Thursday, July 2, 2020

A Dark Month for Seattle

There have a lot of complaints about our cloudy weather of the past month--and it has been a bit depressing.

Yes, western Washington typically "enjoys" a cloudy, June gloom this time of the year, as high pressure builds over the eastern Pacific, pushing cloudy marine air up to the Cascade crest.

June 29th

Has this year been particularly bad?  The essential answer: Yes, but not by a lot.

We can start with the total monthly radiation reaching the ground in Seattle at the WSU AgWeather site near the UW, which only goes back to 2012.  June 2020 was the darkest June since 2012--eight years.  June 2015, when high pressure dominated our region, was far brighter.  2012 was abysmal.


Looking at the daily solar radiation numbers in Seattle, one notes some variation in the sunshine reaching the surface, with some decent amounts in mid-month.  But the first half of June and the last week have been cloudy and dark.


Accompanying the enhanced clouds was more rain than normal, something evident at Sea-Tac Airport, where there was about an inch more than normal over the past 4 weeks (see below, cyan is normal, purple is what fell into the gauge there).
Why so cloudy and dark in June?  Blame enhanced high pressure over the eastern Pacific and lower pressure inland.  But this year, the pattern was supercharged.  Let me show you.

Here is the difference from normal of the heights (think pressure) at around 18,000 ft (500 hPa).  Heights were much higher than normal (yellow/orange colors) over the eastern Pacific, but lower than normal (blue colors) over Montana, Idaho and eastern WA/OR.    Such a pattern is associated with higher than normal sea level pressure over the Pacific, lower than normal pressure to the east of the Cascades, and an enhanced onshore pressure difference that pushes cloudy, marine air into western Washington.


The result is day after day we had satellite pictures that looked like this.


Sunny in much as eastern WA, but dark in the west.

Looking forward I have some good news and bad news.  The good news is that there should be more sun over the weekend as the above infernal pattern weakens.  Bad news, because the pattern is forecast to return next week, with lots of low clouds again invading the west (see forecast for 5 AM Wednesday below).





14 comments:

  1. I literally came to this blog today to understand why this is happening. So if this is the case, why not just forecast dark and gloomy instead of faking us out with sunny forecasts every day? This isn't directed at you, but it seems like a cruel trick from meteorolgists. #2020

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    1. I was wondering the same thing. I had outdoor plans today and knew once the forecast had dropped a few degrees from the 77 they had originally showed six days out that we'd have a mid-60s washout. You learn how to read the forecasts here and take weekend sun chances with a huge grain of salt.

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  2. What about wind? At least over here on the west side of the sound, it seems like we've had a lot more wind this spring and summer. Does the wind accompany the gloom naturally?

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    1. I was wondering this same thing. Could it be related to the supercharged pressure systems you mentioned?

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  3. Some models now are indicating that the Four Corners High may be building north after mid month.Could be a sunny and warm second half of July,which historically,has often happened.Don't give up on summer weather yet.

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  4. I live near the Skagit farmlands and drive through most days. Corn here is supposed to be "knee high by the Fourth of July" and this year, it is barely six inches. Late start with all the rain and subsequent clouds and cool weather.

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  5. The clouds do not bother me...I have lived here for over 70 years...but, the silly drizzle has me wearing out the intermittant wiper switch on my cars!...where is the real rain hiding?

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  6. it's 2020. why would we have a nice summer?

    the hits just keep coming.

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  7. This June in Puyallup was the first one in the last 5 that did not exceed 90 degrees. We got all the way to 85 on the 23rd with only 5 days getting past 80. Three days didn't even reach 60! Perhaps a correlation with the phenomenon described here?

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  8. Cliff,

    I'm happy to have cooler weather because I don't like extreme heat. However, I would like some moderate temperatures with plenty of sunshine. We'll have to accept what the weather brings.

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  9. How about that Southerly that blew all day Monday over the Sound?? Higher sustained winds than forecast and gusts peaking at sunset just as I was docking my boat after a choppy 10 hours going from Skagit Bay to Bremerton. Not a very relaxing day on the water.

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  10. High pressure you get rain.
    Low pressure you get rain.
    When does the medium pressure system arrive so we can get some summer?

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  11. We were having a prolonged heat wave in June and July of 2015, if I recall correctly. It was intolerable. And so little rain. I'd like to see the June solar radiation chart back to 1990. I bet there were a number of years when we had less sun that we got this year. I guess I'm a hard-core Northwesterner: I like it around 68F.

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