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.
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 fire 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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