September 08, 2018

The Fall Transition

Summer is over west of the Cascade crest.  Heat waves are done.  Dense smoke in western Washington is finished. 

Take out your sweaters and rain gear.   You will need them.   A reason to be thankful we are living in the Northwest rather than central and southern California, which the threats of fires and smoke will continue for several months more.

Let's take a look at the latest ensemble prediction of the NOAA/NWS GFS model for surface air temperature at Sea-Tac Airport (remember an ensemble is when we run the model many times using different initial conditions or model physics).  The black line is the mean of the ensemble (generally a very good forecast) and you can see the prediction of the various ensemble members. The forecasts go through September 16th.

No heat fact, we expect cooler temperatures, with Monday and Tuesday barely getting into the 60s!    Then a very modest warm up with highs in the 60s.

And with the cooling temperatures, there will be multiple opportunities for light rain (see ensemble predictions for 3-h precipitation).  
Why all the changes?  Because of a major reconfiguration of the atmosphere with the development of a persistent upper level trough over the northeast Pacific.  

To illustrate, here is the upper-level map (500 hPa, about 18,000 ft ASL), showing a major trough right off our shore.

Wednesday at 8 BIG trough over the northwest.  A very cool pattern for us.

 The problem for heat wave lovers among you is the rapid weakening of the sun, which has a big impact no matter what the meteorology is doing.  

We are running out of sun (and time) for warm weather.   To give you an idea, here is the climatological temperatures for Sea Tac Airport, with the yellow line showing the daily max temps.  The vertical green line shows today.  Temperatures have NEVER gotten above the low 90sF for the rest of the month and soon never above 90F. 

The super-extended forecasts (e.g., the NAEFS US/Canada ensemble) show no hint of getting back into the 80s.  The cooler weather should be putting a damper on fire growth west of the Cascade crest, and increasing westerly flow will keep east WA and Canadian smoke away from us.  For me, I suspect the calls about "smokestorms" will be over.   


  1. It was about 65 degrees today, clouds at multiple levels, cool and pleasant wind from the south. What a refreshing relief from the heat and the smoke.

  2. Please no. I'm loving these 70-72F days with a few cumulus clouds!

  3. Soon the wildfires will be out. Horay

  4. I prefer the Central California Coast, Cliff. Cooler summers and warmer winters and way less rain. I no longer dread fall and winter like I did in the Pacific Northwest. Paradise here, Cliff. You may want to consider it when you retire. I am happy that I did!

  5. Cant say the smoke was fun but it was great waking up in the morning with sunlight filtering through the window.

  6. With the exception of Autumn, 1963, returning to school, here in the NW, has in my lifetime invariably been a signal of the end of summer and the onset of cooler temperatures and rain. The rains sometimes started in early September and sometimes not until October. I've always embraced it. 1963 is a standout in my memory. For some reason that Fall I remember being weeks of warm, even hot, days, and no rain, not until October. I've never checked the weather history to see if that's accurate, but I remember getting to swim for an extra month in local lakes.

  7. I much prefer the wet season over the dry. Bring it on! I love some outdoor fun in the crisp, rainy air!

  8. Yay rain! Yay gear for cooler weather! Yay for cooler, cozier times ahead. I’m always glad to see spring come, but that’s equally true for fall in my book!

  9. I have always wondered why our normal temperature curves reach their minimum in late December, while most mid-latitude locations bottom out in late January. If anything, wouldn't you expect that in a marine climate like we have, the date of minimum temperature would be delayed rather than expedited (given that all that water serves as a store of heat)?


  10. The trick with having significant seasons is that you can allow yourself to get sick of one, knowing the next one coming will be totally different. I'm sick of rain by February and I'm sick of sun and heat by September.

    I lived on the equator for a few years, and every day was hot and humid. Every. Day. The daily rain showers were so warm they would hardly cool you down, and then the humidity got even worse when the sun came back out and everything got steamy. Every. Day.

    And if you went into any building, the Aircon was always set so cold that you'd freeze, especially if you were wet. That was a special kind of hell.

    Now I live in a special kind of heaven out on the OlyPen.

    Hooray for September.

  11. Why do some folks come on and blast the weather in the PNW, when they don't even live here anymore? Weird, sounds likes a bad case of projection.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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