westerly, so central Puget Sound gets rainshadow action more frequently.

But what about the Cascades?    Well, the magnitude of the rain shadow effect (the difference in precipitation between, say, North Bend and Ellensburg) really varies in magnitude depending on a number of factors, like wind speed, wind direction, and the stability of the air.  Sometimes a single storm produces radically different Cascade rain shadow amplitude as it approaches and passes the region.

As a frontal system approaches, with strong, moist southerly flow, and relatively stable (layered), deep clouds and precipitation, quite a bit of precipitation passes over the mountains and the rain shadow effect is weakened (though it still exists).  See a UW WRF simulation of precipitation at this stage and the corresponding surface chart:

Then after frontal passage the flow turns westerly (good for rain shadowing since there is a strong wind component up and down the mountain) and air is less stable (which produces a lot of shallow convection on the windward Cascade slopes), with the rain shadow effect being profound. Here is an example of a WRF precipitation simulation and the corresponding surface chart:

Rain shadows are not trivial matters and several of us are studying them at the UW .  One person even has a blog/website dedicated to it.  Lewis and Clark did not understand rain shadows and they had a miserable winter in 1805 along the Oregon coast.   

Dr. Dale Ireland
I am sad to note the sudden passing of Dr. Dale Ireland, the enthusiastic amateur meteorologist and astronomer, whose comments on local weather and his wonderful HD cam of weather over the Olympics were valued by all of us. Dale could always be counted on to tell us about the next meteor shower or shuttle passing, and his observations on everything from mountain wave clouds to submarines over the Hood Canal were both interesting and valued.   The Ichiro statue on his deck that got covered in snow brought us many smiles. A friend of our discipline will be greatly missed.


JewelyaZ said...

RIP, Dr. Ireland. I will miss his camera and appreciate what he did for the weather-curious.

Rain shadows are super-interesting and I recently had an argument with someone back home (Raleigh, NC) who just could not believe that Bellevue gets 10" less rain a year than the Triangle. It's TRUE. I tried to explain the rain shadow effect, and I could see the comprehension was not ever going to come. Oh well! It's important to understand it here, and I appreciate your more-detailed info here.

Bob said...

Ouch. Very sorry to hear about Dr. Ireland. May his family and friends find smiles and peace in their memories.

I had the pleasure of of communicating with Dale a couple years back while trading photos of a noctilucent cloud display over our region. His webcam was a regular stop for me on the web.

SkunkBayWeather said...

We got to know Dale and his wife Suzanne through our website hobbies. We feel honored and privileged to have them as our friends. I just want to share one thing about Dale. If there is one quality that I loved about him, it was his childlike wonder of the universe and environment. I will always remember the twinkle in his eye as he showed me his “man cave” and telescope. His fascination of all the different atmospheric conditions that rolled by his front windows reminded me of the same emotion of a kid on Christmas morning. This was not an academic appreciation. His fascination was driven by an inner child that truly appreciated the world and the never ending discovery of what is out there. He was a passionate free spirit. We loved him and send out best to Suzanne.

Unknown said...


Could you please post a legend with the top precipitation map? It would "scream" a little louder if there was a legend associated with the map, and make it much more meaningful.

Thanks Cliff. Love the blog.

Emily Pfeifer said...

So very sorry to hear about the passing of Dr. Ireland. He will be missed.

Hindu said...

Great post. Here in the Tri-Cities I'm always hoping for a few drops of rain for relief in the warm months. Always thought it was odd when the local weather guys predict (for example) a 40% chance of precip (high for here) and W/SW winds greater than 20mph. Wind and rain seem incompatible here, no doubt due to the rainshadowing/downsloping.

Once I heard that the pct chance of precip is not just that precip will occur in your area, but will actually fall on your head, thus a spatial component. Is that true?

Mike said...

I'm confused. Under what conditions does a rain shadow (no rain) become a convergence zone (enhanced rain)? Is there something about the flow around the mountains, the state of the atmosphere in the lee, can you predict when you'll get one vs. the other?

Unknown said...

Cliff, when will the probcast be working again?