Historic Heat Wave This Weekend; Could Be Hottest Ever in Portland

June 23, 2021

9 p.m. Wednesday…

This is it folks!…for days our weather models have been insisting we have a major heatwave on the way. In fact, for the past three days they’ve been forecasting never-before-seen numbers (hot) for parts of the region. Typically as we approach an extreme event (cold, snow, wind) models clue in a bit more and get more reasonable. But in this case they’ve looked almost the same for days. The extreme numbers aren’t going away. So we’ve gradually been upping our forecast high temperatures for the weekend as confidence grows. After some consultation with the rest of the weather team today, we’ve decided to go with an all-time record high in Portland this weekend. That would be HOTTER than 107 degrees. Here are the all-time records for some of the I-5 corridor cities and the year it happened. Notice they are all between 105-110. Downtown Portland records have the same 107 degree record as PDX.

First, for those of you not wanting to get into the meteorological details…

HIGHLIGHTS

  • What could be the hottest heatwave on record in the Portland area (and much of northern OR/southern WA) arrives Friday and continues through Monday. At the very least, this will be one of the top 3 heatwaves in our area. Very warm temperatures, 85-95 degree temps, continue through the rest of next week.
  • Expect three 100+ days in Portland, and they may all be 105+
  • I’m very confident we’ll hit 106-107, and think 110 is quite possible for the first time here. Records go back to the late 1800s.
  • Humidity will be reasonable at first, but humidity will be up Sunday & Monday…it won’t be a totally dry heat like we tend to get late in the season (September).
  • Overnight lows may be the warmest on record in Portland too. Low-mid 70s Sunday and Monday mornings. That means homes/apartments will remain dangerously warm with no chance to cool off. Typically we see reasonable overnight temps in our area.
  • The only place to cool off will be the coastline…more like 75-85 out there over the weekend. Most likely you won’t be alone if you want to play on the beaches
  • I do not expect any sort of strong wind anywhere in the Northwest. Wind will be relatively light Saturday through Monday

The forecast could still change a bit, but all this is based on what we’re seeing just 2-3 days ahead of the event.

METEOROLOGICAL DETAILS

When I was 12, we suffered through the great August 1981 heatwave. I was living near Mt. Angel at the time. We spent a few days mainly down in the basement. Almost no one had air conditioning back then. Then when my career began in 1991, just out of college, I wondered when we might break that all-time 107 degree record in Portland. It was only a matter of time, but I didn’t think it would take another 30 years! I’m feeling pretty confident this evening that it’s finally going to happen.

A large and very strong upper-level ridge will begin developing just to our west tomorrow

Then by Saturday the heights really soar.

At this time, most models are forecasting 500mb heights over 597dm over southern British Columbia. Meteorologist Ryan Maue had a great tweet last night, mentioning that this would be an all-time record up over that area. And having a closed “dome of heat” just to our north keeps us in easterly flow from high in the atmosphere just about all the way down to the surface. You can see the WRF-GFS cross-section for Saturday afternoon through next Wednesday shows that easterly flow from Saturday through Monday afternoon.

850mb temperatures soar to +21 (C) Friday over Portland, +25 Saturday, and right around +30 both Sunday and Monday on operational ECWMF/GFS models. Ensemble averages from the ECWMF keep Sunday and Monday around +27 to +29. Anything above +28 is an all-time record for our area. The 850mb ensemble chart from the 12z Euro run shows the massive heat spike with the ridge. Very high confidence of +27 or above Sunday PM.

Down at the surface, a textbook perfect thermal trough develops Saturday. This is the WRF-GFS. Weak easterly flow through the Gorge, weak onshore flow right along the beaches. Almost no wind in the valleys. This, along with totally sunny skies, plus very dry air overhead, should lead to a very hot day.

Then Sunday…the thermal trough is right over the western valleys with a (10-20 mph) easterly wind through the Gorge.

At that point 850mb temps are somewhere between +27 and +30. THIS should be the all-time record hot day. Then on Monday things change a bit. A southerly surge of cool marine air is moving up the coastline and beginning to pour into the southern Willamette Valley. This COULD be a day where Eugene is 10 degrees cooler than Portland.

Onshore flow picks up that evening and we drop at least 10 degrees Tuesday. This is all pretty straightforward. But what has been shocking meteorologists is the surface temperature forecast from all the models. The ECMWF operational model looks like this. Crazy hot, exceeding Portland’s all time high by 7 degrees Monday afternoon! Most shocking to me is that the Euro generally has cooler surface temps than expected during warm spells. For example we’re going 97 on the day it’s forecasting 92. It’s hard to believe that would hold with those extreme temps Sunday/Monday, it’s not going to be HOTTER than 110 or 114.

Much better nowadays is to use ensemble forecasting…in this case 51 different members of this Euro model. This is just about what we are forecasting, accounting for the typically low Euro readings by adding a couple of degrees to most days over the next week.

Hopefully I’ve given you some reasoning for my “hottest forecast ever” here. Buckle up for a crazy hot weather ride!

I’ll be working right through the weekend and next week. So make sure you are following me on Facebook: @marknelsenweather and Twitter: @marknelsenKPTV. I will be updating those much more frequently than this blog. Most likely I’ll get a fresh posting done again Friday morning, if not tomorrow evening.

Chief Meteorologist Mark Nelsen