September 11, 2013

Temperature Surge and Thermal Trough

The Puget Sound region and much of western Washington are about to be hit by what I will term a "temperature surge" as strong downslope flow over the western slopes of the Cascades causes a rapid warming and drying of the air at low levels.  This is basically the same phenomena associated with the Brookings heat wave earlier this week, Santa Anna conditions in southern CA, and Foehn conditions in Europe.

During the last day, strong easterly flow has developed aloft, something that is evident from the winds shown on the Seattle profiler.   This strong easterly flow is associated with warm air (27F, 80F centered at around 600 m, if you look closely)

 This morning there is a veneer of cool air with very warm air above, separated by a sharp inversion here in Seattle (see graphic)  During the next few hours this cool air will get mixed out and the temperatures will surge upward...towards the upper 80s in many places.
The warm, sinking air is causing pressure to fall, allowing the northward development of what we call the thermal trough.  A forecast map for 11 AM shows this thermal trough quite well:
Amazingly, this is the best defined thermal trough of the summer.  Typically, we would have seen a half-dozen of these by now...but not in this anomalous summer.   As the thermal trough moves up the coast, one often sees a wind reversal (to southerly flow) along the coast, accompanied by low clouds.  It is happening this morning!

 The transition to warm, offshore flow was evident at some hilltop stations during the past day.  The home of Peter Benda on Bellevue's Summit Community (around 1200 ft) experienced a rapid warming and drying in the middle of the night as southeasterly flow developed:

 And at Larch Mt. northeast of Portland at around 1200 ft in elevation, temperatures zoomed in the 80s yesterday morning and relative humidity dropped to near 20% yesterday morning and stayed there with little change (see graphics).  You will not that all this action occurred when their winds switched in the southeast, with gusts to 20-40 mph.

 Enjoy the warmth..tomorrow will cool a bit (mid 80s) as the thermal trough slips into eastern Washington.


  1. Thanks Cliff! I just landed at Harvey Field in Snohomish and noticed my oil temperature, which is always 100F over Ambient, had really jumped as I cruised through 1700' MSL. I also had some interesting little turbulence descending through 2000'MSL over the Snohomish River Valley at 11am. Smooth above and smooth below. Looking south, a very definite line of brown haze over Seattle, at est. 1500'. Clear delineation. Wasn't expecting an inversion with air coming over the Cascades and mixing/ compressing!!!!

  2. We had a heat blast out at the coast today too. It was sweltering from about 8:45 to 9:15 AM. I almost passed out. ;)

    Today's graph is unusually clear cut:

    It is like someone flipped the switch and turned on the on air conditioning at 9AM.

  3. Really hoping the long range forecasts are correct and this is the last blast of heat for the season. I'm hoping the farmers almanac is correct and we have a cold, snowy, windy winter this year.....I can always dream, can't I?

  4. Not really a comment, but could you discuss where you get your charts such as the one showing the thermal trough. I have hunted the web and can not find a clear source.

    Thanks, David Smith

  5. Wow, Seatac hit 93 yesterday. Boeing Field was only 89 and I recorded 88.9 for a max yesterday at my place in Green Lake. I think the Seatac temperatures are definitely skewed on hot dry days like yesterday.

  6. Thanks for this, Cliff. Excellent explanation.


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