April 19, 2023

Increased Wildfire Danger over the Western U.S. from Wet and Cold Conditions

There is an enhanced potential for wildfires over the western U.S. this coming summer and early fall, but it is not due to warming and drying.

In fact, just the opposite.   A cool, wet winter has created an increased wildfire threat by producing a bountiful volume of flammable grasses. 

A threat that extends from eastern Washington into southern California.

Eastern Washington Near Thorp, WA, July 2022

When most people think of wildfires they usually refer to forest fires.  However, grass fires are just as important in the West, if not more so.  Furthermore, burning grass plays an important role in many forest fires. 

Many of the most damaging fires in the western U.S. have had a large grass contribution.  For example, the eastern Washington town of Malden, which was destroyed in September 2020, and the 2018 Camp Fire around Paradise, CA.

The distribution of seasonal grasses in the West is shown below (indicated by green colors).  There is a LOT of grasslands across eastern WA and OR, and over central and southern California.  And many of the forest areas have an understory that includes grasses and other seasonal vegetation.

The western U.S. has a Mediterranean climate, with winter precipitation and dry summers (yes, this includes Washington State).   Precipitation during the winter encourages seasonal grass growth and is followed by a summer warm/dry period, resulting in substantial flammable fuels by mid-summer.  All that is needed is an ignition source and strong winds, which result in rapid fire spread.

Research studies indicate that precipitation is most critical around November (which supports germination and early growth) and in March/April (which provides moisture as the strengthening sun encourages rapid plant growth).

Interestingly, western grasslands and rangeland have become much more flammable over time, as explosively flammable, non-native grasses such as cheatgrass have spread around the West.  This is NOT due to global warming.  And there are far more sources of ignition these days, including huge increases in electrical infrastructure and population at the urban/wildland interface.

To get an intuitive idea of how important grass fires are, here are maps of historical fire locations for central CA.  Lots of large fires away from the forested mountains.  Importantly, extensive grass near and within forested slopes can play a major role in initiating and spreading wildfires.


Numerous and extensive wildfires have also been found in grassland areas around eastern Washington State (see below).


The Situation This Year

The percent of normal precipitation during the present water year (from Oct 1 to now) is shown below. Most of California was wetter than normal as was southeast Oregon and the eastern slopes of the Cascades.  Surprised that the Cascade easterly slopes were moist?  Blame the easterly, offshore-directed flow that made the rest of the region dry!


Importantly, higher than normal precipitation has continued during the critical last month over California, Oregon, and yes, the eastern Cascade slopes.


All this moisture has encouraged luxuriant grassland growth around the West.   

Do you want to see some proof?

Below are two NASA MODIS satellite images for southern California: the first is for today and the second is from two years ago (2022), also on April 21.  

Look closely.  A LOT more green today.    When that stuff dries out....as it WILL dry out.... there is potential danger.



But let us get more quantitative about the wildfire threat.  

A wonderful USDA website called fuelcast.net uses precipitation and other data, manipulated using machine learning, to predict the dead fuel load (mainly dead grasses) later this summer (see below).   As noted by grassland/range expert, Dr. Matt Reeves, when the standing dead fuel load gets to about 800 pounds per acre, the wildfire threat is substantial.  

As evident below, substantial portions of eastern Washington, eastern Oregon and particularly California will reach that level.

The bottom line:  there is a substantial grass/rangeland wildfire threat this summer after the West dries out, as it always does.



There is a lot of talk in the media and others that wildfires in the West are mainly the result of heating and drying due to global warming.   The truth is perhaps a bit more nuanced and complicated.  Cool and wet winters and early spring can greatly increase wildfire threats.


___________________-

The Northwest Weather Workshop agenda and information are online.  If you want to attend you must register (on the website).   A few speaking slots are still open for those interested in presenting.








21 comments:

  1. "The truth is perhaps a bit more nuanced and complicated." What a wonderful statement. I find this be true in nearly all contexts and a sentiment I try my best to convey during any scientific conversation.

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  2. When is there not a high wildfire risk? I mean, what conditions would lead to a headline of “Wildfire Risk Below Normal This Summer”?

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    1. When people ask me what the fire season will bring for the coming summer my reply is "Ask me again in October and I will be able to tell you what it was".

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    2. Probably a summer like 1993 or 1999, which were cloudy and dismal. The best climates alternate frequently between rainy and sunny, a regime which these west coast med climates just can't muster. Here, it as feast or famine.

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  3. In the late 1960s, before the internet and email were invented, US Forest Service Ranger District Fire Control Officers (FCOs) had two letters in their desk drawer.

    When it had been a dry winter and spring they would dust off the "Dry fuels" letter and mail it to all employees. This letter said that due to a lack of rain and snow over the winter the fuels were very dry and fire danger would be high so expect to have a busy fire season.

    When it had been a heavy snow winter and wet spring they would dust off the "Tall grass" letter. It said that due to the wet conditions the grass was very tall so expect a busy fire season when it dries out.

    Tom Jones, retired Forest Service Fire Fighter

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    Replies
    1. That was a much better system than what we have today - an overcomplicated mess.

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    2. In essence: wild fires are a risk every summer. Always take care not to start one.

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  4. Thanks for bringing attention to an issue that is near and personal. I live in the transition zone (ecotone) of east-slope Cascades forest and the shrub-steppe.

    "... at the urban/wildland interface"

    This is generally referred to as the "wildland urban interface" (WUI) pronounced "woo-E".
    https://www.usfa.fema.gov/wui/what-is-the-wui.html

    Various programs have been introduced to lower the risks. One example is FireWise :
    https://www.nfpa.org/Public-Education/Fire-causes-and-risks/Wildfire/Firewise-USA

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  5. Is there a difference in the quantity of smoke or the danger to life and property from a grassland fire as compared to a forest fire? I had always heard that grass fires were over faster and generated less smoke than a forest fire. Is this incorrect?

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    Replies
    1. scott, forest fires produce more smoke. Both can be produce great danger, with grass fires often moving much faster. And grass fires can be associated with forest fires..cliff

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  6. I have had about all the chilliness I can take. I don't see how the grass can grow in this cold. The grass in my yard has only just started to grow and I have yet to mow for the first time this year. Small consolation! Bring me a small dish of climate change, please, topped with sunshine.

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  7. As in nearly every issue today, given the over-whelming sources of data and relatively shallow level of comprehension, fear is the tactic government agencies use to restrict and punish. "Managing healthy forests is too costly; forest fires produce healthy forests; put masks on; get your 26th booster; drive less to save the earth; give up your gas stove to save the planet; pay your taxes or else....." Fear drives compliance. Compliance

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    Replies
    1. Skeptic great point! They blame CO2 for everything and it's all our fault.... they don't tell people that the majority, about 94 percent of co2, is from Plankton in the Oceans and humans produce maybe 3 percent, Volcanic activity does the rest. One question I always have is if co2 is so bad then why do the legal 502 state grows pump co2 into the rooms to produce twice the yield from plants? All plants and trees take carbon and release oxygen which in return produces healthier bio diversity. Fires do the same thing as a natural process always has always will.

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    2. oh no, "drive less" what a horrible sacrifice! pay taxes? Outrageous!

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    3. Ok, then set an example for the rest of us, and pay more in your own personal income tax. Just you first, then maybe others will follow.

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    4. No restrictions, no obligations, and no accountability. Perfect laissez faire. Pretty sure that is what most neolibs want. Only the markets judge. So if not paying taxes and driving more is OK by market conditions then those who want less driving and taxes are wrong. Where do you think the USA slots in? People love their cars and hate taxes.

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  8. The luxurious growth of grass this year is certainly a concern with regards to fire potential this summer, just as it has been for several past summers. As Cliff states, the grasses and small fuels in Eastern Washington nearly always dry out during summer bringing this fire potential.
    However, we do not always get large fires every summer. Grasses and small fuels are very responsive to changes in humidity and will not ignite or burn readily if the humidity is too high or if the small fuels are cool and damp. Our large fires, especially forest fires, in Eastern Washington are nearly always associated with extended periods of warm, dry weather so the major factor of whether this summer will be a bad fire year will likely be if we have extended warm, dry periods.

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    Replies
    1. Since it is almost always dry during the summer, even more important is wind and ignition.

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    2. Well, that is on us to a great extent to practice a little common sense. Heh, we're doomed.

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    3. Wind is absolutely required for a large grass fire and dry lightning has been the ignition source of many of our large forest fires so these factors are important. However, a strong wind is not always required for a forest fire, burning in steep mountainous areas, to become large. Many of our larger forest fires have started and become large in relatively light wind conditions but in hot, dry weather, spreading by convective burning up steep slopes and expanding due to spotting. However, because these forest fires usually burn for several days, wind usually eventually becomes a factor. Often our windier summers are our cooler, more moist summers, especially in the higher elevation forest areas so are not our serious fire years. Most of our large forest fires and also many larger grass fires have been preceded by several days or longer of hot, dry weather.

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