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.

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