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 waves..in 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 AM...wow... 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.   

from Cliff Mass Weather and Climate Blog https://ift.tt/2oRPeyE