September 06, 2020

Extreme Wildfire Potential for Western Oregon and Washington

When one thinks of major Northwest wildfires, the region east of the Cascade crest normally comes to mind.

But major wildfires can strike the western slopes of the Cascades, the Olympics, and the coastal mountains of Oregon, when conditions are just right.

Eagle Creek Fire in the western Columbia Gorge on September 2017

And late Monday and Tuesday, extreme wildfire conditions, driven by powerful and unusually strong easterly (from the east) winds will occur over parts of western Oregon and Washington, with the potential for explosive and life-threatening fires. A historically rare set-up.  The meteorology will be there and the fuels are dry--the only question is whether humans will ignite a fire.

It is up to all of us to make sure this doesn't happen.

West-side Wildfires

Fast moving, extreme wildfires are not unknown over western Oregon and Washington. The most recent example was the Eagle Creek Fire of September 2017, which burned roughly 50,000 acres over the scenic western side of the Columbia River Gorge (see above). This fire was ignited by some kids throwing fireworks on September 2nd and was supported and spread by powerful winds from the east. Other major west-side fires include the Yacolt Burn north of Vancouver, WA (1902; 248,000 acres), the the Great Forks Fire (1951; 38,000 acres) on the Olympic Peninsula, and the Tillamook Burn (1933; 240,000 acres), the Bandon Fire (1936; 287,000 acres, the Biscuit Fire (2002; 500,000 acres) of western Oregon.

Major west-side wildfires, which hit specific locations only every few hundred years, are the features that determines the longevity of the trees in much of the region.  Many of the oldest trees of the west are found in areas protected from fire, such as the extreme old growth of Mt. Rainier's Grove of the Patriarchs, located on an island in the Ohanapecosh River (see below).  

Protected from west-side fire.  Picture by Granger Meador

The Perfect Set-Up for West-Side Wildfires This Week

Nearly all of the big fires west of the Cascade crest have occurred in September, when the surface "fuels" are at maximum dryness after our normally dry summers.  On top of that, this summer has been drier than normal (see below for departure from normal for the July 1-Sept. 4 precipitation, orange and yellow are drier than normal).  With a drier than normal summer, the surface fuels in our region are drier than normal.

USDA Forecast Service maps show that the "dead fuels"  are desiccated and highly flammable.  For example, the 100-hr dead fuels (1-3 inch diameter) have moisture contents below ten percent (orange) over the western slopes of the Cascades--dry enough to readily burn.  The finer fuels (e.g., grasses) will be even more flammable once the dry eastern flow starts on Monday.

Since we have dry fuels, to get a big fire we need a source of ignition, dry conditions (low relative humidity), and strong winds.

The source of ignition will have to come from humans:  a poorly managed campfire, crazy use of fireworks, poor forest practices, sparks from firearms use, or arson. 

Strong winds provide oxygen to a wildfire and push superheated air and embers forward, helping to drive rapid fire growth.  Low relative humidity helps dry out the fuels further.   West of the Cascade crest, the only way to get dry air is for the flow to come out of the east, descending the Cascades and coastal mountains.  As as the air descends it can warm, dry and speed up.

And on late Monday it will all be in place.

The Meteorology of the Upcoming Event

On Monday and early Tuesday, a very strong (and unusual) area of high pressure will move southeastward over Idaho and Wyoming, while low pressure develops along the coast.  As a result, a HUGE pressure difference will develop across the Cascades and the westside (see forecast pressure and temperature map for 5 AM Tuesday).  This extreme pressure difference (high east of the Cascades) will drive powerful easterly winds over the western slopes of the Cascades and northwest Washington. You see all the lines (isobars) over the Cascades?  That is what a large pressure difference looks like.

The predicted easterly winds are going to be very unusual and very strong.  For example, the predicted wind gusts over the western slopes of the Washington Cascades at 2 AM Tuesday are predicted to reach as high as 50-60 kts (57-69 mph).  The winds on the southwest side of the Olympics will be almost as strong.   Amazing for this time of the year--almost unprecedented.

For western Oregon, the winds are even stronger! (see below--reds are 70 knots!)

Not only will the wind be strong, but the relative humidity of the air west of the Cascade crest will be extraordinarily low.   To illustrate, here are is the predicted relative humidity at 2 PM Tuesday (see below).  Below 10% in much of western Oregon and Washington.  Amazing.

The bottom line of all this is that there is the potential for large fires in western Oregon or Washington, particularly south of Olympia.  The fuels are dry, the relative humidity will be low, and the easterly winds strong, if not extreme.     All it will take is a careless ignition.....which we must do everything to avoid.


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  1. Thank you for this explanation. I saw the Red Flag Warning on my weather app. But the warning lists "Fire Weather Zones". Perhaps in some future post you could do an explanation of the various fire weather warnings, what the zones are, and how to find out what fire weather zone one lives in. Perhaps as you develop your podcast, you can have interviews with people who are experts on these types of things. That would be a cool expansion and extension of your Friday podcast the Seattle weather forecast and then interviews with others about relevant and pertinent weather subjects. Thanks.

    1. If you go to the NWS Seattle home page and click on "fire weather" you'll find a map of fire weather zones. Fire weather is a fascinating and complex facet of meteorology and there is tons of interesting info out there.

  2. That you for this post, sir. I’ve noticed that the long-range models STILL don’t have precip through the third week of September. Will this be the driest September on record?

    1. Ya this highly amplified large scale pattern is stuck and likely won't change much before months end. There have been Septembers with little to no rain before. 2012 was a dry one.

  3. In my telling, there was a stormly fire on Mt. Si in the 50s--don't find anything on the itnerweb about it though. There was little timber on the mountain's face back then.

    1. That fire was in May of 1958. It started east of Mt. Si, on the south facing slopes above I-90 up toward Snoqualmie Pass and burned westward threatening North Bend. I was a student at the UW then and they dismissed the forestry class so they could help fight the fire since this early in the season they were not prepared with the usual summer fire crews to fight it. An early season fire resulting from a low snow pack and a period of dry east winds.

  4. I can almost hear the wind storms with sheets of fall rains as I take another bite of my nutmeg induced pumpkin pie from the local coop.

  5. Might be worth mentioning that the Eagle Creek fire of 2017 started in almost exactly the same spot as the Yacolt Burn of 1912, and the cause was the same each time: Kids playing with fireworks. And they both started in early September. The difference was that we're now better at fighting fires -- much more equipment -- than we were then.

    At least neither you nor (yet) any of your commenters have tried to blame all of this on global warming. Wildfire is an ever-present summer threat in the Pacific NW, including along the coast at times. In fact, when it comes to the redwoods of Northern California and far SW Oregon, fire is necessary for their natural propagation: It opens the pine cones and permits the seeds to germinate in newly-cleared forest floors.

    1. The Yacolt fire of 1902 may have started in Oregon and spotted across the Columbia River, but when I was living in Carson, WA about 1950, an elderly lady who grew up there remembered the fire starting in the hills in back of Carson on the Washington side. There may have been more than one initial fire which burned together resulting in the larger Yacolt fire.

  6. I have advised friends, one of whom planned on going to Mt Rainier Tuesday that they should not. Should an evacuation be ordered the traffic, at best, would be horrific. And at worst the only roads out could be closed. Why risk it when low land trips with multiple escape routes are available for those few days?

    I remember seeing one of the final outbreaks of the Tilamook Burn, most likely '51, but possibly '45. We drove a fair distance west from Skyline Blvd. outside of Portland. It was pretty spectacular.

  7. The WRF appears to notably exempt the Strait of Georgia/northern inland waters (Samish Bay to Boundary Bay) from the exceptionally low RH predicted to desiccate the vast majority of the remainder of the region. Assuming this prediction verifies, an explanation of the mechanism which acted to limit low RH values over that area would be of interest for this local resident...

  8. Interesting how we are now getting a west side downslope wind event similar to the California Paradise fire, except in September instead of November. Lets hope our powerlines have been better maintained.

  9. Your math is pin point accurate! It will be historical!! Are you smoke ready??πŸ‡ΊπŸ‡ΈπŸš’

  10. Winds just started clocking from the NE here near Twisp after strong gusts from the NW all morning. Fire bear Omak Lake and one near Manson.

    A campfire was reported near Buck lake North of Winthrop yesterday. Without a concerned citizen speaking with those happy campers and a call to 911, that carelessness could have been a major fire by now.

  11. Winds just started clocking from the NE here near Twisp after strong gusts from the NW all morning. Fire bear Omak Lake and one near Manson.

    A campfire was reported near Buck lake North of Winthrop yesterday. Without a concerned citizen speaking with those happy campers and a call to 911, that carelessness could have been a major fire by now.

  12. 25 minutes ago: UPDATE: The Cold Spring Canyon/Pearl Hill fire has prompted Level 3 evacuations for the entire town of Mansfield.
    Again, Level 3 Evacuation, or 'Get Out Now' notice, for the entire town of Mansfield.
    Due to poor visibility, there is no route out of town. Residences are asked to securely shelter in place if possible, or evacuate to the Mansfield School Gym.
    The Douglas County Sheriff’s Office says the blaze has burned an estimated 9,000 acres
    Okanogan County Emergency management issued Level 3 evacuations for areas northwest of Omak Lake south to SR 17 along the Columbia River and west to Highway 97 north to Malott.
    A level 2 'Get Ready' evacuation notice has been declared for the city of Bridgeport.
    The fire was first reported about 9:30 p.m. Sunday just south of Omak.


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