September 21, 2021

Are Northwest Summers Getting Drier? The Truth May Surprise You.

We have had a dry spring and summer and people are concerned.  Completely understandable.

And people's discomfort and fears have been unnecessarily worsened by the incessant drumbeat in the media and some politicians that Northwest summers are been getting drier.   And they are not shy about suggesting the cause:   climate change or what was previously known as human-caused global warming.

But is this really true?  Do observations REALLY show that our region is getting progressively drier over the past decades and century?  

The facts are clear:  there is no long-term trend towards drier summers in the Northwest.  And the effects of climate change (global warming) will be very small, with some locations receiving more summer precipitation.  But as in real life, there are details and nuances that are generally not provided in the media.....let me provide them to you here.

Fact 1:  The Pacific Northwest is climatologically one of the driest locations in the U.S. during summer.

Few places in the U.S. get less precipitation during the warm season than the Northwest, with our midsummer "drought" evident in the plot below.  Eastern Washington also has its precipitation minimum during the summer.

Seattle Precipitation Climatology

As explained in my recent podcast (see below), the origin of the dry conditions is clear:  during summer, the storm-bearing jet stream moves north of us, and the cool Pacific and an east Pacific high-pressure area lessen our chances of getting thunderstorms.  Weeks or more without rain is not unusual in our region.  It is why native plants are adapted to summer droughts and our eastside forests DEPEND on wildfire to remain healthy. 

Fact 2:  There Is Little Long-Term Trend in Summer Precipitation over the Northwest

Let's start with Seattle.

Recently, the Seattle Times did a story on how Seattle was experiencing the driest summer on record.  Strangely, they published this story before the end of the summer and when a heavy rain event was in the forecast.  Like doing a story on a low-hitting baseball game in the sixth inning with known sluggers soon at-bat.  

The truth is that this summer (20 June 20-21 September) was the 24th driest summer in Seattle, considering observations going back to 1894.  Not so impressive.

Let me show you a plot of Seattle's summer precipitation prepared by Dr. Joe Zagrodnik, a recent graduate of my department and currently a scientist at WSU. The light blue bars show the summer total precipitation each year.  The black line shows an average (or smoothing) of annual summer precipitation.

You will note a few things. There are a lot of ups and down... or what is called interannual variability. This is not unexpected during a dry summer when a chance shower can make a big difference.

There is little long-term trend in Seattle's summer precipitation... the last 30 years is about the same as the early 20th century.   

You will also note some long-term ups and downs:  wetter in the 70s, drier around 1990, wetter in the early 2000s, a bit dier recently.   

What about extreme dry years in Seattle?  Is there a tendency to have more extreme dry years recently as claimed by some?   To answer this question, below is a plot showing the distribution over time of the top ten driest years in Seattle.

Wow... most of the extremely dry years were early in the 20th century and there is clearly no upward trend in extreme dry summers recently.  In fact, we seem to be having LESS extremely dry summers in Seattle.

What about the rest of Washington State?  

Let's look at the NOAA/NWS climate division data, which unfortunately is only available on a monthly basis (below).  This plot shows the July through September precipitation from 1900 through 2020 (blue line), plus a running average/smoother (red dots) to take out some of the variability.

Lots of ups and downs, but with little long-term trend.  Some periods of high and lower precipitation as in Seattle. Perhaps slightly drier in the end.

Where did the years of driest summer conditions distribute over time for Washington State?  Here is a plot of the 20 driest years for the state.  The early part of the 20th century was dry, but there are few Northwest residents old enough to remember that period.  Clearly, no long-period trend towards more extreme dry conditions.   

Climate Change and Northwest Summer Precipitation

The implications of global warming on Northwest weather/climate is an area in which I am actively working on, under support from a variety of sources (such as Amazon's Catalyst program and the National Science Foundation).  We (including Professor Salathe of UW Bothell, and UW scientists Rick Steed and Jeff Baars) are running state-of-science high-resolution simulations of the impacts of increasing greenhouse gases on our region.

Let me show you some regional climate forecasts using an unrealistically large increase of CO2 over this century (called RCP 8.5, often termed the business as usual scenario).  So the actual changes should be less than this.

Here is the difference between the late 20th century and the middle of this century in terms of June-July-August precipitation.  Small drying in the west and little change over eastern Washington.  Eastern Oregon gets wetter.

Again, keep in mind that this forecast is using a very aggressive change in CO2-undoubtedly too much.

What does the model forecast for Seattle? 

We ran an ensemble of many climate simulations starting in 1970, which are shown by the colored lines.  The average of all these forecasts is shown by the green line and observations are indicated by black dots (the y-axis shows inches of precipitation).    Not much change...just a very slight decline over time.

I could show you a dozen more of such predictions, but climate simulations provide a consistent message:   

Global warming will not change Northwest summer precipitation by much, perhaps with a very slight decline in the west that will hardly be perceptible.  And east of the Cascades might get wetter as more southwest monsoon moisture moves northward.

Now temperatures will warm slowly during this century, which will cause more evaporation, so soil moisture during summer could well decline.   But there is no abrupt, end of the world, "existential" drought threat in the offering.  

Sometimes atmospheric variability randomly gives us a very dry summer, like this year, but that is entirely natural and expected.

Any media or individuals telling you otherwise is not following the science.


  1. Interesting, is the trend similar for fall/winter/spring as well?

  2. I wonder how much the extremely low runoff in some of the basins this water year and the impacts or threatened impacts to irrigation, fishing, etc. impact perception about precipitation. It looks like the NWRFC is projecting below average runoff in the 2022 water year in part, as I understand it, because the dryer than normal soils will hold on to more of the precipitation when it does fall. More of a psychology/sociology question, but I am curious how water use impacts perceptions about supply.

  3. Had hoped that Inslee would have been voted out, but no, not the case. Never did like the guy as a Governor (Spellman was way better) but to be fair, Inslee's Covid policies were not terrible and we kept out numbers reasonable, but still could have done better.

    Anyway, I also agree that some of this is what the media is also screeching, but also I think a lot of people coming in from elsewhere and not used to our very dry summers and feel it's a drought when in fact, it's not one and as Cliff has pointed out, it all depends on whether we have enough water going into the dry summer months or not. Plus, I also feel these same people may be doing the old Chicken Little thing with the sky is falling routine and thus helping to beat the drums of drought when in fact it's not the case.

    Being one that is native to this area, and Puget Sound specifically, while our temps are not brutally hot usually, with temps averaging in the 70's much of the time, but it is very dry and for those of us who grew up here, I feel some may have forgotten how less dry it was about 40-45 years ago as I recall the occasional showers in August from time to time some years, and we have not have much if any of that in recent years, but overall, never felt our summers were all that much dryer, but it feels worse due to how dry it often gets in the spring and early summer here, with many years, the grasses beginning to dry out as early as May.

    1. My brother and sister attended Jimi Hendrix's last Seattle concert, in July of 1970...It was very rainy!
      And I certainly remember going to Green Lake to watch the July 4th fireworks, back in the early 60s...It rained a few times then also!...But I am just pointing out the obvious...NW weather manages to be both stable, and wobbly from time to time.

  4. Just wondering what weather station is responsible for all this data? I have noticed several times this year some giddiness over some rain that fell in the northern part of western WA But here in the southern part at least where I'm at we never got a drop.

  5. Cliff, how do you generate all these amazing graphs? Is there a free dataset others can access to analyze the data.

  6. Maybe it's time to do a IPCC report blog explaining what the "science" means. Climate change advocates constantly scream this report proves catastrophic weather events are climate change driven , yet I think they just hear a pundits interpretation of it. I have a sister that is big time "will my son be able to breath" scared (I know is irrational), but the "science" tells her that I guess. Unfortunately, when I suggested following you. She scoffed and said she won't listen to a right wing Trumpster. Odd isn't that anyone contradicting their views is labeled as such. Thanks for being the calming guiding voice of science.

    1. Same is true for rational voices regarding Covid. For example Bret Weinstein. He often gets labeled a conservative.

    2. I don't think Weinstein should be considered a rational voice on much of anything these days. He's plainly become an online grifter. I don't think he has much of ideological bias but rather seems overly committed to being a contrarian.

  7. Are the trends the same if you look at meteorological summer (June 1 to August 31) as opposed to astronomical summer. I've looked at data for meteorological summer for Victoria, BC and the last decade has been the driest since the very dry 1920s. At the same time, there have been a number of wetter than normal Septembers recently. Perhaps using astronomical summer (which extends well in September) offsets the drying trend seen for June to August?

  8. Garyoak,
    Meteorological summer around here really start after July 4th and extends into mid-September. June is often cool and cloudy (June gloom).... In any cases, there is not radical change using June 1 to August 31st...cliff

    1. Okay, but if you look at average rainfall at Sea-Tac for June 1 to August 31, which is the commonly used definition for meteorological summer, there does seem to be a modest downward trend. I also looked at a rolling 30-year average for 1946-1975 through 1992-2021, and again, there is a steady downward trend - about a 15-16% decrease over that period.

    2. that is not meteorological summer....but if you want to use that, you should start in 1894, when the seattle record starts...

    3. It actually is considered meteorological/climatological summer (NOAA -, Private Industry -

      Doesn't take away from the need to use the entire record for analysis but does highlight that using different somewhat arbitrary (but hopefully self consistent) divisions of the calendar can lead you to slightly differing conclusions which needs to be highlighted in any discussion of trends. You sorta allude to this with your baseball - sixth inning analysis.

  9. Cliff, I would very much appreciate your opinion on the widely used USHCN (i.e., adjusted) climate datasets versus the raw data. I was recently dismayed when I ran some summaries using the raw downloaded data, compared them with the USHCN data and found completely different long-term trends. USHCN shows increases in temperature over the past 40 years that are nowhere to be found in the raw datasets. This is a real shock to me because USHCN is so commonly used in the scientific community.

  10. Cliff says, "Any media or individuals telling you otherwise is not following the science."

    There in lies the problem with today's government officials and the media covering them.
    Dishonest officials are supported by dishonest media.

    If sound science were guiding policy makers there would be no climate crusade at all.
    But the left has long ago fully invested their feelings into the AGW movement requiring messaging to be continually enhanced with more and more deceit.
    It's why every climate conversation or debate goes off the rails. There is no going back to honesty abandoned. Too much of the sphere of human communication is available to endlessly perpetuate the stream of lies.

  11. I grew up in Seattle, born in 1949, raised in Shoreline. I raised my kids here, mid-80's to the 21st century. My summer memories of these periods seem to be consistent with your graphs. As a kid, it seems we couldn't rely on dry weather when planning family events in the summer, and once we got back into school after Labor Day, it would get drier and hotter. I remember my kids getting rained out on summer day camp events, and lots of mud at Scout camp for both them and me.

    1. Jeffnorman, you bring up a good point. Back in the late 70's the first 2 sessions of Camp Huston, located just east of Goldbar would often be wet and cool. Third session was when it would begin to dry out and be warm as it came after the 4th and the truth was summer didn't "begin" until after the 4th of July. I recall a few fourths where it was cool early in the day and never got much beyond the mid to upper 70's and maybe partly sunny, but be dry enough to do the fireworks.

      In recent years, that seems to have changed some when we are more likely to be dry and warm on the 4th instead and recall the occasional rains that would swing through mid to late August for a day or two, then get warm and dry, with often the fair running and cool nights and warm, sunny days and in recent years, it seems the fair can be wet.

  12. California, the nations fruit and vegetable basket, is getting drier and the water table is dropping. So is much of the west. Why not pipe water from areas that have too much such as the east coast?

    1. Because the pipe would have to be the size of the Colorado River to have much effect. You think the Panama Canal was hard to build. Imagine a river many times the size of the canal running through the Rockies, and all the other upthrusts between here and where you think you can get a major river's worth of water.

      Much easier to move the people... or far better yet, get the consumption of water way, way down and recycle it.

      CA has to accept that what water they have is pretty much what you will ever have, unless Mother Nature decides otherwise. And adjust accordingly.

    2. Yes and no. A minor 3,000 cfs diversion of either the Columbia or the Mississippi to CA/NV would provide nearly 2.2M acre-feet of water per year, or about half the current capacity of Lake Shasta. It doesn't solve the buffer problem--how one ensures water is available when it is needed. Restoring Shasta Dam to its original intended height (200 ft higher than it is today; it 'shrank' as it was being built in WW2 due to resource constraints/re-allocation) would triple its capacity (4.5MM acre-feet to 13.5MM acre-feet). A series of constantly-fed micro-reservoirs (say, 20 of them, each a 2 mile square and 30 feet deep) up and down the Central Valley could serve as groundwater recharge sites, with each one holding over 76K acre-feet, and in aggregate over 1.5MM acre-feet. Most importantly, these could help stop/reverse the subsidence that's happened over the last 100 years. I agree re recycling (and other capture) technologies (incl. cisterns for rooftop rain capture)--it's maddening to just watch the water run to the sea.

  13. Pffft, there you go again, Cliff. Using actual statistics and meteorological expertise to back up your points. None are so blind than those that refuse to see, etc.

  14. Unfortunately the average person does not have the time or capacity to deep-dive these nuances. While I agree sensationalism isn't helpful, I fear that these "debunking" posts serve as confirmation bias for climate change denialists. The underlying theme in Dr. Mass' climate posts is that climate change will be minor, that it's is inevitable, and that no action needs to be taken.

    1. Not minor....but slowly developing, so we have time to do this in a deliberate, logical way. I have NEVER said there was no need for action. You are saying things that I have never expressed.

    2. Jeffrey Anderson - That's a pretty elitist statement, "the average person doesn't understand."
      Do you understand the complexities of climate? Can you explain them to "an average person"? Cliff has never said "no action needs to be taken".

  15. Interesting. So given the science, in your opinion, with what level of urgency should we address climate change? For example, with regard to burning coal?

    1. We need act in a deliberate, logical way. Push technology development that will solve the problem (e.g., fusion power, safe fission, next generation renewables), and work on adaptation (fixing forests, protecting coastal areas, increasing AC). This is slow developing problem and we have time to deal with it without panicking folks.

  16. Great post. I'm curious why 1976-77 didn't make it in the driest 20. Cliff, any thoughts there?


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