A Serious Wildfire Threat for Western Oregon and Southwest Washington

There is a serious wildfire threat for western Oregon and southwest Washington on Friday and Saturday.

Not the equal of September 2020, but serious enough that we need to be careful. This blog will describe the situation.

There is a serious wildfire threat for western Oregon and southwest Washington on Friday a A Serious Wildfire Threat for Western Oregon and Southwest Washington

Historically, major wildfires west of the Cascades crest occur in August and September for two reasons.  

First, the surface "fuels" are dry after our typically arid summers.  

Second, this is the season of strong easterly (from the east) winds as cool air --and associated high pressure--begin to move into the continental interior. Air accelerates from the interior high pressure towards lower pressure along the coast.  Strong easterly winds dry as they descend the western slopes of our regional mountains.  Strong winds can initiate fire and stoke it.

The USDA Forest Service created the Hot-Dry-Windy (HDW) index, which correlates strongly with fire growth.   This index is essentially wind speed times a measure of dryness (called Vapor Pressure Deficit, VPD).   

Here are the predicted values of Hot-Dry-Windy for 5 PM and 11 PM on Friday evening. Anything over 300 (green) is concerning.  Anything over 500 (orange and red) is dangerous.   The coastal mountains and the western slopes of the Cascades are problematic.



The threat moves northward on Saturday morning, reaching southwest Washington (see HDW map for 11 AM Saturday below).


As noted above, surface fuels are relatively dry right now.  The relatively large 1000-hour dead fuels are down to 11-15% moisture content over NW Oregon, and the smaller 10-h fuels (like grass and small bushes) are under 10% (which means ready to burn).
 


The winds in the threatened area will be from the east to northeast and very dry.  To illustrate, below is the predicted relative humidity at 5 PM Friday.  Wow.  Large areas of western Oregon will have an RH of less than 10%.

There is a serious wildfire threat for western Oregon and southwest Washington on Friday a A Serious Wildfire Threat for Western Oregon and Southwest Washington

The "good news" in all of this is that the winds, although strong, will probably be below the threshold to significantly damage power infrastructure and start electrical fires (see wind gust forecast for 5 AM Saturday morning--highest gusts around 35 knots).

There is a serious wildfire threat for western Oregon and southwest Washington on Friday a A Serious Wildfire Threat for Western Oregon and Southwest Washington

However, any preexisting fire (e.g., the Cedar Creek fire of the central Oregon Cascades) could explode.  And we must be very, very careful not to start any fires, which could rapidly grow.

Modern weather prediction is now good enough to provide accurate warnings of fire threats.  But we need to respond to prevent any further fire ignitions.
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I will be giving a talk in Portland at OMSI on the great Columbus Day Storm and Modern Weather Prediction Technology on September 24 at 10 AM (this is free).  The 60th anniversary of the greatest storm to hit the NW in over a century will be on October 12.

For more information:

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