Beautiful Weather; Last of the 80s?

September 26, 2018

6pm Wednesday

Today has been another fantastic fall day.  A chilly start with lows in the 40s and lower 50s, then all areas west of the Cascades in Oregon made it well into the 80s.  Check out those lower 70s on the coastline as well

Todays Observed Highs OrWa 2017

I saw someone mention somewhere on social media that this is summer weatherwise.  Most definitely not.  The long nights and low sun angle give a totally different feel.  It gets warm between about 2 & 6pm in this pattern, but cool during the lengthening nights, mornings, and evenings.  Even here in the city it was still only 69 at noon.

Todays Observed Temps in Portland 3D City Earth Scene 2017_2

Strong high pressure centered to our west over the Eastern Pacific remains in place through Friday and that guarantees us two more days above 80 degrees.  Then lowering upper-level heights plus a strong onshore push drop us 10-15 degrees Saturday for a more typical late September day.

So is this the last of the 80 degree temps?  It could be.  Check out the past few years…specifically the last date we hit 80 each of those years.

Last 80 Degree Day Heatwave

The average for Portland (in any year) is September 29th.  Last year we hit our last 80 on this date (the 26th).  After this Friday there’s no sign we’ll hit 80 in the following week, although the ECMWF ensemble forecast highs for Portland do hint at warming again late next week.


That said, models have not been doing real well handling a split-flow setup Sunday and beyond so I have low confidence beginning that day.  I’m not a fan of putting question marks on the 7 Day forecast (and we don’t do that here) but early next week would be the time to use them!

Here’s a plot I use regularly…the ECMWF 850mb temperature ensemble plot.  This is temperature in celsius up around 5,000′.  Each thin line represents one of the 51 members of the “ensemble forecast system”.  Basically each line is one run of the model and it’s done 51 times every 12 hours.  The blue line is the actual “operational” model run and has a bit higher resolution.  Red line is the average of all the ensemble members.  You can probably guess what the yellow/green line is…climatology.  Temperatures up around 5,000′ drop quickly the first two weeks of October.  You can glean a bit of info from this chart; I noticed two items today.

tseries_850t_000-360_Portland (1)

  1. Good agreement among ensembles (all the lines close together) through Sunday…the 30th, then they are all over the place.
  2. Temperatures are well above average through Saturday AM, then plunge to below/near average after that time.  Looks like we’ll be all done with the 80s down here at sea level after Friday.
  3. BUT, look at that range the middle of next week!  That could give us a high of 85 or as cool as 58, best to just keep the 68-70 for now until the solutions come together.

By the way, today is the 71st day we’ve hit 80 this year in Portland

80 Degree Days Yearly

As we all know, the past 5 warm seasons have been VERY warm; quite a contrast with those much cooler years 2010-2012.

Chief Meteorologist Mark Nelsen