After a toasty late July and early August, cooler weather has recently returned to the Pacific Northwest.  Except for a 90 on Monday the 10th, every other day since August 5 has stayed below 85 degrees at PDX.  That’s allowed those of us without A/C to cool out our homes, after a rough time a couple weeks ago.

Just as important as the cooler days have been the cooler nights:  Most nights since the 5th have dropped into the 50s in Portland, with a few 40s in rural locations (including Hillsboro Airport!).  Even in the PDX heat island, temps have dropped below 70 degrees very quickly once the sun goes down.  This was after a long stretch of lows above 60 degrees, during our warm/hot stretch.  It’s tempting to think of the last few days as a “sign of the coming fall,” but keep in mind that it’s normal to have occasional cooler periods even in the middle of summer.  Usually it is followed by a return to a warm pattern, after a few days.

And it so happens that this week’s milder pattern, will be followed by a return to hot this weekend.  This is looking like a serious late summer heat wave; temps will be back up around 100 degrees on Saturday and Sunday!  Maybe even a degree or two hotter Saturday.

What is the pattern?  We’re going to see a sudden rebounding of the upper-level ridge now, after a few days with weak troughing in or near the Pacific Northwest.  Today Friday is the beginning of the “event”, so to speak.  It may or may not make it to 90 degrees at PDX; but upper 80s are almost guaranteed.  

But then by Saturday, things get really wild.  The airmass overhead gets really hot, up near +24 degrees C at 5,000 feet elevation.  There is also a lot of east wind in the lower-middle levels of the atmosphere.  That brings extra heat as the wind descends the Cascade slopes and warms still further.  Given that a +20 airmass is usually good for low 90s even without offshore flow, I think +24 means 100 degrees is likely.  With the dry east wind, the odds are in favor of reaching the century at PDX, and perhaps even a couple degrees hotter.  The one exception might be places near the Columbia River Gorge like Troutdale, if there is a bit too much wind.

Sunday afternoon is very hot as well.  Maps show a little bit of marine moderation along the coast, but a hot airmass will still prevail over the Willamette Valley and SW Washington.  We might “cool” off slightly between Saturday and Sunday here in Portland; but what’s the difference between 101 and 99 to most observers?

Finally on Monday we get some actual onshore push into Portland.  It’s still a warm atmosphere with southwesterly flow aloft, though.  So while temps will cool off to “only” the low 90s, humidity may creep up at the same time:

By Tuesday we start to see some disagreement.  I’m not going to post the image here, but the “operational” map shows quite a bit more marine influence than Monday, which would pull us back down into the 80s.  But quite a few members on the ensemble chart keep things relatively hot on Tuesday; more toward the low 90s.  Essentially, depending what happens today and Tuesday, our official heat wave could range from 3 to 5 days.

(courtesy of Weathertogether.net)

The other minor disagreement in the ensembles is Saturday vs. Sunday.  They nearly all agree on a +23 to +24.5C airmass for Saturday afternoon; hence I’m going to go with 101 degrees as my ‘forecast’ due to easterly flow.  Sunday could be just as hot as Saturday, or it could be 2-4 degrees cooler.  

If we manage to hit 100 on both days, it would bring our season total to 4 days.  That has only happened 5 times in 82 years of airport records, most recently in 2006.  There were 8 other years when we had 3 century days.  What’s most interesting is that four of the five ‘super-scorcher’ years (1941, 1977, 1981, 1992) were really not particularly hot summers.  2006 was somewhat hotter than the other four, though still not a record.  Many of these summers had one of the two main months hot, and the other near normal to below.  Sometimes the heat was dominant in July, and sometimes August.

This year is following the same pattern.  The first 6 weeks of meteorological summer were actually somewhat cooler than normal, then we shifted into a relatively warm pattern for about 3 weeks beginning in mid-July.  Now a mid-August heatwave.  That’s about it for intense summer warmth!  This definitely isn’t like 1998 or 2009 or 2015.

The good news is that this will most likely be the last “big” heatwave of the season.  It’s pretty tough to get much above 95 degrees in Portland, after the 21st-22nd of August.  It can definitely happen, as did last year at the end of the month.  But it takes a perfect combination of offshore flow and extremely hot air overhead, to pull it off.

Be careful this weekend…the extreme heat and dry east wind will cause the fire danger to go through the roof.  Avoid all campfires and don’t park in dry grass, where the hot underside of your vehicle could potentially ignite something.

-Karl