November 04, 2014

Super Rainshadow

The Northwest is known for rain, but it really should be famous for its profound rain shadows, which are some of the most dramatic in the world.

Take yesterday.  Here is the precipitation for the 24-h period ending at 9 AM Tuesday morning. Over 4 inches on the southwest side of the Olympics, but only a few hundredths to the northeast of the Olympics.   Yes... more than 400 times more precipitation on the southwest than the northeast side!
Also more than 4 inches on the windward side of the Cascades and a few hundredths on the eastern slopes.

 Interestingly, the driest conditions were very close to the Olympics, on the eastern slopes.  Why?  Because downward motions were very strong there.  To illustrate this , here is a WRF model vertical cross section oriented SW-NE across the Olympics.  The vectors show winds in the cross section, the shading shows clouds, the blue lines is temperature, and the red line in relative humidity.   Upward motion, clouds, and 100% RH on the windward (SW) side of the Olympics.   Virtually no low clouds northeast of the crests.  If you look carefully you can see downward motion and mountain waves downstream of the crest.  Downward motions kill clouds and precipitation.

The rain shadow in the lee of the Olympics was obvious in the weather radar.   Here is a radar image from 11 PM Monday night.  A lot of rain over the region.  The  rain shadow was quite extensive, extending between the lee (NE ) slopes of the Olympics across northern Whidbey Is. to the San Juans.   If you watched an animation, you would see that the rain shadow morphs and changes shape, but generally remains in the same area.

Because of the heavy precipitation on the windward side of our mountains and our strong rain shadows, the Northwest has some of the largest precipitation contrasts in the world.


  1. I'm a pilot for Alaska Airlines and flew into this rain shadow about 8pm Monday night. I was on a heading of 250 degrees north of downtown with a lot of rain on the windscreen when it stopped suddenly and we broke into this tranquil clear space. It was remarkable and I wondered why it was so distinct. The dry space was clearly visible with my airborne weather radar. Thanks for the explanation!

  2. Great explanation. One thing I'd love to know is the Rainshadow Effect on annual sunshine. This is the 2nd time I have read a magazine article about the San Juan Islands having 240-300 days of sunshine a year while Seattle only has 50 (Bicycling and Trail Runner Magazine). Indeed places like Sequim and the San Juan Island have less rain but does that mean they really have more sunshine? From my experience, it seems like they just get a little less rain than Seattle but over the same number of days.

  3. Somebody in Lacey needs to fix their rain gauge.

  4. @Matt, it's true. Those microclimates really DO have more sunshine! That's one of the reasons Sequim has so many retirees... seeking the sunshine without leaving our state or even the greater Seattle area. :-)

    Please sign the petition! We're at 316 and counting... it's taking off now that it's visible to all visitors of the We the People site.
    Spend the $25 million already appropriated on a supercomputer for increased weather-prediction capabilities. We the People White House petition site Don't forget to check your email because you have to click to confirm your signature. Here's Cliff's post on the matter, which I paraphrased mightily to build the petition text: The US is Falling Further Behind in Numerical Weather Prediction

  5. Here's the summary of a study that was done (Olympic rain shadow) using personal weather stations that had a solar sensor installed:

    Report Highlights:

    Winter (Nov-Jan) saw 5X as many mostly sunny days in the shadow vs. Seattle.
    Winter saw only 1/4 as many dreary days in the shadow vs. Seattle.
    Spring (Feb-May) saw the highest number of rain shadow days per month, at nearly 8!
    Summer (Jul-Sep) saw rain shadow areas and Seattle with nearly equal mostly sunny days.
    Port Angeles was definitely *in* the rain shadow, with quite similar benefits to Sequim.

    Our data shows that the climate in the rain shadow was clearly brighter and sunnier than downtown Seattle, with 127 mostly sunny days vs. only 88 in Seattle, fewer cloudy days, and dramatically fewer dreary days.

    Here's the link:

  6. Thank you John! I have to admit though that I'm more curious about the San Juan Islands though. Do you think the results would be pretty similar there? In any case 127 vs. 88 sunny days is very different than the 250-300 vs. 50-70 days that they write about in Bicycling and Trail Running magazine. I love the SJ islands but I think it's really unfair of publications to make people think they are headed to some tropical, sunny paradise instead of dark and grey Seattle when really it's pretty similar.

  7. Cliff - Something interesting happened during the burst of heavy rain that happened this AM. See this trace ( from a personal weather station. Notice how the pressure started back up (frontal passage?) and the worked its way back down? I noticed a similar looking trace with the UW Weather Station, although it is hard to really be sure due to the scale. Any ideas on what caused this? A downburst?

  8. 10404RE: Matt Fisher
    The southern and southeastern corner of the San Juans is mostly in the shadow, but the north San Juans really aren't. And those 250-300 "sunny days" versus 50-70 probably does reflect days you can ride comfortably, as long as periods of light, misty rain and brief sun breaks qualifies as comfortable and "sunny". The difference truly is dramatic in winter/spring.

    More appropriately, we should be reporting "days you'll get soaked riding your bike" as the criteria to differentiate the rain shadow. But that doesn't sound nearly as sexy to as "sunny days".


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