April 09, 2018

West Coast Precipitation Trends

There is so much talk about heavy precipitation and drought along the U.S. West Coast these days, as well as claims about long-term trends, that I thought it would be useful to look at the actual data.

In the figures below, I show the winter precipitation (November through March) for the states of Washington, Oregon, and California for 1918 to 2018 using the NOAA Climate Division data available from the NOAA ESRL website.  The West Coast has a Mediterranean climate,  which means the year is divided into wet and dry seasons, and November through March includes the overwhelming majority of precipitation everywhere along the coast.

First, California.   Quite a bit of up and down, with 1977 being the lowest (about 7 inches) and 1982 the highest (about 33 inches).  Little overall trend.   With an average of only 20 inches during the winter season, lots of variability, a huge population, and an enormous agricultural establishment based on irrigation, you can see why California has invested in a  massive multi-year reservoir capacity.

Oregon also shows little long-term trend in precipitation, with 1977 (a year of a huge persistent ridge) being the driest of the last century.  Mean precipitation is higher than California (about 27 inches compared to California's 20 inches), the population in much less, and there is far less irrigated agriculture.  Thus, they can get away with less reservoir capacity than California.

And then there is Washington State.   Winter average precipitation is about 42 inches and the lowest it has fallen to is about 22 inches, far above the minima of California and Oregon.  Plenty of variability year to year.  There appears to be a slight upward trend in winter precipitation over the past century.

 So what should you conclude from all this?   

(1)  With all the talk of droughts and floods in the media, there is really very little long-term trend in winter precipitation over the U.S. West Coast.

(2)  The West Coast has a lot of year-to-year variability, so having dry and wet years is not unusual.

Why so much winter variability in precipitation along the West Coast?

Several reasons.  The West Coast is greatly influenced by atmospheric rivers, which bring periods of very heavy precipitation.  These features are intermittent and just getting one or two more or less can make a huge difference in seasonal precipitation.

Another reason is El Nino and La Nina.  The oscillation between them, a mode of natural variability, can produce wet and dry years in various portions of the coast. 

The bottom line:  West Coast precipitation is characterized by wet and dry years, precipitation increases northward, and little overall trend has occurred over the past century.  Not sexy, but the truth, nevertheless.


  1. So, we're not totally screwed afterall? All the Hysteria is hyperbole presumably for ratings?
    Deep in my heart I always thought so.

  2. As I've noted in other comments, it's a cyclical drought/drench climate, and always has been. The global warming cult seizes upon every non-average year, which is most of them given the large standard deviation, as "evidence" for its cult.

  3. So not man made but natural fluctuations? Thanks for adding some science to the politicized talk of drought from particularly Washington state politicians.

  4. Cliff why did you not use trend lines in your graphs?


  5. Putting trend lines in such widely variable graphs would make no sense because the error limits are so wide.

    And Cliff agrees that global warming is happening, because he understands the evidence.

  6. This may add to the discussion - the potential impacts on climatology and long - range forecasting could be significant:


    So the science was settled, plants and trees got all of their nitrogen from the atmosphere...except now they don't, they get just as much from the earth's bedrock. Time to go back and do some more modeling, because science!

  7. The polar ice caps are shrinking. Climate is slowly changing. Weather is highly variable. Discount Global Warming at your children's peril.

  8. More grist for the mill:


    The scientific community has been systemically geared over the past two decades towards getting grants, and that has led to the overtly politicized environment it finds itself in.

  9. Andy... two reasons there are no trend lines. The first is that the web site I used to make those plots did not have the ability to do so. But also important...a linear trend line is simplistic and often deceiving. What should there be a simple linear trend for complex meteorological parameters? The world is not linear....cliff

  10. Localgal... Who said anything about discounting global warming and climate change? Cliff has pointed out many times that global warming is real and what the effects will be. Rain on the west coast is not one of the effects due to the Pacific Ocean. Don't blame normal weather patterns on climate change.

  11. Polar ice caps are melting? As you all say, where's your proof? I've found plenty of scientists that have found the opposite to be true. Have you visited the ice caps lately? I know I haven't, so I have to go on second hand information. Care to comment? We live maybe 80 years on a planet that has been around longer than we actually can prove, and yet we think we change the climate? The environment we live in we definitely can and have altered, but the climate? Check out space weather and our Sun for your answers to most of your queries. We are literally ants on a gargantuan pile beyond our scope of understanding. Good luck finding your proof, other than the appeal to some supposed authority showing you so.

  12. Interesting post, just want to add some thoughts for those who are taking this and jumping to conclusions. I think it is important for readers to realize that this is a very basic look into precip trends and the conclusions drawn are also very basic. Big picture problems that many have in mind while discussing climate change are somewhat not addressed.
    - The data may be from a good source, but using an aereal average such as the geopolitical boundary lines of large states doesn't do this type of research justice. Point data from a single location would be a much better (and the least erroneous way) to look at this. Beyond that perhaps basin wide within specific river basins may be the next most logical step.
    - It is implied that all precipitation is equal. A deep, healthy snowpack produces predictable runoff that can be calculated and managed much easier than high elevation rain events that cause flooding. There is much thorough research out there that addresses this.
    - The Pacific Decadal Oscillation isn't mentioned but the 30 year cycle has been referenced by many peer reviewed papers to have an effect on precipitation on the west coast, as well as AR's and ENSO.

  13. I find this post to be slightly disingenuous because:

    You are based in WA and state that in WA "There appears to be a slight upward trend . . .". It is easy for even me to see from the graph.

    Then end with: "The bottom line: . . . , and little overall trend has occurred over the past century."

    The personal twist that discounts a clearly visible, with 100 years of data, trend line showing an increase in precipitation seems like human bias.

    I got no skin in any game, not trying to be mean our confrontational. Just giving feedback about my response as I finished the post.

    Then I read the comments and Capitalism reared it's ugly head throwing zoological competitive nastiness in with objective facts. Next thought? Science has definitely hit a wall if it's all based on gathering greenbacks. The next big thing will be free energy. The domination matrix that limits rational debate will be turned upside down

    Thanks for your wonderful work Cliff!!

  14. One trend that I do see on this blog is how it consistently attracts a number of contributors who assert that climate change and global warming is mostly a product of politically influenced scientists. I think we can now draw great comfort that the phenomenon is not real, that there is very little consensus among "objective" scientists, and that any scientific agreement is driven by the desire to obtain government funding.

  15. Stating precipitation amounts for a entire states like this is unusual, since actual amounts vary so much, such as between the west side of the Olympic Peninsula and the dry central Washington area.

    Are the figures stated area weighted averages or how are they derived?

  16. Can you post the data? It would be fun to play with.

  17. I'd be curious to see the trends for B.C. and South East Alaska as well.

  18. Cliff, Thanks for the response.

  19. Hi Cliff. I am a fan of yours from California.
    I am interested to get your take on the idea of 'marine heat waves', as discussed in this article- https://www.wired.com/2016/08/marine-heatwaves-spawning-unprecedented-climate-chaos/
    Do you think this is a legitimate way of thinking / terminology?
    Do we have accurate ways of tracking subsurface ocean temperatures trends?
    Thank you for all you do.

  20. Hm, looking at the full year data for WA from 1917 to 2017 shows an upward trend of .42"/decade with an approximate 11% increase in that time frame.
    Looking at Puget Sound Lowlands the increase is .58"/decade for an approx 15% increase.
    On the coast increase is .95"/decade.
    These percentages are consistent with Dr. Mass's predictions for precip increases due to climate change.

  21. Cliff - Sorry if this is off topic but I don't know how to contact you any other way. Since you are my most trusted source for information on climate change - and your opinions and information are pretty much apolitical, I wonder if you can comment on this story about LA painting their streets with a white coating to reduce the effect of urban heat islands:


    This seems to me, and many others a total waste of tax payer money that could be more effectively spent on other projects that would have more positive effects. Thank you for all you are doing to enlighten all of us.

  22. Researchers at the University of Washington, including Cliff Mass, have projected a big increase in extreme winter precipitation along the West Coast due to atmospheric rivers through the end of the century. Average winter precipitation will increase at a more modest pace, according to 2015 research published in the journal of the American Meteorological Society. https://journals.ametsoc.org/doi/abs/10.1175/JHM-D-14-0080.1

    WQR's comment above says that "Cliff has pointed out many times that global warming is real and what the effects will be. Rain on the west coast is not one of the effects due to the Pacific Ocean." The 2015 journal article contradicts WQR's understanding of Cliff's research. Increased rain on the West Coast will in fact be an effect of anthropogenic climate change, but the more important metric is the number of extreme precipitation events rather than annual averages.

    From the abstract of Cliff's 2015 journal article: "Winter mean precipitation along the west coast increases by 11%–18% [from 4% to 6% (°C)−1], while precipitation on extreme IVT days increases by 15%–39% [from 5% to 19% (°C)−1]. The frequency of IVT days above the historical 99th percentile threshold increases as much as 290% by the end of this century."

    Posted by Steve Thompson

  23. Drought is related to both precipitation and evaporation. Analysis of one component does not say much about the long-term drought trend.

  24. The "wetter winters" scenario has long been forecast for the northwest and the trends, such as they are, do seem to corroborate that.

    The question of the day, at least here on this thread topic, is to what degree California is part of that expectation.

    Considering that damn near a quarter of all American ( and Canadian!) agriculture comes out of central California, I'd say that is a pretty interesting wild card which incidentally is a climate not particularly well aligned with Seattle.

  25. Please include Vancouver, BC next time. This is a Metro area of 2.5 million just 140 miles from Seattle. Great blog!

  26. The "wetter winters" scenario has long been forecast for the northwest and the trends, such as they are, do seem to corroborate that.

    Could you please tell everyone where you get your mushrooms? We'd like to see in colors too.


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