Greetings world!  The month of September has been a rough one here in the Pacific Northwest.  Western Oregon endured what is most likely the worst outbreak of wildfires in recorded history.  The historic easterly windstorm on Sept. 7-9, resulted in the conflagration of FIVE “megafires” over and west of the Cascade Crest:  the Lionshead fire near and west of Warm Springs; the Riverside Fire in the upper Clackamas Valley; the Beachie Creek Fire in and around Detroit; the Holiday Farm Fire in the McKenzie Valley; and finally the Archie Creek Fire east of Roseburg.  

 

The two biggest fires – Lionshead and Beachie Creek – actually combined over the Cascades and created a single burn zone of nearly 400,000 acres!  All in all, over 900,000 acres have burned since September 1 in the state of Oregon.

Prior to this year, western Oregon had never seen even two megafires (100,000 acres or more) in the same season, let alone five!  Between 1920 and 2019 there were a total of 6 such fires.  They included the three Tillamook Burns in 1933, 1939 and 1945; the Bandon Fire in 1936; the Biscuit Creek Fire in the Siskiyou Mountains in 2002; and the Chetco Bar Fire in 2017.  While some of the fires were born earlier in the summer, all of them were conflagrated in August or September by dry easterly winds.  (The Yacolt Burn of 1902 in SW Washington, spread during the 2nd week of September as well.)

All the smoke from these fires blocked out the sun here in Portland for eight consecutive days.  Surface temperatures plummeted, due to the “nuclear winter” effects of dense smoke.  The cool dense surface air became stagnant – just like a winter inversion – and didn’t get blown out by the usual westerly breezes once the east wind stopped.  Weather models had serious trouble predicting when the smoke would dissipate, in part because they failed to clue in to the cool air and the inversions caused by the smoke.  

Fortunately there are plans to update the models in the near future, such that smoke-induced cooling becomes more “endogenous” to the forecast.  

 

WERE THE MEGAFIRES CAUSED BY CLIMATE CHANGE?

This is a complicated question to answer.  It is almost certainly true that the warming and drying climate across the Western U.S. over the last couple decades, has contributed to a general increase in wildfire activity in all these states.  But not all fires are megafires, and not all megafires happen in Western Oregon or Southwest Washington.  

Cascadian megafires (i.e. west of the Cascades and north of California), ALWAYS occur in late summer under a dry east-wind scenario.  The seasonal window is very narrow here; most happen between mid-August and mid-September.  In each case, early-season Canadian cool air was moving into the Northern Rockies and Northern Plains at the same time the wind and fires broke out in Oregon.  That creates the strong continental high pressure, which gives the east wind its punch.  

None of the climate-change models suggest that late summer/early fall east wind is expected to increase in the future; if anything, it may become slightly less prevalent.  At the same time, the jet stream is expected to become somewhat more amplified in the future, which might make the east winds we do get a bit stronger.  Less frequency, but more intensity?

But it’s also possible that as our summer dry season continues to get longer and hotter, that we may no longer need strong east wind to get megafires in Western Oregon.  Increasing fuel flammability alone, may do the trick.  

 

Why five fires this year?  A few likely culprits:

1. This was a historic east wind for the seasonal timing.  It was the strongest ‘Virgo’ east wind event (late Aug-mid Sept.) in at least 50 years, if not 70.  Some people may try to claim that climate change made the jet stream more amplified than it otherwise would have been, and that contributed to the intensity of the wind.  But I wouldn’t lean too heavily upon that theory; it just looks more like a “perfectly bad” pattern setup to me.
2. Population growth and infrastructure growth are creating more opportunities for fires to get started.  There are many, many more powerlines in Oregon today than there were 50 years ago.  That means more total downed powerlines in a windstorm – and in this pattern, more fires get started.  Bigger population also means more opportunities for human stupidity in the form of fireworks, campfires and cigarettes (and sadly, arsonists).  None of this is caused by climate change.
3. Human firefighting activity has increased in the past 100 years.  Back in the early 20th century we didn’t have the means to fight large wildfires like we do now.  In the shorter term, this more aggressive suppression helps protect our growing towns from burning down in a major blaze.  But in the long term, fire suppression can lead to a buildup of fuels – making the inevitable conflagration, when it finally happens, that much more devastating.  Fire suppression is not caused by climate change.  
4. Western Oregon has been experiencing ongoing drought conditions.  This is the one factor where climate change may have played a significant role.  I was making a big deal about this earlier in the spring, but stopped paying close attention after our cool/wet June.  But apparently, that wasn’t enough to extinguish the drought caused by two consecutive dry winters.  The map released the same week of the fires, showed severe drought conditions across most of the Cascade foothills and EXTREME drought in southwestern Oregon:

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In any case, the fire danger was largely squashed by the past week’s fairly heavy September rains.  Portland has seen 1.62″ of rain since Tuesday; and undoubtedly the Cascade foothills have seen more.  When you add to the rain from the thunderstorms two Fridays ago, our monthly total is now 2.06″, which is significantly wetter than normal.  That likely spells an end to our 2020 fire season…unless we somehow get a LONG period of dry/warm weather in October with multiple east wind episodes (like 2+ weeks).  It never ceases to amaze me how after months of bone-dry weather, a couple inches of early fall rain can put an end to the fire season.  I guess it’s a reminder of how brief our Cascadian fire season is; the wet and the green are never too far away here.  Especially not in the mountains.

 

Looking ahead…guess what?  We’re going back to summer as we end September and begin October!  Well, at least a brief stint of “second summer” or “Indian summer.”  A strong upper-level ridge will rebuild along the West Coast beginning today Sunday, and intensify for most of the upcoming workweek.  

850mb temps are expected to climb to about +21-23 Celsius over Portland by midweek.  The all-time record for the entire month of October is about +24, by the way.  So this is just about on par with past seasonal episodes that brought record warm temps.  Of course a “heatwave” now means high temps 85-90 degrees, not 95-100 like in midsummer.  Nighttime lows should be in the upper 40s to mid 50s, depending on proximity to the Portland heat island.  

By the way, our record highs at PDX for 9/28 through 10/2 are all between 88 and 92.  It would be a bit foolish to forecast record-breaking highs, though, unless the airmass was looking historically hot.  But mid to upper 80s are very likely, and a few spots in NW Oregon could well touch 90 on one of the days, perhaps Wednesday or Thursday.  

 

There will be some easterly wind through the Gorge for most of this week, but not nearly as strong or as dry, dewpoint-wise, as in early September.  And the ground is well-irrigated from the recent rains, so a major flare-up of the fires is unlikely.  We’ll probably just see a little bit of hazy light smoke in the air as the week progresses, not enough to affect our temperatures in any significant way.  California will not be so lucky, as they haven’t had rains to tamp down their fire danger.

Enjoy the sunshine this week!  -Karl