August 09, 2021

Major Heat Wave Coming This Week

A major heat event will occur over the region this week.

But let me make it clear at the outset, it won't be nearly as warm or lengthy as June's event.  Seattle and northwest Washington will have a far easier time.

The Willamette Valley will fry and some places in eastern Washington will get near 110F.

The Set-Up

So what is going on?  Another major upper-level ridge will develop over the eastern Pacific (see 500 hPa, 18,000 ft map below for 11 AM Wednesday).  But the amplitude will be weaker than in June and in less than an ideal position.

At low levels, this upper-level feature will be associated with high pressure over the Northwest interior, producing easterly (from the east) flow that will warm as it sinks over the Cascade western slopes.   

As the lowlands warm, pressure will fall west of the Cascades producing the familiar thermal trough of low pressure, pushing up from southern Oregon.  The map for 5 PM Thursday shows this feature, with solid lines indicated sea level pressure, wind barbs indicating winds, and shading showing low-level temperatures.

This situation represents a more typical once-a-year heatwave for our region.  We also have another advantage over June:  the sun is considerably weaker and nights are longer.  Furthermore, this will be a relatively rapid warm-up/cool-down event west of the Cascade crest.

The Forecast

Let me show you the latest forecasts.  Today and tomorrow will be well-nigh perfect around the region, with moderate temperatures and little smoke.  Wednesday will start the warming, but it won't be bad.  

Here are the temperatures at 5 PM from the UW ultra-high resolution model.  Over 100F in the lower Columbia Valley and upper 90s in the Willamette Valley.  Puget Sound and Northwest WA will be comfortable

Thursday will be the first unpleasantly warm day west of the Cascade crest.

The temperature at 5 PM will be torrid around Portland and in the lower Columbia Valley, with highs getting to 104F+.   Mid-90s around the Sound, but higher away from water.  Also warm in the Fraser Valley.

The latest forecasts suggest that Friday will be cooler in the west as marine air starts to move in, but there is still some uncertainty with that.   Eastern Washington will stay warm for another day (see the forecast for 5 PM Friday)

The good news is that the latest model runs suggest massive cooling from Sunday onward (see Seattle temperature forecast below from the NOAA GEFS ensemble system).

With the weakening sun and the cooler temperatures expected next week, we will be running out of time for another major heatwave.


With reasonable temperatures the next few days, cool your house or apartment at night with window fans (still about $20 dollars!).   Block the sun from entering through windows.  Make lots of ice.   And remember, for many in western WA the water (e.g., Puget Sound) is cool and so is the air downstream.


  1. Thanks Cliff. I think many of us are still a little traumatized from the last heatwave so even the idea of temps over 90 is a little harrowing. I ended up covering cardboard with foil and putting that in my windows. I saved it all and will do it again. Will we have any breezes this time around?

  2. i sure appreciate your thorough and accurate explanations of our weather. many blessings!

  3. It is August Cliff...How about that first La Nina report...I'm seeing lots of moisture coming our way starting fall 2021

  4. You mention "little smoke" on Wednesday, but does it appear that the smoke will hold off throughout the extended period?

  5. Cliff, Do you think we could have another hot smoky September like last year?, I fear this might not be our last heat event of the year.

    1. That’s what I want to know too. The way I see it, we didn’t really have our first heat spell until mid-late July, so the hot/Smokey September was like a “siummer tax” if you will.

      Experience living here has taught me that the sooner it’s hot, the sooner it lets up in the heat.

  6. I love your blog. I have enough vision to get something out of all your charts especially since your text tends to include a decent summary of what the pictures, especially the maps indicate. But when I read your blog with my screen reader the images just read with a long series of numbers. it's not annoying enough to deter me from visiting your blog, but could you consider doing a little more with alt text to both illuminate the content and give the screen reader soemthing to read with each graphic besides long messes of numbers/

  7. In general, the W. Washington area experiences a small handful of 90+ degree days most summers...Like around 3-5 days a year...and we are, right now, in our statistically hottest time period. And I certainly can remember a few summers here, where the temperature never made it to this upcoming event, like Cliff suggests, is a "normal" weather event...but "normal" ion this area can be fickle...I remember going to see Jimi Hendrix play in the Seattle Center Coloseum, on 23 May 1969---the temperature that day hit 90!...and that record held for many years.

  8. City of Newport, Oregon, on the coast, has implemented water use restrictions. This is due to low water in the Siletz River due to drought conditions.

  9. (Now, Wednesday morning): Prof ...your forecast seems to be unfolding just as predicted. So far this week, here near Mt Baker, we've had chilly nights and very typical daytime highs (24 hr morning reports - 8/9 L 56.3 H 64.4, 8/10 L 55.2 H 72.7, 8/11 L 56.5 H 82.0). So I do expect a much warmer 'today' and a scorching 'tomorrow' (Thursday). Kept the windows open for a chilled-down house and outbuilding. Here we go! Actually seems like a very normal summer.

    Thanks for the brilliant forecasts and posts. The UW radar and satellite pages are wonderful too.

  10. With the smoke and haze in the air do you expect lower temperatures than forecasted or have the models improved?


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