On July 9th, the temperature surged to 98F at Seattle-Tacoma Airport. This was a record for the day and 22F above normal.
But that is not what surprised me.....SeaTac had gotten that warm before.
It is was HOW it got that warm.
I have completed a lot of research on Northwest heatwaves and published roughly ten articles on this subject in the peer-reviewed literature. Historically, to get that warm in Puget Sound you needed easterly (from the east) winds over the Cascades, winds that descended the western slopes of the Cascades. (see schematic).
Easterly winds are good for Puget Sound lowland warmth in many ways. First, they remove the influence of the cool Pacific Ocean, which is only around 50F. Second, the air comes from the generally warmer interior of the continent. And finally, air sinking down the western sides of the Cascades warms by compression.
Strangely, the situation on July 9th did not fit the traditional heatwave situation.
Why?
Because there was little evidence of easterly flow. Seattle Tacoma Airport got very warm without the powerful assistance of atmospheric compression.
I was not a little bit shocked by this realization. Could I prove this was more than a fluke?
So I decided to do a little research...which I am going to write up this year. I found all the times when the temperatures at SeaTac reached 95-99F. (Remember that the highest temperature on record there was 108F in 2021).
Then I examined the regional weather patterns associated with the first five events at these temperatures and the last five events achieving the same (again 95-99). Here are the two collections of dates.
The first group occurred before global warming was significant.
The second group after human impacts would have been expected.
For each of these collections of dates, I found the average sea-level pressure patterns and 5000 ft winds (actually 850 hPa pressure winds).
The results were stunning.
Let me show you the upper level (again around 5000 ft winds) before and after global warming for such events. For the earlier events (below global warming, GW), major heatwaves required easterly winds (look at the little arrows). The shading gives the wind speed. After GW, the warming assist from strong easterly winds was not necessary.
Why not necessary? The atmosphere starts out warmer for a start, with summer temperatures now 2-3F warmer than 50 years ago. And there are other potential factors associated with global warming as well, such as drier soils and alternations of weather patterns. Furthermore, SeaTac Airport temperatures have undoubtedly been warmed by several degrees due to the construction of the third runway, airport development, and increased urbanization in the area.
Then I asked another question: What about the really extreme heatwaves, like the 2021 event where SeaTac got to 108F or the 2009 event where 103F was reached?
Do they require easterly flow?
I checked.....and the answer is yes. To get uber heat, an assist from downslope compressional warming is required.
Anyway, this is a fascinating topic: how are the weather patterns required for extreme heat changing? One I plan to examine in great detail.
Interesting as always Cliff. I wonder if similar less east flow for warm weather would be seen at other western Washington stations. I have to admit I don't know where the Sea-Tac weather station is located at the airport. Could an increase in pavement around the station make the difference?
ReplyDeleteThe FFA claims it is here: 47.445, -122.314
DeleteAbout 100 feet from the center of a runway.
You should consider the change in cooling degree days that's occurred in the populated areas of Western Washington since local weather records began.
ReplyDeleteAt KBLI, 6 of the top 10 years for total annual cooling degree days have occurred since 2000, while only 2 of the bottom 10 years for total annual cooling degree days have occurred since 2000. At KOLM, 7 of the top 10 have occurred since 2000, while only 1 of the bottom 10 have. These are great examples of locations where urbanization has had little effect so that variable does not confound. You'll need some elite statistics to make comparable the records of other locations where the encroachment of development is likely to have had a more substantive effect but I suspect that a similar pattern would be revealed.
Summers here are hotter now, drier too; and noticeably so (with all the attendant meteobiophysiological phenomena) - we see it in the affects on our health, physical comfort, our utility bills and, in general, our quality of life.
I'm curious about any differences in humidity without the assist from easterly winds. Seems like it gets warm and also is more humid without the easterly winds. Humid at least for around here.
ReplyDeleteIt gets quite muggy during hot weather when there is no outflow - particularly north of Everett where there is no low pass over the North Cascades through which easterly winds can be channeled. Many locations in western Skagit and Whatcom Counties had dew points in the 70s today. Dew point temperatures in the 70s used to be unheard of even in the more humid areas of the I-5 corridor but they have now become a nearly annual occurrence and August is the peak month for such uncomfortable conditions.
DeleteThis is really interesting. Thanks for the clear explanation of your research on this. The nuanced pattern with super heatwaves still needed an assist with easterly winds is a neat finding.
ReplyDeleteI'm also very curious to see how this comment section plays out
We have been measuring a system for 80 years that has been moving and changing for hundreds of millions of years. Is 80 years of measuring long enough to draw conclusions?
ReplyDeleteYes, I think an 80-year period is adequate, because the exponential rise in atmospheric CO2 started about 300 years ago (start of the Industrial Revolution), and has really accelerated for the 50 years, overwhelming any "natural cycles".
Deletehttps://climate.nasa.gov/vital-signs/carbon-dioxide/?intent=121#:~:text=Since%20the%20onset%20of%20industrial,ice%20age%2020%2C000%20years%20ago.
Was there a similar rise 356,487 years ago? What? There may have been one but no one knows for sure? What many people don't know is just how little they know.
DeleteLook at the link I shared. They do know for sure - there was indeed a rise about 350Kya. But not nearly as great as the one we're experiencing now. As I said previously, the current increase is overwhelming the "natural cycles".
DeleteWe are definitely starting the day from a higher temperature base in recent summer days in the Wenatchee area. The average summertime (June thru September) minimum temperature at the Tree Fruit Research Center in Wenatchee for the past five summers is now almost 6 degrees warmer than the long term average for this station. There has been a steady rise in the average summer minimum temperatures since about 1980.
ReplyDeleteThe discovery of getting very warm days without an east wind in the Seattle area also may have an affect of the frequency of wildfires on the west side. As you have pointed out, very large fires on the west side require east winds, and the frequency of strong east winds may be decreasing due to climate change, but due to the increasing frequency of warm days without east winds, this may lead to an increase in smaller fires over there, since there is a pretty good correlation between warm temperatures and fire starts.
You need to also look at how those temperatures are recorded and when they converted from mercury thermometer to electronic. The electronic thermometers record instantaneous temperature with no inertia smoothing like the mercury thermometers. Also look at exactly where they are installed. Are they influenced by jet engine exhaust or a gust from a hot source? How has the concrete/asphalt/rock installation changed over the years? How has the urban heat island effect changed over the years?
ReplyDeletePre GW heat waves usually included a refreshing north to northeast breeze, with heat waves ending after two or three days when the coastal surface low moved inland followed by a marine cloud layer with little or no afternoon burn off. Post GW the heat waves seem to be warmer and longer with little or no wind which makes the heat less tolerable. No more strong marine pushes to end the relentless >80F heat. Looking forward to hearing more from Cliff on what has changed.
ReplyDeleteI'm curious about the claim that there have been "several" degrees of warming at KSEA since the runway was built. July 9th hit 96f in Normandy Park, over grass with a sheltered fan aspirated thermometer. July 9th Seattle WFO was 97f and Boeing Field also 97f. The Normandy Park location is usually several degrees cooler than KSEA (much closer to the Puget Sound).
ReplyDeleteIs there any data to back this claim up about KSEA? Because if KBFI and the Seattle WFO are hitting 97 at KBFI then it doesn't take a 3rd runway to expect it would hit 98 at SeaTac it would seem.
Great write up otherwise.
This is solid evidence for this....you have to look at trends to see it clearly. SEA became increasingly warmer compared to other locations as development occurred there. For example, since the late 1940s, the max temps have warmed up twice as much at SEA and Olympia...cliff
DeleteHaving through the major heatwave of 2009 in Seattle, in a concrete slabbed mid century apartment with no AC, that was brutal, but thankfully, a corner unit who's living room had a north facing window that opened, and a slider to a balcony so there was air circulation. My bedroom faced north, living room north and west and I could then leave the slider open at night as I was on a higher floor and left my bedroom door cracked and the place did eventually cooled off after midnight as the concrete acted like a sponge and absorbed the heat of the sun, plus, I was on the top floor so the roof likely added to it all.
ReplyDeleteThen moved to Tacoma in 2016 to a little stick built house with spotty insulation, bedrooms up front having the most insulation, the back porch, come laundry with absolutely no insulation anywhere within it, the rest eh, at best moderately insulated. I do have 2 window AC units to help out when it gets unbearably hot.
I recall the unusually warm nights of 2019 where the blob in the Pacific kept us in the mid 60's all night long, but the daytime highs were barely warmer than our normal 77/78F so do wonder if the overnight lows are not as low as they normally are, thus leaving the daytime heating a little higher?
This morning, it's already 68F at 7:50AM, seems a bit on the warm side than normal, though I don't think our humidity is all that much different, but hard to know for certain without some digging. I was out earlier getting my recycling and garbage out before 7:30 as it's trash day and it was quite pleasant out, I was in short sleeves and it was not chilly in the least so likely didn't get below 60?
Most nights, I've noticed that we drop to the upper 50's much of the time and it seemed we would drop to the low 50's to at most the mid 50's in the past, so cool nights for sleeping, unless it's a heatwave, often I find that AC helps early on when one first goes to bed as my bedroom which faces south and is fairly well insulated can be on the warm side when I go to bed but will cool down as the night rolls on. This has meant difficulty falling asleep some nights.
Anyway, will be interesting to see what you find as you dig deeper into all this in the next year, assuming our country does not fall into chaos come Nov.