There is a severe wildfire threat for western Oregon and southwestern Washington on Friday and Saturday.
Not as bad as September 2020, but serious enough that we need to be careful. This blog will describe the situation.
Historically, major wildfires west of the Cascades have occurred in August and September for two reasons.
First, the surface “fuel” dries up after our typical arid summer.
Second, this is the season of strong winter winds (from the east) when cool air – and high pressure combined – begins to move inland. The air accelerates from the high pressure inside towards the lower pressure along the coast. Strong winter winds dry them as they descend on the western slopes of the mountains in our region. Strong winds can start a fire and burn it.
The USDA Forest Service has created the Hot-Dry-Wind (HDW) index, which closely correlates with fire development. This index is basically wind speed multiplied by a measure of dryness (known as Vapor Pressure Deficit, VPD).
Here are the predicted values of Dry-Narrow Temperatures for Friday 5pm and 11pm. Anything above 300 (green) is concerned. Anything over 500 (orange and red) is dangerous. The coastal mountains and western slopes of the Cascades are problematic.
The threat moved north on Saturday morning, reaching southwest Washington (see HDW map for 11 a.m. Saturday below).
As noted above, the surface fuel is currently relatively dry. Relatively large 1000 hour dead fuel has moisture content reduced to 11-15% compared to NW Oregon, and smaller 10 hour fuel (such as grass and small shrubs) less than 10% (meaning ready to burn). ).
Winds in the threatened area will be from east to northeast and very dry. To illustrate, below is the estimated relative humidity at 5 p.m. Friday. OH. Large areas of western Oregon will have a RH of less than 10%.
The “good news” in all of this is that winds, although strong, are likely to be below the threshold of causing significant damage to electrical infrastructure and starting electrical fires (see 5 a.m. gust forecast Saturday morning). – highest gusts of about 35 knots).
However, any pre-existing fire (such as the Cedar Creek fire in the central Oregon Cascades) can explode. And we have to be very, very careful not to start any fires, they can grow quickly.
Modern weather forecasts are now good enough to give accurate warnings about fire threats. But we need to respond to prevent any further flare-ups.
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I’ll be speaking in Portland at OMSI about the Great Columbus Day Storm and Modern Weather Technology on September 24 at 10 a.m. (free). The 60th anniversary of the biggest storm to hit the Northwest in more than a century will take place on October 12.
For more information:
