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

Reposted from the Cliff Mass Weather Blog

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.

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 NorthwestLet’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.

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September 22, 2021 6:06 am

Some level of statistics instruction needs to begin in primary school but I suppose that lottery officials and gambling business would be against it.

Reply to  Scissor
September 22, 2021 6:11 am

The commandant in Olympia would also oppose it.

Clyde Spencer
Reply to  ResourceGuy
September 22, 2021 9:55 am


This needs to be removed.

Tom Halla
September 22, 2021 6:14 am

Rejecting statistical reasoning is a requirement for liability lawyers and other persons claiming dread trends. They should know better, but it gets in their way. I will not grant advocates any pass for their indomitable ignorance.

michael hart
Reply to  Tom Halla
September 23, 2021 12:46 pm

I think a large part of the problem resides in the difference between Legal reasoning and Scientific reasoning.

Legal reasoning is required to come to a decision, not to be correct. Hence the legal opposition to arguing in the alternative.

A lawyer will argue that if we have five continuous conclusions at nearly 51%probablity, each contingent on the previous conclusion, then the final conclusion is ‘correct’.
The scientist will argue that 0.5 raised to the power 5 is about 0.03. There is almost no chance that it is correct.

I recall I have even seen Steve McIntyre arguing for the lawyers position, which I do not agree with.

September 22, 2021 6:21 am

I know where your missing rain is this year .. it’s here in the southeastern US, particularly in the Gulf Coast area such as Florida where I live. We’ve had an abnormally high amount of rainfall this rainy season which happens to correspond exactly to the dry season in the Pacific Northwest. Yesterday my locality received 5 inches of rain in just two hours – whereas the average rainfall for the entire month of September is just 9 inches. We’ve been getting drenched near daily all month long but for a stretch of just a few days when it finally dried out.

It’s called “weather”.

It’s also just like the Alan Jackson/Jimmy Buffet song, “It’s always 5 o’clock somewhere”.

In the matter of weather, if you’re getting very little rain in one part of the country, someone else in this country is getting drenched. And vice versa. If you’re getting hot in one part of the country, you’re getting cooler than normal in some other part of the country. And vice versa.

Weather, see?

Reply to  Duane
September 22, 2021 1:34 pm

Midwest too… In MO we have 65% more than normal rainfall this year so far (56 inches versus 34 normal)

Reply to  menace
September 22, 2021 1:39 pm

oops nix that, I found my usual source of info on NWS hasn’t updated the graph in last 6 months, grrr

spot checking monthly data we’ve been slightly above normal rainfall this summer

September 22, 2021 6:53 am

I told you that it would get drier.

It was there before, remember?

Ron Long
September 22, 2021 7:18 am

Nice to see actual data presented here at WATTS, however, I rely on anecdotal data provided by my twin brother who lives in Eugene, Oregon. He says his annual attempt, in the backyard garden, to grow pumpkins up to 150 pounds is well underway and did not require more watering than normal due to good rains spread out through the summer. On a different level of anecdotal information, CNN has just reported that the United States, China, and Britain are in agreement about Climate Change, which is not believable as only two of the countries have even the slightest level of credibility.

Brooks H Hurd
Reply to  Ron Long
September 22, 2021 10:06 am

The biggest of the 3 appears to be on the verge of a national property collapse. Evergrand is the most visible, but it is far from an outlier.

Sorry about the OT, but China’s agreement on climate action is a joke. Perhaps China will find that “Climate Chnage” is causing their property bubble to burst.

Reply to  Ron Long
September 22, 2021 3:45 pm

Okay, I give up. Which of the countries have even the slightest level of credibility?

Ron Long
Reply to  chickenhawk
September 22, 2021 6:39 pm

Ethiopia and Guatamala?

September 22, 2021 8:01 am


The trends shown in graphs seem to track the long term trends in the solar cycle, with the driest years associated with the weak solar cycles of the early 20th and 21st centuries, and an absence of droughts during the solar maximum in the mid-late 20th century.

This would make sense if weak solar cycles result in less solar energy going into the Pacific Ocean, and thus less evaporation from the the ocean and less moisture in the atmosphere.

If this is the case, then we should expect more dry years over the next couple of decades.

September 22, 2021 8:01 am

Just a heads up to my WUWT buds.

The severe drought farmers in the US and Canada went through this year will result in the worst wheat crop in 30 years.

Now is the time to buy a few cases of your favorite pasta because retail prices will likely increase by 50% in the months ahead…

What’s worse, is that we’re in a rare back-to-back La Niña cycle, so Western states may likely suffer through another year of insufficient rainfall next year, too.

Reply to  SAMURAI
September 22, 2021 9:11 am

Coffee prices may go up to, because Brazil had a coffee killing frost earlier this year, even though NOAA/NASA said it was record warm there.

Steve Case
September 22, 2021 8:05 am

Here’s what NOAA’s Climate at a glance says for summer precipitation in Spokane:
comment image

As far as I know, NOAA doesn’t mess with precipitation like they do with temperature.

Reply to  Steve Case
September 22, 2021 9:11 am

Yet …

Brooks H Hurd
Reply to  Steve Case
September 22, 2021 10:08 am

Rainfall is easier to measure.

September 22, 2021 8:22 am

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

That seems to be the general trend among people who constantly scream about “following the science”

“THE science”. Right.

4E Douglas
September 22, 2021 8:46 am

My family came from Missouri to Washington state in 1909.
Had a dryland wheat farm near Starbuck, WA . Grandpa talked about how dry it was. Heading
to NE Oregon in 1916. Combination wheat and cattle.
About 3500 acres.some timber too. Nobody around to remember the “Big Burn” of 1910 in Idaho and the”Tillamook Burn” of 1933
the gave problems through 1951. On my Mother’s side they went through the “Dust Bowl” in Kansas.
Weather. Period.

September 22, 2021 9:06 am

The NOAA data for California shows about the same trivial trend.

Clyde Spencer
Reply to  eck
September 22, 2021 10:01 am

Then it must be getting wetter somewhere else!

September 22, 2021 9:10 am


I read:

  • ‘Statewide total March and April precipitation ranks as the driest on record for Idaho, 2nd driest for Oregon, 3rd driest for Montana, and 4th driest for Washington since 1895.
  • There are also long-term precipitation deficits within the region. Specifically, the Lower Columbia Basin in Washington, southern Oregon east of the Cascade crest, and south central Idaho have 2-year precipitation totals that are only 50%–70% of normal. 

There’s certainly a severe drought in the NW and explaining it away won’t solve it

Reply to  griff
September 22, 2021 9:59 am

Imagine, deserts are dry too. 😀

Brooks H Hurd
Reply to  griff
September 22, 2021 10:12 am

Failure to put current weather in historical perspective allows our political critters to demand more control and Trillions in taxes to fight a problem that they have created from partial, cherry picked and ‘adjusted’ data.

Reply to  griff
September 22, 2021 10:42 am

Hmm, this is hard. Griff’s unreferenced “data” versus Cliff Mass’ well-referenced data. Plus Mr Mass, is a trained, experienced meteorologist, and griff is …

Reply to  griff
September 22, 2021 11:06 am

You don’t “solve” a drought, you live with it. Everyone has droughts or prolonged drier periods from time to time, as well as prolonged wet spells. It is called “weather.”

By the way, the article is about SUMMERS, presenting long term data, Griff, so you are off topic. What’s the point in responding to Griff. He/she/it never engages in discussion, but just tosses comments and then runs.

Pat from kerbob
Reply to  griff
September 22, 2021 11:20 am

Read about pallisers triangle

Grasp that we alternate through drought cycles

Also reflect that these drought are shorter and less severe than in the past

Reply to  griff
September 22, 2021 12:13 pm

Droughts were worse in the 1970’s but that doesn’t count because CO2 levels were much lower then and most people understood the difference between Weather and Climate.


Dave Fair
Reply to  Sunsettommy
September 22, 2021 12:53 pm

There was no money in climate then. We now have whole industries based on CliSciFi.

Reply to  griff
September 22, 2021 1:54 pm

Griff thinks “normal” equals perfection when in fact normal is just average and nothing to be excited about.

Reply to  griff
September 22, 2021 5:58 pm

Question for you griff: how did the previous record get established? And the one before that? And the one before that?

Frederik Michiels
Reply to  griff
September 24, 2021 9:26 am

There was a time in June 2021 that the Belgian media was agreeing with you. The summer of 2021 would be dry and our groundwater would be extremely low.

Sorry griff we just came out of the wettest summer ever recorded since 1833.

Oh yes half of our country is completely wiped from the map by the worst floods since 1920.

So far the drought narrative for Belgium.

September 22, 2021 9:55 am

Surprised? C’mon, man. You’ve been doing this for how long?
They say something, you think the opposite, and YOU are right based on the facts.
See, that’s why I am never surprised.

Clyde Spencer
September 22, 2021 9:57 am


It looks like more spam from “Sara.”

September 22, 2021 10:59 am

I’m not suggesting anything deliberate here, but I found the presentation in this post a little sloppy. Unless I’m reading this wrong the second graph is described as:

The light blue bars show the summer total precipitation each year. The black line shows an average (or smoothing) of annual summer precipitation.”

But the graph itself is for # of rainy days, not what most people would think of as total precipitation.

The next graph is the top 10 driest years, but the post was to this point about summer precipitation.

The next graph then gets back to the topic of summer precipitation and the post recovers from there with the main points well made.

Thanks for all your work on this post and others. Hopefully you’ll take this as constructive feedback.

Pat from kerbob
September 22, 2021 11:13 am

Same here on the canadian prairies
It’s a drought year omg omg

Yes, that is why it’s prairie with the only trees being the ones we plant or the clumps that hold on in river valleys

Had it pretty good for a couple decades but it’s due for switch which will then be proof of AGW
Omg omg

No, just natural cycle swinging back

Read up on Palliser triangle

Pat from kerbob
September 22, 2021 11:14 am

Strange that I made a comment and is awaiting approval

Are there new trigger words that I didn’t get memo on?

Citizen Smith
September 22, 2021 11:16 am

Hey Cliff Mass, why no mention of the Pacific Decadal Oscillation?

September 22, 2021 1:43 pm

Press Climate Propaganda is nothing but a long list of lies. I cannot recall a single “Climate Crisis” Press report that is truthful…and no retractions are ever provided when they are called out on their lies.

The preponderance of all reporting (about everything) is now mostly lying propaganda and leftist opinion disguised as news.

Who are all the idiots that still provide an audience for all the lying?

Reply to  DocSiders
September 22, 2021 1:57 pm

The press no longer reports the news – just the latest chapter of the narrative.

September 22, 2021 1:46 pm

But wait. Tax funded NPR on the other thread says just the opposite of what Dr. Cliff Mass says.

What’s up with that?

September 23, 2021 2:15 pm

Northern California has had 2 rainy days over the last several weeks. Nice to see these early rains. Back in the 1950s/60s it was common to see the first rains arrive in September. Then that started to change in the 1970s. Over the last 8 years here in NorCal the first rains often did not arrive until November. There were several years recently where the first rains arrived in late December or in January.

Michael Hammer
September 23, 2021 3:17 pm

“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.”

There is another way of looking at this that some may find easier to appreciate. Warm moist air at the equator rises. When it reaches the tropopause is spreads out poleward but as it does so 2 things happen. Firstly, because the Earth is a rotating sphere the surface speed diminishes as latitude increases so the air from the equator ends up moving faster than the surface. The effect of this is for the faster airmass to be pushed back towards the equator until it can slow down through friction. At the same time the GHG components of the air mass allow it to radiate energy to space meaning it can cool down and then descend again. This balancing act between slowing down and cooling determines at what latitude the air descends again and it is around 30 degrees. The descending air having ben cooled to around -50C at the tropopause is extremely dry and the descending air forms a high pressure region at the surface. Thus at around latitude +-30 degrees there is a permanent high pressure band of very dry descending air which is why most of Earths deserts are at around that latitude. Mariners in the past called these the horse latitudes because so many ships were becalmed in the descending air stream and ran out of water forcing them to throw the horses they carried (which drink a lot of water) overboard. This is the Hadley cell a major driver of weather on Earth.

However, the center of the Hadley cell is not precisely the equator. It is the point of maximum insolation and that moves with the seasons, moving north in July and south in January. That means the edges of the Hadley cell also move. For a location at near the equator it makes little difference with a hot wet climate year round – the tropics. At latitudes around 20-25 degrees they are well within the Hadley cell during their summer so hot and wet but close to the edge of the Hadley cell in their winter so a dry winter – monsoonal climate. For locations around 35-45 degrees, they are under the edge of the Hadley cell in their summer so a hot dry summer but well outside the Hadley cell during winter coming instead under the influence of the Ferrel and polar cells which yield cold wet winds from higher latitudes so a cold wet winter – meditteranean climate. Since the air in the Hadley cell has considerable mass, there is a time constant to its movement and that means the actual movements are delayed from the movement of the insolation peak by some weeks.

Interestingly, if there were no GHG’s in the atmosphere the air at the tropopause could not cool and thus not descend. Therefore there would be no convection, and thus no weather. Without convection there would be no lapse rate so a static isothermal saturated atmosphere. No evaporation of water, no clouds, no weather. Without convection and evaporation the surface temperature would follow the insolation level albeit with a time constant determined by the thermal mass of the surface but as we know from experience that is surprisingly short. Consider how quickly a sidewalk or beach sand heats up on a summers day. Temperatures at noon in summer in the tropics would climb above 100C and even at around latitude 30-40 they would get to 70+C. Of course at night the temperature would fall far far below freezing. So GHG’s do not so much warm the planet, rather they dramatically reduce the daily temperature excursions and thereby make life on earth possible.

Frederik Michiels
September 24, 2021 7:46 am

Hmm and while the media in Belgium were hyping at a short warm spell that the summer will be drier and the ground water will take a hit for the worse…

reality: wettest summer ever recorded and half of the country flooded claiming 54 victims. One of the worst flood disasters ever.

now that turned out a bit differently then expected…

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