August 04, 2022

Heatwaves in the Northwest: Are Extreme Heat Events Increasing Rapidly?

 My Friday podcast on extreme heat events in the Northwest stirred up a lot of comments and some controversy, so perhaps I need a detailed blog that puts the details in front of you.

My basic point is that the peak temperature, duration, and frequency of extreme heat events are not rapidly rising in the Northwest.

And that mean/average temperatures or minimum temperatures are rising faster than the extremes.

Let me explain.

First, you have to be careful to use reliable observing sites

The Seattle Times and some climate activists like to look at temperatures at Seattle Tacoma Airport, but SeaTac is the LAST station you want to use for climate studies.  Why?  Because of the profound growth of the airport, with the addition of a third runway, many additional buildings, and lots more concrete.  Plus, massive urbanization around the airport.  Plus, the observing record only goes back to the late 1940s and the sensor location has been moved.

SeaTac Airport.  The weather sensors are at the black dot

Based on the recommendation of Mr. Mark Albright, past Washington State Climatologist, let's consider Olympia Airport instead. No additional runways, a much less developed environment (see below) and the record goes back to 1941.

Below is a plot of the highest temperatures in July and August for the entire record at Olympia.  

Do you see much of a trend in the extreme high temperatures?  I don't.

There are more very cool years in the earliest part of the record, so if you calculate a linear trend line, you get a slight upward tilt (about 1.2F over the period).  Again this is not driven by the temperature extremes but by more unusually cool years early in the record.

Now let's look at the trend in the daily average temperatures for those months.  This is the average of the daily highs and lows.   There is more of an upward trend: about 2F over the entire period.

Next, let's examine a different this case Lind 3NE, which is found in a totally rural area in eastern Washington (see a picture of the surroundings below).  This station goes back to 1931. No urbanization here.  An area of natural conditions or dryland farming.

Here is a plot of the maximum temperatures for July and August at Lind.  Mama Mia!  The extreme highs are GOING DOWN.  Many of the warmest years are early in the record.

On the other hand, the daily average temperature is going up (by 2.4F over the entire period)

Other well-exposed, mainly rural, observing sites show similar behavior.  In contrast, highly urbanized or disturbed sites (like SeaTac), particularly those with sensor exposure issues, show much more of an upward temperature trend. Obviously, these observations are not suggesting an existential threat...just a small upward trend of mean temperatures and less upward trend in extreme warmth.

Six Day Heat Waves

A lot has been made in the press and by certain climate activists about SeaTac achieving six days above 90F for the first time.  This kind of frequency above a threshold can be very deceiving, since very small warming (say .1 F) can cause one to cross the threshold.

It is far more meaningful to look at the actual temperatures during the warmest six days for each year.

So let's do that!

For Seattle, the six warm days last week came in third for the period of record.  Note that many other years were right behind.   And remember all the recent artificial warming due to the third runway and more.  Without all that development, SeaTac temperatures last week would have been much further down the list.

For Olympia, last week falls to number five, and was over two degrees cooler than the "winner" in 1981.  1941 was also warmer.

It was mighty warm in eastern Washington last week, so what about Lind (see below)?  The heatwave last week did not even make the top ten.

Were we unusually warm for an extended period last week?  You bet.  Did global warming contribute to it?  Quite possibly by a few degrees.   Was this the record six-day warm period at any of the observing sites in the region?  No.

There are some folks that get concerned when I put our summer heat waves into historical perspective, but it is critical to do so.  Heat waves are nothing new in the Northwest.  Although our mean temperatures are slowly increasing, the extremes are rising more slowly.  And when it comes to impacts, the extremes are really what count.

Why does global warming affect the means more than the extremes?

This is a topic I will cover in a future blog in some detail, but there are many reasons why global warming influences mean temperatures much more than extreme high temperatures.  

This is not surprising.  Radiative effects are stronger for minimum than maximum temperatures, some local wind circulations supporting heat weaken under global warming, irrigation causes cooling during the day, and many reasons will be discussed.


  1. How about the lowest temperatures recorded in the Seattle area? I have read accounts that Green Lake Seattle froze over in I think the 1930s, and people drove cars onto the lake. And I think once a long time ago they recorded a temperature of 0 degrees F in Seattle. As for record highs rising not so much as the average, does that relate to the "fourth power rule" for radiative heat transfer?

    1. Green Lake has frozen over 3 times in the last few years, though it probably wasn't thick enough to drive cars on any of those times. There were a lot of people walking on the lake in the shallow end one night in February 2019 though.

  2. Thanks for the analysis. It would be interesting to see June and September included for a more complete seasonal view.

  3. There are several ways to make heatwaves more severe: higher temps, and same-temp-lasts longer. Higher overnight temps for a few nights in a row is a big setup for heat stroke fatalities.

  4. Your own paper based on regional climate models ( shows perhaps a 50-100% increase in the frequency of modestly high summer temperatures in 2022 as compared to 1970. Since the paper also predicts more rapid increase in 2025, isn't it a pretty safe bet that the *frequency* of extreme events is already increasing?

    The paper also shows an increase in average daily maximum summer temperatures from 1970-2022 of perhaps 1-2 deg C at Seatac. Since these are modeled, not observed, heat island effects wouldn't apply.

    You can view the plot from the paper at if you can't get through the paywall.

    Summary from the paper: "During the historic period, the model simulations closely follow the observed values, with modest increases through 2017. Significantly, the simulations capture the observed range in interannual variability as well as the multiyear trend. Starting around 2025, the percent of days exceeding the 90% value starts to increase more rapidly, and by the end of the century the maximum temperatures exceed the historic 90th percentile on about 75% of the days."

    1. Fred... in our work the major changes in extremes are in the future...not during the past decade. And I have to admit that our assumed greenhouse gas assumptions (RCP 8.5) were too large...way too much warming. Not realistic. We are redoing it now with something more realistic (RCP4.5)..cliff

  5. Wouldn't something like the standard deviation statistic give a better indication? Not saying I am some statistical dude. Just saying that They are averages. Maybe, what is the standard deviation on a rolling 25 years look like. Just wanting to see if the amount of extremes that are occurring since about 1980....probably not much change before 1980.

  6. The recent late July heat wave may not have been very impressive at Lind, WA, as you said, not making it into the top 10 there. However, at the Tree Fruit Research Center in Wenatchee, the average maximum temperature of 107.6 for the period July 26-31 of this year, was second only to the 109.2 six-day average set in June 2021 from June 25-30 with record back to the late 1930s.

  7. Please tell me why the official weather station is at SeaTac airport that is not even in the city?

    1. It was common practice in the '40s to move the official recording stations from downtown (usually at the fire department) to the local airport, which are almost always outside city limits. Sea-Tac is not unique in that regard. The Olympia Airport station is in Tumwater, about five miles from downtown.

  8. Another reader pointed this out recently, but irrigation is a huge factor dampening extreme high temperatures in Eastern WA. Irrigation has greatly increased over time. I would suspect that is at play with the Lind data.

    1. There is little irrigation around Lind, but it is true that some portions of eastern Washington are cooled by irrigation.

  9. @Cliff Mass,
    At a glance, a few things that seem wrong to a non-specialist:
    1 - Why are you using only a few stations to analyse extreme heat events in the Northwest, rather than all stations?
    2 - If your analysis intends to look at extreme heat events generally, why are you restricting your extreme heat events to July/August when extreme heat can happen any time of the year?
    3 - If your analysis intends to look at extreme heat events in Summer only, why are you restricting the analysis to July/August when international definition of Northern Hemisphere meteorological Summer is from 1st of June and ending 31st of August?
    4 - You state at the start of the post "My basic point is that the peak temperature, duration, and frequency of extreme heat events are not rapidly rising in the Northwest.".
    4.1 - How is a graph plotting peak temperatures per calendar year and a few tables showing 6 day peak temperature averages per calendar year supposed to analyse duration and frequency of extreme heat events?
    5 - More broadly,what is your definition of "extreme heat event"? How can you claim to analyse something you do not define?

    1. Lowlander....happy to answer your questions. (1) I simply wanted to show the results of two well exposed, long-term stations. Other stations are just like those (I checked). The blog would have gotten a bit long if I showed the plots for hundreds of stations (2) July and August are the warmest months of the year in the NW. If one does it for all months, you get the same results (3) Same answer as (2). June is generally a cool month in the NW (June gloom month) (4) Obviously, I can't have everything in a blog. The high temps are of interest and there has been a lot of talk about the six days of hot weather. (5) I am not claiming anything. Showing plots of some key parameters related to heat.

    2. At Olympia Airport, in the past 10 years, 5 of the hottest days have occurred in June. And the record set last year on June 28, 110 deg F, surpassed the previous record by 6 degrees. Would be interesting to see the high annual temp trend for the Olympia station with all of summer included, instead of trend for just July and August.

    3. Dee Aurora.... have done this for the summer and whole year. Same story. You do get the peak in 2021...but that is only one extreme event and really doesn't change the trend of the high temps...cliff

    4. Thank you for the reassurance that 110 deg F extreme will not likely become routine! For us apartment dwellers, when and how to install AC in a rental is a hot topic. Predictions are extremely useful.

  10. You suggest in the text that the Lind plot is annual maximum temperature, but it is the maximum in July and August. If you plot the annual maximum, the highest temperature on record occurred in 2021 (114).

    1. Jim.... you are correct...fixed the typo. Just like Olympia...July and August...cliff

  11. "This kind of frequency above a threshold can be very deceiving, since very small warming (say .1 F) can cause one to cross the threshold."

    That's rather the point, I think. You have to draw a line in the sand somewhere, and insinuating that 1–2° is somehow unimportant because it's a small number is misleading. On days where it's nearly intolerable in my house, I measured the indoor temperature at 85.5°F. On warm days where it was acceptable, the temperature was 83.4°F. At some point, a threshold is passed where a tiny change makes a big difference.

    Heatwaves do indeed seem to be growing more frequent, however. The maximum mean charts are dominated by recent heatwaves, and even the not-so-recent years that make an appearance are separated by decades in most cases.

    1. There are no lines in the meteorological sands. Arbitrary thresholds....are just that...arbitrary without meaning or importance. The question is not the frequency above some threshold but what percentage of a heatwave is natural and what percent is man-caused.

  12. I have a question about terminology. At Sea-Tac Airport station, concrete and urbanization are likely increasing the recorded temps in summer. Likewise, urbanization throughout Seattle and PNW region are having an effect locally on summer temps. I am confused whether the man-made urbanization heat effect is considered another facet of modern climate change. Urban heat effect is an important issue for the millions experiencing its effects on top of greenhouse warming. And running our air conditioners contributes to urban heating!


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

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