May 31, 2012

Portland is Hot, But Seattle is Cool

As we enter the warm portion of the year,  differences between Portland and Seattle temperatures are again becoming evident.  Seattle has a far more maritime climate than Portland and thus is considerably cooler during the summer.   In fact, the Willamette Valley as a whole has a far more continental climate than the Puget Sound basin. 

To illustrate this situation, consider some topographic maps.  Seattle is clearly open to the influence of cool waters, with Puget Sound at its front door and a sea level path to the Pacific Ocean.  With water next store and high pressure offshore, constantly pushing marine air into the Puget Sound interior, it is hard to get the Puget Sound lowlands in the 80s or 90s during the summer.  Only when we have strong offshore and downslope flow can Seattle and vicinity get toasty.

But Portland and the Willamette Valley are another story!  The Willamette River or even the Columbia is no Puget Sound, and the Willamette Valley is surrounded by mountains or high hills on all sides, thus isolating it from the marine influence during the summer--a period when the Pacific cool, marine layer is relatively shallow and unable to surmount the terrain.  Thus, it is far easier for Portland to bake...and it is not unusual for the high temperatures from Portland to Eugene to be 5-10F warmer than Seattle...sometimes much more.

Lets begin by looking at the monthly average maximum temperatures at Seattle, Portland, and Salem.
Consider July.   In that month Seattle has an average high  of 75, Portland gets to 79, and Salem, a torrid 82F.  Not need to head to down to Salem.  But look at the winter high temperatures...there is very little difference among these three stations...and that is not surprising. With strong, deep flow coming off the Pacific and lots of clouds, we all suffer with cool, wet conditions.

But the mean monthly temperatures tell only part of the story...what about the extremes? Below is a plot of daily means and  extremes...lets consider the daily extreme high temps (yellow colors).  Seattle has only gotten above 100F once---103F in July 2009.  But for Portland, daily records above 100F are numerous and above 90F commonplace over most of the summer....very different than Seattle!

Lets illustrate with last summer:  here is the plot of temperatures at Portland (green line) and Seattle (red line for July 1, 2011 to Sept 1, 2011.  Portland is warmer on virtually every day, sometimes by as much as 10F.  They just don't have Seattle's natural air conditioning.  But they do have better food trucks, several of which sell cooling drinks and ice cream.
And if you think the Willamette Valley is warm during the is nothing compared to the next basin to the south...the one including Medford and Ashland.  They laugh at 100F temps down there....but that is a story for another day, and one that includes fog during the winter.


  1. Speaking of Hawaii, anyone else been noticing the muggyness of the air today? Feels great. There's been a few DP readings of 60. Thats impressive for the PNW.

  2. Hi Cliff,

    I see that the "June Gloom" has arrived in Seattle right on time. At least it is rather warm this year.

    Speaking of Medford, I remember driving through there in the early eighties, in my Fiat Spider with the top down. 108 degrees. As Roseanne Roseannadanna used to say: "I thought I was gonna die".

  3. I'm jealous of Portland's warmer summertime temperatures.

  4. Well, the grass is always greener (or browner?) on the other side... As a Seattle native who has lived in Portland for 10 years, I miss the cooler (but still sky blue) summer days of Seattle as compared to the sometimes days on end of 90 or even 100+ heat that can happen in the Willamette Valley. But yeah, the Rogue River basin is hotter yet, and California's northern Central Valley? Hell on earth in summer!

  5. Cliff, how far south do you have to go to escape the tendency to June Gloom? At what point does June get sunnier, as well as warmer than May for the same location?

  6. Having grown up in Camas, I know well of the differences. I much prefer the cooler Seattle area weather. I also remember the "Silver Thaw" well.

  7. Cliff,

    Always happy when you get into a bit of Oregon and Portland talk. Your blog is NW weather and understandably tends to be a bit SEA/WA centric as far as case-studies for specific Wx situations. Not a criticism, to be clear, but appreciation when you turn your focus this way :)

    That said I think your post here should have a little bit of a caveat about the 'cooling' impact--not just talking about daytime highs. Diurnally Portland seems nearly identical to Seattle when you look at the lows. We get the marine push aplenty each evening in the summer, giving us pleasantly cool nights. Thus it isn't entirely 'hotter' down here, just during the day.


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  9. How about Vancouver, BC...often make the drive from here (Vancouver) down to Seattle, or vice-versa...and the weather tends to change dramatically.


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