Historic Heat Wave Begins Monday

July 30, 2017

7pm Sunday

The first few days of August will feature the hottest temperatures we’ve seen in at least 8 years west of the Cascades.  Just two years ago we had our longest heat wave on record here in Portland, but we didn’t hit 100 degrees.  This time the heat sure won’t last as long but it’s going to be hotter.

I’ve been forecasting in Portland for 26 years (and lived here most of 48), so I’ve seen lots of heat waves.  The forecast numbers from the models for this coming week, specifically Wednesday and Thursday, are right up with or even higher than anything I’ve seen!


PLEASE check in on your older friends the next few days, the heat is especially hard on elderly folks if they don’t have air-conditioning.

I expect 6 days of 90+ weather beginning Monday.  We’ve done that 12 times in Portland weather history.  It’s unusual, but not an extreme event.

We’ll get at least two, possibly up to 4, 100+ days.  I see Tuesday-Friday (4 days) having the possibility of reaching 100 degrees in the western valleys of Oregon and extreme SW Washington (including the metro area).  I’m VERY confident we’ll go way over 100 both Wednesday and Thursday, with Friday being the best bet for a 3rd 100 degree day.  Tuesday we might be a degree or two shy of triple digits.  It’s quite rare to get more than 3 days at/above 100 as you can see from the stats, we haven’t done that since 1981.


Overnight lows will be “reasonable” for this kind of heat.  We’re lucky that relative humidity will be near normal through the week and we won’t see any cloud cover.  That allows temperatures to cool down into the 60s for most of us.  During the 2015 heat wave humid weather and occasional cloud cover kept us very warm.

The all-time record of 107 degrees in Portland could be threatened either Wednesday, or more likely Thursday.  All through my career I’ve watched other cities reach new all-time highs (Seattle, Reno, Olympia) and wondered when PDX would finally hit 108.  It’s not like any of us WANT it to hit 108, but it’s a numbers game I suppose.  I have forecast 105 & 107 for Wednesday/Thursday here in Portland.  But to hit 107/108 everything would have to just right (or just wrong…right?).  During the 2009 heat wave we thought it might happen but there was a little too much easterly wind and that may have stolen a degree or two.

The Oregon Coast will have some hot weather too, but not as extreme.  Right now I’m thinking 80s Tuesday & Thursday, with a few lower 90s possible Wednesday only.


Why so hot?  A strong ridge of high pressure (a hot blob of air) develops right ON the West Coast this week.  You may recall earlier in the summer that hot ridge was to our east, over Utah.  Now it’ll be right over the top of us.  For the geeks, it appears to be a 594-595 decameter high center.  That strength of a ridge isn’t unusual at all centered farther to the east, but it’s almost right on top of us for this week.

This is the 3rd day models are showing 850mb temps (the temperature in celsius around 5,000′) in the +27 to +28 range Wednesday and Thursday afternoons.  They also did that during the July 2006 heat wave and we remained below 105 degrees.  When we get this extreme, forecasting an exact high is…a bit inexact sometimes.  Anything over +26 is quite rare.  In fact we’ve only hit +27 (over Salem on the balloon sounding) a few times in the past.  The all-time record is +28.3 on September 2nd, 1988.  We hit 105 that day, which was just amazing for early September!  We now have the possibility of seeing that during the hottest part of the summer with a higher sun angle.  If that +28 actually shows up, it’ll be unprecedented for early August, thus my forecast of 107 Thursday.  Note the excellent agreement among the ECMWF model and its ensembles through Thursday:


Also note the temperatures stay above average after our heat wave ends next weekend. It’s a bit of an understatement to say that at least the first 10 days of August will be warm!

As for records, just by chance the first 3 days of August have never been above 100 at the airport weather station.  That means it’ll be easy to beat those records:

Mark Hot 3 Day PDX Forecast

The National Weather Service is forecasting a high (114) warmer than anything in the past 25 years down in the Rogue Valley.  Wednesday (circled numbers are records for either day or all-time) forecast highs:


With more records to come around the region on Thursday:


I’ll keep you updated this week on the blog, but remember you can follow me on Facebook and follow on Twitter for more frequent updates.  You see those feeds on the right side >>>> over there >>>

Chief Meteorologist Mark Nelsen