Heat Wave Update: Hottest of Summer & 90s at Coast

9pm Tuesday…

It’s gonna get hot!  Okay, that’s a bit of a cliché but it’s true, I see a real scorcher coming up the 2nd half of the week.


  • Thursday-Saturday highs will be somewhere between 95-103 in the Willamette Valley and Portland Metro area.  That’ll be the hottest 3 day stretch of summer
  • Strong east wind arrives Friday not only in the Gorge but also west slopes of the Cascades and down into the valleys.  That means extreme fire danger and maybe some downed corn stalks too!
  • Fire danger will be extremely high west of the Cascades Friday and Saturday with the dry east wind.  Any fire that starts could spread very quickly.  Historically the largest fires west of the Cascades have spread in just these conditions here in western Oregon.
  • Friday may be the warmest (hottest) day in almost two years at the Oregon Coast.  For one day (Friday) and possibly part of Saturday, east wind will make it to the Oregon Coast.  Assuming that occurs, high temperatures will be in the 90s (yes, 90s!) in many spots out there.

Things have changed a bit in the last 36 hours.  First, a hot upper-level ridge is going to develop just to our west Thursday-Saturday, in the offshore waters of the Pacific Northwest.  No change there, but now models show a small but potent disturbance dropping down the east side of the ridge, bringing a taste of fall into Montana.  The next question is “what does that have to do with us?”.  That cool air is high pressure, so now all models are in agreement we get a strong easterly flow over the Cascades and through the Gorge.   Here’s the Friday 5am surface pressure map from the ECMWF.  The yellow line is the lowest pressure, air will flow toward that “thermal trough” as we call it.  Notice it’s on or even off the coastline at that time = east wind all the way to the beaches.


That easterly flow is the “icing on the cake” for a “perfect” heat wave.  850mb (5,000′) temperatures are forecast to remain near +22 to +24 (celsius) from Thursday through Sunday morning.  Such hot temperatures would typically push us well up into the 90s or even about 100 degrees this time of year.  Add in the extra heating as the dry wind races downhill off the Cascades and you can add another 3-5 degrees.  As a result there is no reason we can’t get to 100 degrees or above at some point between Thursday/Friday/Saturday.


On Thursday the offshore flow is just getting going in the afternoon, so I kept the high temperature down to “only” 98 degrees.  It’s sure possible we hit 100 degrees though.  Friday is the trickier forecast because we may have so much east wind blow here in the metro area that it might keep our high temperature “down” a few degrees.  As a result I’ve gone with a 95 for PDX.

Saturday (at this point) looks like the hottest day to me; hot airmass in place, easterly wind to start then it goes mainly calm in the afternoon, and full sunshine.  I put a 100 for Portland that day due to those factors.  Sunday we lose the offshore flow so even though it’ll be hot it should be well below 100 degrees.

Hitting 100 past August 15th is tough to do in Portland due to decreasing sun angle and about 2 hours less daylight than late June.  But check out 3 past episodes including Portland high temps and 850mb temp measured over Salem.  Notice how the atmosphere was overhead in 1977!

August 16th/17th 1977

102 deg / 850mb temp +24.6
105 deg / 850mb temp +27.0

August 14/15th 2008

102 / +24.6
100 / +26.6

August 18th/19th 2015

96 / +23.4
97 / 20.6

The coast is always a very tough forecast during hot weather episodes but this is what I’m thinking right now:


Saturday is the tough forecast day as we get a flow reversal and its southwesterly surge of fog and cool air pushing up the coast.  Friday will likely be the hottest day we’ve seen at most locations along the Coast since September 2014.  That month we saw a day with high temperatures in the low-mid 90s out there!

This whole episode is a bit extreme, but definitely not something “unprecedented” or unheard of in mid-August.  It just doesn’t happen that often.  In fact there have only been 5 days in Portland’s history where the temperature has made it to 100 degrees from the 18th of August through early September.  Interesting that 4 of those happened in 1986, 87, & 88 isn’t it?

Chief Meteorologist Mark Nelsen


20 Responses to Heat Wave Update: Hottest of Summer & 90s at Coast

  1. pappoose in scappoose says:

    Area Forecast Discussion
    National Weather Service Portland OR
    841 AM PDT Thu Aug 18 2016

    SHORT TERM…(Today through Saturday)…Big change starts today and continues for a 3 day stretch as an upper ridge builds over the
    region, making it hot, hot, hot! Some may say sunny and miserable.

  2. Tyler Mode in Battle Ground says:

    Low of 51.7 here in BG. That means close to a 50 degree spread coming up this afternoon. If it hits 98.8 it would be largest spread of the year so far.

  3. Paul D says:

    100+ three days in a row = boring!

    I just drove from Northern AZ through most of UT and went from sunny and 90 to pouring rain and 52 in about 20 minutes! Now that was some fun weather!

  4. Will be interesting to see how the heat wave plays out for me with the strong easterly surface flow. In the past, that regime has sometimes been associated with some of the most dramatic temperature differences between Seattle and Bainbridge Island.

    The hot air passes over the Sound, cools down, and the resulting sea breeze holds temperatures down for me at the same time time Seattle roasts under a hot east wind descending the Cascades. I’ve been as much as 12 degrees F cooler here under such situations.

    But the above only sometimes happens. Other times, I get no more than the normal 3 to 7 degrees F difference on warm days. More turbulence and mixing disrupting the formation of a strong cool layer, perhaps? Who knows.

    Gonna be toasty at any rate. NWS has my highs for the next three days as 86/85/81. With Seattle at 89/93/93, 80s are pretty much a sure bet for me; that should happen even if the maximum difference situation happens each day (which is unlikely). Note that the NWS models have the maximum-difference situation on Saturday.

  5. boninepaul says:

    This is going to be a real challenge for plants and gardens in general. People tend to ignore their plants when we have cooler or even normal summer high temperatures. They may even think that a little rain or drizzle actually HELPS plants. Well, it may temporarily halt positive evapotranspiration but the ground it still bone dry and getting drier. Now we’ve gone almost two months without real beneficial rain and the combination of heat and wind is going to really stress garden plants. The same way it exacerbates the fire danger its going to be harsh on ornamental plants. As a nursery owner I’m not looking forward to this but the word for people is to water, water, water now and at least once during the heatwave. Soak plants well. Its better to water deeply and infrequently than to water shallow and often. I’m glad the season of heat is drawing to a close.
    If it is as hot as 1977 or even July 2009 things will really, really need it.

  6. colbyworthylake@gmail.com says:

    So you’re telling me that when I go to Newport on Friday, I still won’t escape the heat?

  7. I’m glad Mark included data from August 16-17th, 1977. I’ve been looking back at that month recently and the current long warm dry pattern reminds me of what it was like back then for most of that month.

  8. Scooter says:

    I remember a very hot day in late September, it must have been 15 to 20 years ago and was around the 25th of the month and we hit 95 degrees here east of Oregon City.

  9. Scott J says:

    The 85-95 high temp is impressive but nothing like the 108° I experienced in Brookings on July 8, 2008. That was one very unusual day.

  10. runrain says:

    No morning clouds in PDX today. Could we catch 90 today and see 5 days of 90+ in this heat wave? Sunday is now looking like the day we may not get to 90, however.

  11. I don’t like this forecast. Fix it, all powerful weather guy.

  12. Would this same pattern / exact same temps, be “unprecedented” if it occurred 10 days later in the season, Mark? I seem to recall you blogging a few years ago, about how mid-August (e.g. the 3rd week) is about the limit for the really top-tier heatwaves.

    • Mark Nelsen says:

      I think it would have to be in early September. We’ve seen +23 850mb temps and gusty offshore flow through the end of August. Now if I see a +27 on September 10th and a high of 100, that’s a different story.

    • Anonymous says:

      Sept 2 1988 hit 105. Wonder what the 850mb temps were that day?

    • I think the limits imposed by a weaker late August sun are more appreciable in The Dalles – because east winds in late summer don’t benefit us. We never see more than 2 or 3 days 100+ during a late August / early September heat wave, and they are almost always low 100s.

    • Mike near Clackamas Town Center says:

      Anon….28.2C 😉

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