February 03, 2024

The Real Story of the 2021 Northwest Heatwave

 In addition to my public outreach activities, such as this blog, I have a "day job" as a professor of atmospheric sciences, where I not only teach a collection of classes but do research and publish.

During the past few years, I have been actively studying the 2021 Northwest heatwave with my students and staff (and outside researchers), and have published two papers in the peer-reviewed, refereed literature on this event (see below, including links to read them yourselves).

The conclusions of this peer-reviewed research are somewhat different from some of the extreme claims by some in the media and by some activist groups, and I want to show you some of the evidence.

The bottom line:  natural variability was the key cause of the heat wave, with human-caused global warming playing only a small role.

Let me show you some of the most interesting graphics from the paper.

One strong finding was that the June 2023 extreme temperatures were unique and crazy extreme.   The heat was NOT the result of slow planetary warming that one would expect from increasing CO2 for human emissions.

The 2024 paper demonstrates this fact in several ways.   Below is a plot of the annual warmest surface temperatures for the entire record at Quillayute, on the Washington Coast, and Lytton, BC, where Canada's all-time high temperature was observed.

Note that high temperatures at both stations were absolutely unique:  not only the highest in record but radically higher than previous high temperatures.  I have also placed a trend line on there as well.  

There is a small upward trend on average (about 1°C over the past 40 years)...and that could be due to human forcing.    But this trend hardly explains the huge warm anomaly in 2021.

Another way to see how anomalous June 2021 was is to plot the frequency of annual high temperatures over the entire record (see below for Portland, Quillayute, and Lytton, BC).   The red arrow shows the value in 2021.

Amazing.  The 2021 high temperatures are from a different world.  Nothing is even close.

In my paper, we tried a different, independent approach to determine whether that 2021 heatwave was unprecedented.  My co-author, John Christy of the University of Alabama, a world-leader in the use of satellite data to measure earth's temperature,  looked at lower-atmosphere temperatures from 1979 to today, all based on sophisticated satellite measurements.  

The map below shows the difference from normal of the temperatures for a five-day period encompassing the 2021 heatwave.  

Wow.  A HUGE warm temperature anomaly over our region. Nothing like it anywhere on the planet.

Then he found something startling.  This extreme warmth over our region was the warmest s anomaly during mid-summer anywhere in the northern hemisphere for the ENTIRE observational record.

The 2024 paper also determined the ingredients that produced the insane 2021 heatwave.

We showed there there was the strongest ridge of high pressure aloft of the entire meteorological record (see figure below at 500 hPa...about 18,000 ft) over our region.  We also note that there is no scientific evidence that global warming produces stronger ridges ( I have published papers on this issue)

A recently published paper (see below) traces this extreme high pressure to a long train of atmospheric waves originating in the western Pacific.  These waves were initiated by a very unusual configuration of sea surface temperature and moisture.

At low levels, the sinking and warming of the ridge aloft were enhanced by offshore-directed easterly flow forced by an approaching low-level trough (see below for around 5000 ft, 850 hPa).

In short,  a lot of random pieces came together to create an unprecedented heat wave, with background global warming only contributing about 10% of the extreme heat.

What about dry conditions before the heatwave?  Did dry soils play a major role?  

We completed a paper exploring this issue (Conrick and Mass 2023).    We ran two simulations of the event:  one with actual (dry) soil moisture values and the other with normal/moister (or climatological values).    The result is shown below.  The modestly drier than normal conditions caused about 1.5C warming in eastern Washington and very little in western Washington.   

In short, previous dry conditions had only a small impact.

Finally, our paper described the near-perfect forecasts of this record event, even 3-5 days before.   Such forecast skill can greatly reduce the impacts and death tolls from extreme heatwaves.

Forecast temperature at Seattle from the NOAA GFS ensemble model, the red dot 
shows the observed highs.

In summary, the June 2021 heatwave was one of the most extreme such events in the observational record.   Natural processes dominated, with multiple factors coming together in an optimal way.

Not unlike thrown dice and all coming up sixes.  Global warming contributed very modestly to this event:  perhaps 10% of the warmth could be attributed to human greenhouse gas emissions.  Thus, much of the claims in the media (e.g., Seattle Times, Washington Post), by local politicians, and several advocacy groups of global warming dominance were scientifically inaccurate.


  1. It's a useful reality check to note that this one heat wave, in this one corner of the world, could have happened without global warming. But if you find yourself having to make this argument month after month, year after year, then doesn't that point to a larger pattern? This is exactly what global warming was projected to look like. Worldwide, the ten warmest years on record have all occurred in the past decade.

    1. How many decades old is the earth? How many of those decades were similar to the last decade? You know the answer without even having been there: millions of them!

    2. Jerry, I think that's been Cliff's blog mantra. He continually, time and again, has to talk about the larger pattern of media and politicians over exaggerated response to single events. Plus, without anthropological climate change, justwith natural variability if the earth was warming even by a 10th of degree each year above "normal" it would be the HOTTEST ever on record. Which "hottest" is a catch word to scare you. The earth warmed a little. Yes man made climate change added, however the fear mongering has to stop. Time change via natural and man made. We can solve the man made part without bankrupting and going back to stone ages if we allow science and the will of us to conserve and help follow natural course.

    3. I agree that individual events can't be attributed to climate change, and the media certainly blows these events up beyond what they merit. There is a lot of fearmongering from both sides of the argument, and I agree it needs to stop. But I think his question is very valid. For people who paid attention to "global warming" science long before it became a culture war conflict, we did expect it to look like an increase in extreme weather events. It FEELS like there is a constant flow of extreme events that require the argument that the latest one wasn't caused by climate change. It FEELS like we're seeing an increase in these events, as was predicted. I acknowledge that this feeling may just be manufactured by media sensationalizing everything. But I'd love to see a critical analysis that supports or refutes this in a scientific manner.

    4. Booger.. Science is not based on how things Feels. It is based on hard numbers and real data. Human "feeling" is notable inaccurate and subjective...The critical analysis with real data is in my papers....please take a look at them..cliff

    5. Booger - Looking globally, beyond the 2021 heat wave in the Pacific Northwest, there is substantial evidence that climate change is making extreme heat events hotter, longer, and more common. So your feeling is not off-base.

  2. Cliff,

    I appreciate your typical consistent approach to look through the data to identify trends and only after analysis make declarative statements on what has happened and why. I truly respect the effort that you put in to make sure these stories are communicated to the public with scientific accuracy.

    There have been and there will be no events that can be labeled as solely caused by global warming, which is the nuance that the Times, WaPo, politicians and advocacy groups miss in their stories and statements. I understand how that could be difficult for you because to be accurate the global warming aspect should be either a footnote for the heat, wind, and flood stories or there should be a general piece that occasionally runs saying Seattle will continue to see an increasing number of 95°+ days, slightly more rain, less lowland snow and an eventual decrease in mountain snow.

    Let's also face it, even with the heat wave being 10% human caused those extra ~4 degrees (10% of +36° from average) probably impacted more plants and animals than we would expect. Probably time for them to start adapting because that 10% isn't going away.

    The problem with global warming will be the upcoming decades of lost glaciers, decreased snowpack, melting polar ice caps, rising sea levels, shifting weather patterns and the costly adaptation that humans will have to enact.

    The reason I am posting is because I interpret your "well actually" posts as damaging the movement against global warming just as much as the media, politicians and advocacy groups do when they say "this is global warming!". The posts come across as implying that global warming isn't a problem because it is just a few percent.

    I would love if you followed up some of your "well actually" posts with one that highlighted an area that will be significantly impacted by climate change in the near term (I know they are out there) or even add a short note on your Climate Change view and link to your post and podcast from 9/4/23.

    I do want to thank you for being a prominent voice on the weather in Seattle because I appreciate there being discussion and analysis about our neck of the woods.

  3. It seems to me there's another way of coming to the same conclusion, though no doubt less rigorously and far more provisionally. The record highs for six states occurred in the first 23 years of the 20th Century, and in another six states for the first 23 years of the 21st Century. It therefore would seem to cherry-pick to posit the records in two of those states -- Oregon and Washington -- as being caused by AGW. It is mildly surprising that more states haven't set records the past 23 years, given the undeniable fact of warming.

    1. Why dwell on states at all? The 50 United States combined are less than 2% of the earth's surface. Worldwide, we know that the past ten years have been the warmest on record. That's what's melting polar ice, raising the sea level, driving climate refugee movements, etc. Not what's happening in Western Washington.

      Western Washington has had its share of problems from global warming (do you remember choking on wildfire smoke every single summer when we were growing up)? But the biggest problems are the global ones.

    2. Jerry..... the effects of global warming are similarly elsewhere: heatwaves are dominated by natural variability, but with a small (2F) boost from global warming. Wildfire smoke is predominantly NOT a global warming effect, but poor forest management, invasive grasses, human ignition, and the like.

    3. Cliff, I understand that poor forest management has played a huge role in the USA's big fire seasons. But wildfires are increasing worldwide, not just in the USA but also in Canada, Europe, Russia, Australia, and elsewhere. In most of these places, forests were never "managed" the way we think of it here. Due to climate change, fire seasons begin earlier, end later, and see hotter weather. During Canada's record wildfire season last summer, they also experienced >100F temperatures above the Arctic Circle for the first time ever. It would be a stretch to believe that record heat doesn't play a significant role.

    4. Jerry, It is NOT TRUE that fires are increasing worldwide. https://earthobservatory.nasa.gov/images/90493/researchers-detect-a-global-drop-in-fires

    5. The above referenced article by Cliff states that fires have decreased world-wide due to significantly less man-caused burning in the tropical grass and savannah areas but that fire has increased in most of the forest areas of the world outside of the tropics in recent years.

    6. Correct, wxman, thank you for pointing that out. Also, the article only covers the period from 1998 to 2015. It excludes most of the past decade which was the hottest on record.

  4. Thanks Cliff.
    I do enjoy knowing about these episodic weather events.
    I think my high temp was 116°F. This winter I had -17°F.
    I am not much concerned with the high temperatures. The
    coldest ever for me was -33, many years ago in Iowa. When
    approaching about 15 (above 0), worry sets in.
    I realize there has been warming since about 1850, but not
    much and not climate changing. Where I live, any change
    noticeable can be linked to people and cattle.
    [I believe – no proof – there has been a large increase in the
    number of Washington hawthorn trees. The search of images with this phrase will show why: 'washington hawthorn tree thorns size' ]

    1. John, I'm in the process of doing a rough analysis of how much wind & solar power plus battery storage must be added to the Northwest power grid to fully replace the four hydroelectric dams on the lower Snake River in Washington state in a way which supplies 3000 Mw of 90% nameplate generation capacity 24/7/365 to the region, doing so for the next seventy-five years.

      The BPA's web site provides an excellent body of power generation data which can be used to follow the patterns of intermittent wind and solar generation as these patterns occur inside the region throughout the year.

      Just a quick visual look at the pattern of intermittency for the year 2023 indicates that supplying that 90% capacity factor 24/7/365 would require a wind & solar overbuild of between three and six times the current W+S nameplate capacity of 2833 Mw, plus installing the backup battery storage needed to fully buffer the intermittency.

      As compared with the four lower Snake River dams -- which have a service life of several hundred years with proper maintenance -- current wind, solar, and battery technologies have a service life of twenty-five years at best. Which means that over the next seventy-five years, the renewable systems installed at the outset would have to be fully replaced at least three times.

  5. This comment is not met to counter Cliff's assertion that climate change had only a small affect on the record high temperatures of June, 2021 but just to illustrate the recent warming at one station. The temperature record from the Tree Fruit Research Center in Wenatchee was summarized for the period 1935 to 1993 and listed the highest and lowest temperature recorded for each of the 366 (leap year) days during that 58 year period. Since 1993 this is what has happened during the summer months of June thru September (containing 122 days) from 1994 to 2023: Since 1993 the high temperature on 50 of the 122 days has been equaled or exceeded. On 10 days the record was equaled or exceeded more than once. Nearly half of these record highs have been set in the last 10 years. A record low temperature has been equaled or exceeded on only 15 out of the 122 days during this period.
    A record high for the month of June was set in 2021, the record high for the month of July was set 2022, the previous August record high was tied in 2023, and the previous September record high was tied in 2020. This only speaks of the high temperatures during the summer, but the overnight summertime low temperatures have probably shown an even more significant warming during this period.

  6. Well...speaking of weather over the last two weeks or so...right after a decent cold snap, we experienced several days of unusually warm weather temps,...considering we are in the dead of Winter!....60* seems way out of line, but right now, we are slowly cooling down to normal Feb temps....does this strange little time period mean anything? Is it a precursor of another very warm summer?

  7. It's pretty obviously silly to search for the fingerprints of global warming in relatively short-duration weather events. Climate change is manifested in longer term trends. Much more sensible to just look at monthly record high and low temperatures. For example, Quillayute has set 6/12 monthly record high temperatures during the past 10 years but not a single monthly record low temperature. While, at ~57 years, Quillayute doesn't have the longest period of record, it's nothing to sniff at considering the lack of urbanization. Similarly, BLI, with it's 75 year period of record and relatively rural setting, has set 7/12 monthly record high temperatures during the past 10 years but, again, not a single monthly record low. The "Clearbrook" NWS COOP weather station, located in rural north Whatcom County between the towns of Lynden and Sumas, has set 5/12 monthly record high temperatures in the past 10 years out of a whopping 121 year period of record and, predictably, has not set one monthly record low temperature during that time.

    One mistake that climate activists tend to make is to focus on heat, which, while telling, is rather less convincing compared to cold...or lack thereof. BLI and Clearbrook haven't set any monthly record low temperatures this century. Clearbrook is an especially valuable case as it's situated in an area that has experienced very little change to the physical environment due to human development during its long period of record. That period of record is of sufficient length to capture data from the decades featuring some relatively warm weather during the ~second quarter of the 20th century. What this shows us is that, in the extreme and on average, it doesn't get that much hotter nowadays than it did prior to WW2 but at the same time it simply doesn't get nearly as cold as it once did (monthly record low temperatures for Clearbrook are all below freezing). While the warming that has occurred during the instrumental climate record is certainly to some extent the result of greater heat extremes, it's really caused in large part by the lack of extreme cold.

  8. Near Kettle Falls, WA in the afternoon of 29 June 2021, taking photos of an old mercury US Weather Bureau thermometer positioned in the shade and more than a foot from the corner of a small building, I recorded just over 48.1 degrees C, though it probably doesn't mean much to the record keepers.

  9. What I'd like to know is whether not climate change is making these anomalous events significantly more likely.

    1. Good question. There is not evidence that climate change is causing any kind of amplification of the atmospheric patterns that produce such extremes.

    2. Leaving aside causation, is there any evidence for a global trend of extreme weather events happening more frequently now than, say 100 years ago?

    3. It would be interesting to see the measurements in a decade by decade comparison. I love looking at extremes going back to the late 1800's. Just as a comparison. From the records I have browsed, I would guess the 1930's seemed to have an over representation of both highs and lows. I lived in Portland for over 30 years and have seen stories of the Willamette freezing over around 1920 +-. Comparing the trend by decades would be another snapshot.

    4. John, you might need to define "extreme weather events" - the response would probably be different for different kinds. e.g. high temperature records vs cold temperature records.

    5. In your referenced paper discussing the June 2021 heat wave you show that the extreme heat of that period had only a minor influence from global warming, and you also state that there are studies that show that the frequency of weather patterns conducive to extreme warm periods has not been increasing in recent years. The record from the Tree Fruit Research Center in Wenatchee seems instead to show an increasing frequency of summertime record breaking daily maximum temperatures, which must be due to an increasing number of weather patterns conducive to hot weather.
      In the NOAA Technical Memorandum NWS WR-224, Climate of Wenatchee, Washington, the record maximum and minimum temperature for each day of the year for the period 1935 to 1993 is listed. Since 1993, the record maximum temperature for the summertime days June through September (122 days total) has been equaled or exceeded on 70 occasions, on several days more than once during this recent 30-year period. If you divide this 30-year period 1994 to 2023, into three 10-year periods, it is found that 10 summertime maximum temperature records were set from 1994 to 2003, 15 records were set from 2004 to 2013 and 45 records were set from 2014 to 2023. In other words an increasing frequency of record breaking daily maximum temperatures in the recent 30 year period. On the other hand, only 17 minimum temperature records have been made during this same period with 11 of them between 1994 and 2003, 6 between 2004 and 2013 and none in the last 10 years of the record, a decreasing number of minimum temperature records during the past 30 years. One would expect as the period of record of a station gets longer, the frequency that daily records were broken would decrease, and this does not seem to be happening lately with the Wenatchee summertime maximum temperatures. I will also state that June 2021 saw the highest June maximum temperature of record, July 2022 the highest July max of record, while August, 2023 and September 2020 tied the previous record highs for those months.

  10. Keep fighting the good fight, Cliff. As my favorite Nobel - prize winning physicist, Richard Feynman said - "It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong." He also said that the "science is never settled." Something that far too many scientists never practice these days.

  11. Hi Cliff, The Jet Stream has almost made a complete circle around several times recently. Has it ever, and do you have visuals? A rare event or no? J.E.

  12. Our current Mt snow pack has declined recently, should we be concerned?.

  13. Thanks for the great work on the heatwave of 2021, Cliff. The graphs are really interesting and certainly convince me that it was a one-off event, and not a trend. It was 111°F in Olympia on June 29. I will never forget that day, as I needed emergency surgery (not heat related).

    I appreciate your public outreach here in this blog, and I have learned a lot about Pacific NW meteorology over the years.


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

How unusual is it for aircraft to exceed the speed of sound?

Announcement:  Doing an online zoom session with Patreon supporters at 10 AM tomorrow (Saturday) ______________________________________ The ...