July 31, 2021

Monsoon Moisture Reaches Washington and Oregon

When one thinks of the Northwest during the summer, the word monsoon is not the first thing that comes to mind.  But during the last day, Southwest Monsoon moisture has spread from the southwest U.S. into our region.

It has already rained at Sea-Tac Airport and clouds have spread over western Washington.  

The water vapor channel of the NOAA GOES satellite clearly shows the plume of moisture heading directly over the Northwest.

And the visible satellite image this morning highlights the associated clouds over western Washington and northern Oregon

The radar image around 7AM indicates some showers moving up into our region.   My newspaper was a bit soggy this morning.

This moist air will be around this weekend and the air is unstable enough for more showers this afternoon, particularly over and east of the Cascade crest.

Over the 24h ending 5 AM Sunday, most of the showers will be over eastern Oregon (see 24-h accumulated rainfall below.  And some of these showers will be associated with thunderstorms. A few light showers could hit western Washington.

But Sunday is a different story, with northeast Washington and southern BC getting more moisture. Serious rain over some of the wildfires near Winthrop.

After today, there will be four warm days before the bottom drops out of our weather.   

Below are the forecasts for the NOAA GFS ensemble system showing the temperature at Seattle.  In an ensemble system, the model is run many times, each slightly differently.  

Today gets to around 80F, and the next four days are slightly warmer.  But then comes Thursday and BOOM...a big decline (the black line shows the average of the many forecasts).   There is some uncertainty in the forecasts, with a range of solutions, but most are MUCH cooler.   Same for Friday and Saturday.

Rain?  Chance of some very light errant showers over the weekend, but the rain chance really revs up on Thursday onwards, with many of the simulations showing rain.

As I noted early, this is quite a vigorous cool/wet event for midsummer, but I suspect it will be welcomed by many.  And it will reduce the wildfire threat for a while, assuming we don't get many lightning starts.


  1. Thank you for calling this in your previous post: my friends thought I was nutty when I said rain was coming.

  2. What will the winds be like, especially in and around the San Juan Islands?

  3. Where is that predicted "early normal" storm that was going to hit us from the West?,,this Monsoon stuff was going to only affect Eastern Washington, but I guess it has wandered a ittle bit off-course.

    1. It’s still coming as predicted for Thurs thru Sat.

  4. I have read your book Cliff, but I am at a loss to apply what I learned to this "monsoon." Next week, I will be hiking the Olympic Coast, from North to South. Will the Olympic Mountains mitigate rainfall on the coast given that this system is coming from the SE, rather than the SW. That is, does the "rain shadow" work in reverse?

    Need help here, Cliff. Do I pack the rain gear, or just plan on drying out after a few days.

    Sincerely...Singapore Math

    1. Always pack rain gear on the coast. If you get wet and it gets windy you will be glad you did.

    2. The Thurs-Sat storm isn't coming from the SE. It's likely to be closer to our typical Fall/Winter fronts. Those usually clobber the coast and the west and W and SW sides of the Olympics the hardest. Although there was a hint in the Forecaster Discussion this morning that the heaviest rain might enter eastern WA was the NW.

      Lots of confusion here since we are moving from the somewhat unusual SW Monsoon of this weekend to a Gulf of Alaska autumn-style low in less than a week. They are very different beasts, and this autumn-style low is very unusual in being so early. Assuming it develops as predicted. I presume being early means it might behave a bit differently than usual.

      Regardless, preparing for rain on the coast is always the smart move.

  5. Thanks Dr. Mass for providing your expertise and your blog! Many of us are keyed in to the current fire situation in north central washington. Any chance you could educate us around the weather issues related to wildland fire and how a large fire generates weather. I'm particularly interested in what it takes for pyrocumulus to form, how the convective activity is similar as well as different from frontal related thunder and lightning. Your comments on what we might expect in the Methow valley in the next ten days would also be greatly appreciated. Thanks

  6. So was the haze today all monsoon moisture, or is that tracking wildfire smoke with it? Seemed sort of high altitude for smoke initially, but it was more obviously so up near Mt. Rainier

    1. The WA smoke blog is a great resource for this kind of information (although Cliff also just posted an update confirming high level smoke over western WA.


  7. Looks like high-level smoke today. The filtered sun is reminiscent of past fire seasons. Has some of the smoke from Okanogan County made its way west of the mountains?

  8. Here on Sinclair Inset we had a little smoke this morning at sea-level. I would say we would not have been able to make out windows across the bay, but could looking over to Manette.


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