Hot & Smoky Ahead; Then “Fall” Arrives

August 19, 2018

8am Sunday

I’ve ignored weather maps during my weekend, but this morning I see three highlights while perusing weather maps with my coffee.


The first is that we’ve got another brief period of hot weather coming, although once again the heat won’t be too extreme.  We don’t get long heat waves in late August and this will be no different.  Highs reach right around 90-95 or so Monday-Wednesday.  Today should be fine.  Just a bit warmer than yesterday with highs in the mid-upper 80s.  You’ll notice a little more fire smoke than yesterday, and in parts of the metro area the air quality has gone downhill again.  We’ve got weak onshore flow today, keeping those temperatures in check.  Notice the easterly flow on the cross section forecast for Tuesday.   The colored areas are relative humidity above 70%, see how the marine layer disappears Monday PM and Tuesday AM.  “21/12” on the chart refers to 5am Tuesday.  “20/12” is 5am Monday.

But later Monday and all day Tuesday we get an offshore/easterly flow coming down out of Eastern Washington, through the Gorge, over the Cascades, and into NW Oregon.  That helps things heat up Monday and Tuesday, but brings a problem back…


Yuck!  I just checked the HRRR smoke modeling and it shows smoke from all those fires in northern Washington pouring back down over us just like it did last week.  The loop is here:  Here’s the image for right now.


and then the image for tomorrow morning around 8am…you see the northeast flow has brought the smoke right down over us again.


The smoke will be with us Monday-Wednesday (3 days), then a HUGE change is showing up on all models the latter half of the week…looks like FALL!


Yes, I know it’s not technically fall but it’s the biggest change we’ve seen since late spring.  All through July/August we’ve had a semi-permanent ridge of high pressure either over us, just to the east, or just to the west/northwest.  Here’s the ECMWF ensemble average of upper-level heights for Wednesday; warmer colors are above average heights and blue are below average


and then one week from now, Sunday the 26th


A cool upper-level trough over Western Canada and the Pacific Northwest.  This screams “end of summer weather” for at least 4-5 days starting Thursday.  A thick marine layer Thursday and beyond means you’ll need to get used to lots of morning (or all day) clouds and spots of morning drizzle at times.  High temps will drop at least down into the mid 70s.  This late in the season a thick marine layer with AM drizzle can keep us in the upper 60s too.  Here’s the ECMWF ensemble meteogram for Portland; quite a change eh?


Looks pretty comfortable!  It’s not like we’re entering a cool & wet period, but the chance for showers definitely goes up at times beyond Thursday, or maybe it’s better to phrase it as “guaranteed dry weather goes away for a few days.  Notice temps do rise slightly beyond Sunday, back to normal which is in the upper 70s the last few days of August.

I’ll be back at work today (I work Sunday-Thursday), forecasting on the 5, 8, 9, 10, & 11pm shows…I shouldn’t be hard to find!

Chief Meteorologist Mark Nelsen