June 25, 2021

The Reason for the Extreme Warmth on Monday--And My Podcast on the Heat Wave is Out

 I think I understand why the temperatures in western Oregon and Washington will be so stunningly extreme on Sunday and Monday.

A unique combination of factors will come together to make the unthinkable possible.  Forget the "heat dome" explanations found in the Seattle Times and some media outlets, or those saying that the extreme heat can only be explained by global warming.

I will call the phenomenon a downslope heat surge on the western slopes of the Cascades.   

A relative of the extreme heat associated with Santa Ana winds in southern California, but with a twist.

An Unusual Collection of Ingredients

To get this amazing event, a series of ingredients had to occur at the same time and same place.  To put it another way, it is like throwing several dice and having all of them come up with sixes.


Ingredient One: An unusually strong area of high pressure aloft over our region (known as an upper-level ridge), associated with sinking air and unusually warm temperatures.  

At the surface, this feature is associated with high pressure to the east of the Cascade crest, which tends to produce weak offshore (easterly) flow.  Such easterly flow keeps the cooling influence of the Pacific Ocean away.

Why did we get this high amplitude ridge?   It is associated with a highly amplified wave pattern in the eastern Pacific, which may have been caused by a tropical system interacting with the jet stream (see below).  This is the result of natural variability ( I did a paper exploring this issue with climate models)


Upper level (500 hPa pressure, about 18,000 ft) for 5 AM this morning. Red indicates higher than normal pressures/heights, blue below normal.   Note the highly amplified wave pattern

Ingredient Two: An Approaching Trough of Low Pressure That Creates Strong Easterly/Downslope Flow over the Western Slopes of the Cascades

The key to this situation is that there will be high pressure inland and an approaching area of low pressure (called a trough) that will approach our coast.  Between these two features, a zone of very large pressure difference will be created, which will be associated with strong southeasterly flow.

To illustrate, there is the weather chart for around 5000 ft (850 hPa pressure) that shows the key features.  A narrow zone of strong southeasterly flow will be created that will descend the western slopes of the Cascades.


The air will start off warm, with origins from the desert southwest, but will warm further as it descends the Cascades into western Washington.  Why warm more?  Because the air will be compressed as it descends into western Washington.

The absolutely unbelievable temperatures at 5 PM Monday illustrate what will happen.

Surface temperatures will get above 112F over and near the western slopes of the Cascades.  Large portions of western Oregon and Washington away from the water will be above 104F.


All this is happening during a favorable time of the year (the sun is powerful and days are long).  

Check out my podcast for more of the story:

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48 comments:

  1. Cliff, will record lows be as common Sunday night as record highs will be Sun/Mon?

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  2. How do the factors influencing this event compare with the 2009 heatwave? The dramatic temperatures in that event seemed much more concentrated on the west side of the Cascade while this time the whole state will be affected.

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    1. Good question. I recall we had a huge ridge, but not a deep low off the coast to draw air from the south east. That’s my best guess. I hope Cliff will chime in here.

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  3. Cliff,
    One way I am staying cool is by reading your posts from the last couple of Januarys. Doing so got me to ask, what would be an extreme low temp in western Washington that you would consider equivalent to the extreme high predicted here on Sunday and Monday? Have we had an extreme low equal to what we are about to experience?
    Thanks,
    John

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    1. The all-time record high is in danger with this heat wave. The equivalent would be breaking the the all-time record low, which for Sea-Tac is 0F. So something just below 0F for consecutive nights, and under 10 for 5ish nights in a row would be somewhat equivalent.

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  4. It looks like a mirror image of the Pineapple Express, perhaps a Cactus Express?

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  5. Thanks Cliff,
    I love your expert explanations and grounded science!

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  6. Cliff,
    One way I am staying cool is by reading your posts from the last couple of Januarys. Doing so got me to ask, what would be an extreme low temp in western Washington that you would consider equivalent to the extreme high predicted here on Sunday and Monday? Have we had an extreme low equal to what we are about to experience?
    Thanks,
    John

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  7. Cliff,
    One way I am staying cool is by reading your posts from the last couple of Januarys. Doing so got me to ask, what would be an extreme low temp in western Washington that you would consider equivalent to the extreme high predicted here on Sunday and Monday? Have we had an extreme low equal to what we are about to experience?
    Thanks,
    John

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  8. My Windy app said there's going to be some breezy winds on monday. And the graphics you've posted suggest a large gradient between the high and low (low being off the coast). Should we expecting at some breezy conditions on monday? And if so, will it be warmer wind or 'cooling effect' wind?

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  9. I'm wondering if you had a way to describe the temperatures with regards to the standard deviation off of the "normal" temps for this time of year. Is it 2 or 2.5 standard deviations above the mean?

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  10. Cliff, I don't know if you have noticed, but we are currently experiencing an unusual humidity event today with NOAA obs sites showing dew points in the 60-70 degree range and sites in the Bellevue area consistently 65-70 degrees. Any explanation for this high humidity?

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    1. The ground still has the moisture from recent June precip. I think this was noted in one of Cliff’s recent blogs.

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    2. The heat index will be several degrees above the temperature, if the temperature is 110 it might feel more like 115 because of the humidity. Normally we get a dry heat temperatures this high and humidity at the same time is inconceivable.

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    3. There will be an amazing cool down at the end of the heat wave a strong sea breeze will come in the temperature might drop as much as 50 degrees overnight. The cool down might also break some records.

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  11. Funny how the act of assigning a name to things "heat dome", "critical race theory", "last mile problem".. can set things irretrievably off the rails. For instance, one should be very careful that one's detractors cannot easily grab hold of misinterpretations. "downslope heat surge" or DSHS canot possibly be misinterpreted [big wink] Thank you Prof! for informing us through this horrific heat wave ("'wave'? more of a tsunami, ain't it?")

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  12. Please talk about how and when we get out of this event, and when/if we might get another this season. Thank you.

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  13. Hi, just checked a bunch of NOAA obs sites in the region and dew points are at 60-70 degrees all over the place, any explanation for this high humidity?

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  14. So it's like a Chinook? But in summer.

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    1. A little bit like a Simoom wind. I'm praying that we don't have a fire start while the easterly heat surge is occurring. If we do, it'll likely spell real trouble.

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  15. There was a hot easterly wind blowing that night. It was one of those downslope heat surges on the western slopes of the Cascades that come down through the mountain passes and curl your hair and make your nerves jump and your skin itch. On nights like that every booze party ends in a fight. Meek little wives feel the edge of the carving knife and study their husbands' necks.

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  16. We are so lucky to have you in our area. Thanks so much for providing invaluable information. I always go to your explanations during unusual weather events. This is 10X better than TV news or the newspaper. Well done!

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  17. Thanks for your reasonable approach to the weather.....so "refreshing"....and we need it.

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  18. From a statistical point of view, you'd have to reject the null hypothesis that this kind of weather would occur without climate change at a fairly high level of confidence, based on historical temperature distributions.

    Does this mean that climate change is the only possible explanation? Strictly speaking, of course not. Does it, from a Bayesian point of view, mean that climate change is the most likely explanation? Of course.

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    1. Cliff would disagree: "There is no evidence that such a wave pattern is anything other than natural variability (I have done research on this issue and published in the peer-reviewed literature on this exact topic). So without global warming, a location that was 104F would have been 102F. Still a severe heat wave, just slightly less intense."

      Fundamentally, the cause of conditions leading to this event are "natural variability."

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    2. Embellishing your sentence with words like "Bayesian" in this case doesn't do enough to make a strong statement.

      But fine, I can use that word too. From a Bayesian point of view, shouldn't natural causes be the most likely explanation of climate change, given that climate has always changed?

      Also, climate change is not disputed. Forests used to grow where there are now deserts, etc, etc.

      In 1928, a heat wave in Washington State set the current record high of 118F. Climate change caused that also. But natural climate change, or man-made climate change? In 1928?


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    3. Keep in mind that when it comes to weather, the statistical climatological mean is not usually what you actually get. Just look at a time series of daily high temperatures versus "normal" high temperatures. Given this, should you really expect climate change to be realized as the same delta T everywhere, at all times? The difference between the last ice age and today was 6C. Obviously, it was an entirely different planet--it wasn't just that the same weather fluctuations we have today occurred just with 6C lower temperatures!

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  19. A week from Sunday is the 4th of July. Fuels will be dried out from this heat followed by a week of dry hot weather. Yet over here the Suquamish are doing a rip roaring business in fireworks. This is a recipe for disaster.

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  20. Thank you Cliff. My 1975 degree in Meteorology from Utah paid off as everything you said makes perfect sense to me!

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  21. I wonder when rain will come back.... (feels like 'never' now)

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  22. I noticed that you have carefully avoided attribution to global warming, and I get it. Weather and climate are VERY different beasts. However, with the "extreme event" label on this weather pattern, you know it will be counted as such. However, despite disambiguation, it's hard not to think -- here we are. Another extreme event. What will it take before people understand what's happening?

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    1. To be fair, Cliff did address this in his last post. He said that global warming is almost certainly contributing to this event but stopped short of saying it caused it. That more or less jives with what other climate scientists have said; that global warming is making extreme events more likely or more severe. His comments fulfill the later part of that sentence.

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    2. He didn't avoid it. He researched this exact topic and published this conclusion in a peer-reviewed journal: "There is no evidence that such a wave pattern is anything other than natural variability."

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    3. I came here to make a similar comment. I noticed Cliff said that this is not related to climate change... That a lot of this can be explained with normal variations. But the root cause, tropical system interacting with the jet stream in the Pacific, I wonder is that a normal variation or a result of climate change?

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    5. What Cliff didn’t explore in his analysis is the wavy jet stream. Just as extreme waviness brought arctic air to Texas last winter, a similar waviness is bringing Texas air to the PNW today. To see the jet stream go to earth.nullschool.net, hit the “earth” menu and then select Height at 250 hPa. Why is the jet stream so wavy? The jet stream is powered by the Arctic-equatorial heat differential. With climate change, the Arctic has warmed 10X faster than the equator, lowering the differential and making the jet stream weaker. Sorry but this has the fingerprints of climate change all over it. Better get used to a world of record breaking weather extremes.

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    6. JB...that is not true. I have written several papers published in the peer-reviewed literature showing that there is no evidence of global warming increasing waveiness around here. Furthermore, that theory has been thoroughly debunked in the scientific literature.

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  23. I am now hoping for a low weather front to move inshore by wed or thursday, bringing with it some nasty-wet weather...bumping in to our seared state just might set a storm like this off...well, I can dream at least!

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  24. Thanks for you analysis. I feel that I learn something new every time I read your blog.

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  25. Hi Cliff
    What I find amazing is, were this January, we might be talking about an extraordinary amount of snow, if I have understood your analysis of the ingredients needed for that to occur. High pressure in the interior, low pressure approaching the coast, except that this would induce a Fraser Valley outflow event.

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  26. This heat is un-believable. I grew up in California where heat like this was common but we could retreat into an air conditioned house. I'm afraid for all the people who don't have AC, healthy people will be miserable but ok, however this could be alittle dangerous for someone with health issues. I'm also afraid that the heat+southeast winds will create a firestorm. I'm hopeful that everything is still green enough to avoid a major fire outbreak will see. My sister is visiting for the 4th of July and I was hoping the air would be clean enough for good hiking views and I could show her a real driveway fireworks show. She lives in California where fireworks are banned so this would be a real treat. I would light the fireworks in the middle of the street and have garden hose and a few buckets ready not taking any chances when things are so dry.

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  27. From 95F just 2 days ago, the Monday forecast went to almost 115F yesterday. When was the last time a forecast changed by 20 degrees in just several days in June?! Or for that matter, any time in the year? My guess is never!

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  28. Russia is also in the grips of an unprecedented record-breaking heat wave. Is this related to the same effects of the jet stream waves being jostled by that pacific tropical storm, like a downstream effect?

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  29. It's getting bad, I tell ya. Saw a dog chasing a cat and they were both walking.

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  30. I find it interesting that the setup for this heat wave, namely high pressure over the interior BC desert blowing down to low pressure to our southwest, is similar to our classic winter snow setup. Of course there, the low is not a thermal trough but instead a juicy low full of oceanic moisture, and the Fraser outflow is so cold that adiabatic heating is overwhelmed, but the geographic comparison seems pretty close to me.

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  31. Cliff, is it possible this weather phenomenon is driven by a weakening of the Hadley cells and Ferrell cells and the Polar cell? If so, can this reasonably be expected to recur?

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  32. Cliff, is it possible this weather phenomenon is driven by a weakening of the Hadley, Ferrell and Polar cells? Is it possible this could recur?

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  33. If the Cascade downslope is such a big factor, why are we seeing such high temps in Pendleton, on the other side? Downslope from the Blue Mountains?

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