October 18, 2019

Huge Precipitation Contrasts

Sometimes you just have to be amazed by the precipitation variations in our region.  On the southwest side of the Olympics near Lake Quinault there was 6.78 inches of rain during the past 72 hr.   That is more than Seattle typically gets in a month during mid-winter.  But on the other side of the range, there was only .37 inches in Sequim and .19 inches on the southern side of Lopez Island in the San Juans.   With flow from the southwest, there was huge upslope enhancement on the southwest side of the Olympics and profound rain shadow to its lee. 

To put it another way--- there was 37 times more rainfall on one side compared to the other.

 Puget Sound was also  rainshadowed by the Olympics, with a third to one-half inch.  But as the air rose again on the Cascades, the precipitation increased to 3-4 inches over the same period.

The new GOES-17 satellite provided a full color view of the rainshadow, demonstrating that the there can be sunny skies in its core, while everyone else is under clouds.  No wonder folks retire there. Here is the view at 2:10 PM on Thursday, which is a good illustration.

Weather radar can be used to estimate rainfall, and the total estimated accumulation for roughly the last 3 days, based on the Camano Island radar (below), shows a rain shadow that extends down to Seattle.  There is a huge increase south and west of the city--shows how sharp the boundaries of the rain shadow can be.

One of the most exciting aspect of modern high-resolution weather prediction is that we can predict such extreme precipitation contrasts, as illustrated by the 48h accumulated precipitation ending 5 PM Thursday (below) from the UW ultra-high resolution (4/3 km grid) domain.  Big impact of the Olympics, and if you look closely you can see the effects of Mt. Rainier.

This weekend we have plenty more rain, but I have some good news--by the end of next week, upper 50s and dry conditions should return. 
Portland Oregon talk.    I will be speaking next Saturday (Oct 26th) in Portland (OMSI) at 9 AM.  My topics?  The Blob and Wildfire Meteorology.  This is the big annual meeting of the Portland Chapter of the American Meteorological Society.  More information here.


  1. Wed AM - 50 8th graders coming to the beaches of Fidalgo Bay for all morning science projects. We had a box of plastic ponchos, plenty of big black garbage bags with scissors, and all the adults brought extra rain gear. Forecast was for a steady downpour. No intent to cancel.

    Sun breaks, stiff breeze, beautiful fall colors, dry as a bone, and just coolish. A fabulous time by all. Thank you Olympic Mtn. shadowing!!!

  2. Is nobody gonna talk about the 9.99 INCHES OF RAIN IN REDMOND?!... uh... eh...

  3. Cliff
    You noticed correctly about the rain shadowing effect of Mt. Rainer, but did you notice the same effect down south made by Mt. Adams? That rain shadowing by Rainer is why Crystal gets less moisture therefore less snow from storms coming in from the W or SW.

  4. What's the 9.99 on the map over Redmond/Woodinville?

  5. Cliff I grew up in Minnesota and I am not sure how you get Thunder and Lightning when it is 46? I Thought it needed to be warm toe get Thunderstorms?

  6. Cliff, I live in Sequim and have a blog with over 2,200 articles. I periodically post on the weather. How can I create a map like your map with the precipitation in specific areas for a specific duration? I can't seem to find such an option on the Internet. Can you direct me to where I could find the data and create my own map? Thanks a million.

    1. I too want to know where to get that map with the precipitation totals!

  7. The Arlington Airport (KAWO)rain gauge is broken...was when you posted this and is not reporting today.


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

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