June 15, 2017

Heavy Rain and Super Rain Shadow

Announcement:  Atmospheric Sciences 101

I will be teaching Atmospheric Sciences 101 this fall if anyone is interested either as a UW student or the Access Program for those over 60.  This is a general intro to weather and weather prediction.  MTWTh 10:30-11:20, Kane Hall.

There was some impressive rain totals today for June, with the southwest side of the Olympics getting 2-3 inches so far, with 1-2 inches on the western side of the Cascades (see plot of 24h rain ending 7 PM Wed).   But just as impressive was the profound rain shadowing on the northeast side of the Olympics and the eastern slopes of the Cascades.   Amazingly, there were only a few hundredths of an inch of precipitation around Sequim and .02 inches at Friday Harbor in the San Juans.

Take a look at a blow-up precipitation plot for a better view. One location near Sequim had only .01 inches.  Hundreds of times that rainfall fell 50 miles away on the other side of the Olympics.

Why such a difference?   We had a very moist airflow coming into our region, driven by strong winds.   Here is plot of the winds with height at Quillayute, on the NW Washington coast. Winds near crest level were southerly to southwesterly at 30-40 knots (the heights are in pressure, 850 is about 5000 ft).  Quite unusual for this time of the year.

With such south/southwesterly winds, the moist air was forced up on the southerly/SW side of the Olympics (producing heavy precipitation) and then descended (causing drying) on the north and northeast side of the barrier.

The rain shadow was very obvious in the National Weather Service weather radars (see example below) as a "hole" in the precipitation shield.

And the rain shadow was also apparent in the visible satellite imagery this afternoon (see example below)

The heavy rain is causing some local rivers to surge to near record levels for the date--for example, the Wynoochee River near Montesano.

The situation will dry out into Saturday, but rain will return Saturday night into Sunday morning.   Nice for our plants and water bills.


  1. Yep, I saw the rain shadow. Dark grey clouds to the south and east, but I only saw a FEW DRIZZLES in port townsend!

  2. Only measured .001 at our weather station in in Langley on Whidbey Island. If that possible convergence zone today forms to our south, as it often does, I might have to turn on the drip irrigation on our cut flower farm! The trade off is that it would make our farmers market dry today...Oh to be a farmer!

  3. I was hoping for a lot more rainfall on Bell Hill in Sequim. Kept looking wistfully at the radar, hoping the blue hole would close, especially after the clouds settled on us and we were in the fog. Nope. 0.02" at my location. But the fog moistened everything. Good for the foliage, but not the roots.

    In the last three weeks, through several storms, we've gotten a total of 0.25". Soil is drying up.

    While the rain shadow is VERY nice in winter, we can use some water in late Spring. No wonder cactus grows naturally here.

    It all means I need to get my irrigation system going this weekend. Fortunately, aquifers in Happy Valley are full and the Dungeness River is flowing nicely. Other than being a little dry, this has been an excellent Spring so far.

  4. My dog panics when there is thunder, and she hears it before I do.
    Her go to move is to paw open the front door (lock is no obstacle) and run straight and stupid.
    Last time she did this she was hit by a car, and then ran injured for over a mile before someone was able to coax her into their office and give us a call.

    I sure wish there were a thunder alert...

    Seems to me that thunder is fairly rare in the PNW.

    How can I know when the rain will also bring thunder, so I can make sure she's securely upstairs with me where she is safe?

  5. Cliff,

    For those of us who don't live in Seattle - any option for an online Atmospheric Sciences 101?



  6. We had very little rain in Winthrop. Just not enough push to get over the Cascades.

    Cliff, Is your course through the access program going to be available on the internet? Since I live in Winthrop its not feasible to attend class. Thanks.


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