Critical Fire Weather Now Occurring for SW Washington and Western Oregon

A critical fire weather event is now underway for SW Washington and Western Oregon. Satellite imagery shows two large smoke plumes emanating from the Double Creek Fire in Central Oregon and the Goat Rocks Fire in SW Washington, with multiple smoke plumes from other, smaller fires over NW Washington and Southern British Columbia.

Check out the relative humidities and wind gusts over the area as of 5:30 pm. The Portland metro area has relative humidities in the low teens, and Portland Airport has seen easterly gusts in the 30-40 mph range since noon today.

I was blissfully unaware of the impending fire danger this week until Wednesday evening, when I learned that two utilities in Western Oregon – Portland General Electric and Pacific Power – were considering implementing Public Safety Power Shutoffs due to impending high fire danger. Instead, I was extremely busy on Puget Sound Energy’s energy trading desk, as energy prices and volatility reached extreme levels due to all-time record temperatures and loads over California (an event that deserves its own post!). Since then, PGE and Pacific Power have indeed implemented Public Safety Power Shutoffs, with PGE shutting off power to 30,000 customers across all of their official PSPS zones, as well as two additional areas near Silver Falls and Silverdale/Corbett. Pacific Power has turned off power to 10,018 customers, with most of these customers residing in the Oregon Cascade foothills. Not a popular decision, but a very wise one, particularly given the heavy vegetation and tree cover in these areas.

This is a severe event, but it will not be a repeat of the September 2020 Labor Day Fires, which was a ~100-year event and burnt nearly 500,000 acres in 36 hours. Existing fires will experience extreme fire growth, and extreme caution must be taken to not start any new fires, especially in the windiest, highest-risk areas of SW Washington and Western Oregon, but winds will likely not be strong enough to cause widespread damage to electrical infrastructure. 240,000 PGE customers lost power due to high winds during the September 2020 firestorm, and I’d be surprised if that number (excluding PSPS outages) exceeded 25,000 during this event.

This blog is a bit technical, so before I go any further, here are a few highlights.

Fire Weather Event Highlights:

• Fire danger is highest over SW WA and Western OR, particularly the Cascades, Cascade foothills, West Hills, Columbia River Gorge, and Coast Range.
• Fire danger will peak tonight into tomorrow morning for the above areas as pressure gradients stack up against the Cascades and winds in the highest-risk areas reach peak strength.
• This event ends Saturday night as winds become calm, and winds should become weakly onshore Sunday morning.
• The haze currently over the area should persist/increase through Saturday night. Weak onshore flow should help clear lower levels on Sunday but allow upper-level smoke to persist. All smoke should move east by early next week as stronger flow resumes.
• While this is an extremely dangerous event, it will be nowhere near as windy/dangerous as the Labor Day fires of 2020. A better comparison would be the Eagle Creek Fire of September 2017, though this event appears to be stronger.

Pattern Analysis and Short-Term Forecast

The record-breaking California heat wave was caused by a massive ridge of high pressure centered over the Intermountain West. As air rotated clockwise around this high pressure, offshore, downslope flow sunk from the Sierra Nevadas into Western California, with North/Central California seeing the worst of the heat. It’s incredible that Sacramento hit 116 degrees on Tuesday with just 12 hours and 50 minutes of daylight… imagine what would have happened if this setup occurred in late June, like the 2021 Pacific Northwest “Heat Dome” did.

The below image shows the upper-level 500mb anomalies around 5pm Monday 9/5. The red corresponds to areas of ridging and warm weather, while the blue corresponds to areas of troughing and cooler weather.

Despite the ridging aloft, a large “thermal low” resided over California and the Intermountain West. These “thermal lows” occur because hot air is less dense than cold air and is therefore associated with relatively lower pressure at the surface. Since this thermal low was south and east of Western Washington and Oregon, both locations saw cool, onshore flow and did not see the extreme temperatures witnessed over California. The image below shows the temps/pressure/winds at 5pm on Monday, when Sacramento reached 113 degrees (only to reach 116 the following day).

Since Monday, this ridge has amplified and retrograded offshore, while a deep, Canadian trough has moved south. This trough has “nudged” the thermal low over California and the Intermountain West into Washington and Oregon, while bringing cooler, drier, air inland. Here was the upper-level chart this morning.

Because the air with this trough is cooler than the air over Western Washington/Oregon, it is denser and associated with higher surface pressure, resulting in offshore winds for Western Washington and Oregon. As shown at the beginning of the blog, dry, offshore winds have steadily been increasing throughout the day, and they will continue to increase through the evening and overnight.

Below is the surface chart at 5pm today. Note the pronounced thermal trough in the Willamette Valley and the warm, very dry, moderately strong offshore flow into SW Washington and Western Oregon.

Fire Danger Indicies

One of my favorite charts to analyze fire danger is the “hot-dry-windy” index put out by the University of Washington WRF model. The hot-dry-windy index is one of the fire indices developed by the US Forest Service, and it is a composite index calculated by multiplying the wind speed by the Vapor Pressure Deficit, a measure of dryness (thank you Cliff Mass for this tip). It has its limitations – it doesn’t take instability into account (like the Haines Index) and I believe it underestimates fire risk during cold but dry/windy conditions (ex: the Marshall Fire near Denver, CO in Dec 2021/Jan 2022), but it is still a great starting point for analyzing relative fire risk.

As the index shows, the fire danger is much, much higher over Extreme SW Washington and Western Oregon than other areas under Red Flag Warnings. PGE and Pacificorp have lots of electrical infrastructure in these areas, which is why they have initiated Public Safety Power Shutoffs and other utilities, like Puget Sound Energy or Seattle City Light, have not.

Here is the index at 5pm tonight. At this time, gusty winds in the 20-40 mph are impacting much of extreme SW WA and Western OR, highest along exposed ridgetops.

Overnight Friday into early Saturday will be the “peak” of this fire weather event. Although winds will decrease over large swaths of the Willamette Valley as the sun sets, mixing/instability decreases, and pressure gradients stack up against the Cascades, fire danger will increase for the highest-risk locations, like the Columbia River Gorge, exposed ridgetops, and Cascade foothills due to the increase in pressure gradients and wind speeds in these areas. As the night goes on, the highest fire danger will preferentially shift from exposed ridgetops and foothill locations to the Columbia River Gorge, as the cooling air will become more dense and will be increasingly channeled by terrain.

Here is the hot/dry/windy index at 5am. Note how the how the highest fire danger is now concentrated over exposed terrain (like the West Hills and Cascade Foothills) and the Columbia River Gorge.

Here are the sea-level pressure and winds/temperature during that time. Note how a section of the thermal low has essentially “cut off” from the original, consolidated low, and how it is keeping strong offshore gradients and dry/hot conditions in place over Western Oregon and SW Washington. Strengthening offshore winds have also spread northward into Western Washington, bringing increasing fire danger with them.

Fire danger will remain very high Saturday, but it will gradually decrease throughout the day as the atmosphere warms and dries (yes, you read that right!). This is because the warming east of the Cascades will lower pressure at the surface and help decrease offshore pressure gradients and winds. By 5pm, fire danger will still be high due to extremely dry offshore winds, but it will be far less than the threat Friday night through Saturday morning since these winds will be so much lighter.

And by Saturday night/Sunday morning, fire danger ends as winds become calm or slightly onshore. Check out the hot-dry-windy index then… hardly any colors at all!

Fuel Moisture

Severe fire weather patterns don’t mean anything if vegetation is not primed to burn. Unfortunately, that is not the case this year. July and August have been extremely warm and dry, and fuel moisture values are well-below-normal, even for this time of the year. Drier fuel is both easier to ignite and burns more rapidly, as the energy used to burn the fuel goes towards combustion and not evaporating fuel moisture. This is true for both small and large fuels, as small fuels are dry due to the current hot/dry conditions and large fuels are dry due to the hot/dry summer. The below chart shows the “Energy Release Component” of fuels for NW Oregon/SW Washington, which is essentially a measure of how hot a fire can burn and is a function of fuel moisture in all different dead fuel sizes, from dry grass to fallen trees. It isn’t near record values like it was in September 2020, but it is still very high.

This post is getting a bit long, so I’ll wrap it up for now. As mentioned, I’ll have updates throughout the event, both here and on Twitter.

Stay safe,
Charlie