4pm Thursday...

I've been off work the last three days and will be through Saturday. But I want to update you on two important changes ahead. Tonight looks especially interesting.

First, I see almost nothing has changed with regard to smoke. We are in an unprecedented (at least for the past 40 years or so) smoke event over the region. This is the 8th consecutive day with VERY UNHEALTHY to HAZARDOUS air quality in the Portland metro area. I never saw that growing up here and sure haven't in my career that started in 1991. We've been receiving a "double-whammy" of: light wind at sea level keeping surface conditions smoky, plus tons of high level smoke streaming north from Oregon and California. They have conspired to keep temperatures well below what they should be in the current weather pattern. The effect is that this fire/smoke event brought our "summer to an end" on the 10th this year, when under normal circumstances it would have continued through today. We don't have any sort of spell of "summer weather" in sight. Fall is here weather-wise starting tomorrow.

My wife is tired of me saying "if it wasn't for the smoke we'd be sunny and in the low 80s again today". That would be the case today of course. If we didn't have fires burning, this could have been a record warm first half of September. But you see starting a week ago the dense smoke kept us a solid 10-20 degrees cooler than we would have been otherwise.

Air quality numbers at 3pm:

Yesterday evening was interesting down in the south Willamette Valley. That's because a weak westerly push of marine air briefly dropped Eugene and Albany down into the GOOD to MODERATE category. But then a light northerly wind today has allowed the pollution to slosh back into the southern valley.


This is the biggie. We haven't seen this setup since the end of May; a nice nocturnal lightning event is on the way.

  • Anytime after 8pm then on through sunrise tomorrow, ANY of us west of and in the Cascades (including coastline) could see quite a light show.
  • In fact the Portland NWS has the recently burned areas under a Flash Flood Watch since any heavy rain on dry/burned soil will cause all sorts of trouble. Landslides, rock fall, trees drop, etc...

The Storm Prediction Center even has us under a MARGINAL to SLIGHT risk of severe thunderstorms. A severe thunderstorm in this case would be a damaging wind gust over 58mph or large hail. Yes, that could happen during the night. Rare, but I have seen it happen here. And no, no one knows why SPC still uses those categories. Does any normal person know the difference between MARGINAL, SLIGHT, & ENHANCED??? No.

What's the reason? It's the large upper-level low offshore that's been spinning in circles all week. Tonight it makes the big move. Right now, notice the southerly flow in the atmosphere. Imagine how many fires are upwind this afternoon, thus the continuing dense smoke overcast:

Then tomorrow midday, it's about to move right over Portland...

By Saturday midday it has moved to the east, and we have a west-northwest flow overhead. By this point all upper-level air will be coming in clean off the eastern Pacific!

There is a lot of moisture as this system moves in. HRRR says about 1.50" precipitable water west of the Cascades...that's juicy!

The way the low is moving in leads to tremendous lifting in the atmosphere the next 12 hours. Especially 8pm-2am. This IS the classic setup for thunderstorms west of the Cascades. Each model is slightly different with location and timing, but I think the NAM-NEST represents reality well. At 8pm storms are starting to pop just to our south.

But by 1am they are spreading north quickly

And by 6am it's obvious SOME of us will have seen downpours, others relatively light rain

It's important to not focus on specific spots; the point is that over and west of the Cascades we'll see some heavy rain from thunderstorms during the overnight hours. Some areas could see up to an inch of rain, others just a brief wetting.

Beyond sunrise tomorrow, we'll be into a more typical spring/fall showers & sunbreaks pattern. That continues the rest of Friday.


  1. A westerly breeze and thunderstorms should clear out the densest fire smoke by sunrise and we'll just have a smoky smell/view instead of this hazardous stuff. That's an improvement.
  2. Then much cooler air overhead with the upper-level low will finally break the inversion we've been under for 8 days; that should help quite a bit. Lots of the surface smoke should easily mix out after sunrise.
  3. Continuing westerly flow in the upper atmosphere starting late tomorrow through next week should keep California and Oregon fire smoke moving east and away from us.


Enjoy the weather "show" tonight and hopefully we avoid any significant flooding issues in those burned areas. This will be our first measurable rain in about a month; which means roads will be slick at first too. Stay safe!

Chief Meteorologist Mark Nelsen

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