Grass Fires in Eastern Washington: Strong Winds and Human Ignition

June is the beginning of the grass wildfire season over the lowlands of eastern Washington.

By June, the extensive grass and range vegetation of the Columbia Basin has dried out and "seasoned" sufficiently to burn.  Also in June, strong westerly (from the west) winds can develop as weak Pacific weather systems move through.

Dry range vegetation, strong winds, and an ignition source are the ingredients for a large, fast-spreading fire. 

All of this has happened over the past few days, producing several significant grass and range fires, including the one at Juniper Dunes (13,000 acres!).   

The map below shows some of the recent fires.

Here is a satellite image of the Juniper Dune fire from two days ago.   Pretty impressive looking.

The grass fires are now history, but you can see the burned (brown) areas in today's visible satellite image.

So why the fires?  

The fuel moisture of small dead fuels was under 11% and thus flammable (see below).

Looking at the 100-hr dead fuel moisture (for larger dead debris) over the Columbia Basin (below), shows that fuel moisture (solid red line) dropped below normal with our short heat wave (the gray band shows the normal range of fuel moisture).  

Light dead fuels, such as grasses and small-diameter debris, can dry out within hours under the right conditions.

And then there are the winds resulting from the passage of a weak trough aloft.  To illustrate, here are the gusts yesterday---at several locations they exceeded 50 mph!

The strong winds are critical for the rapid expansion of such prairie fires.

It appears that these fires were initiated by careless humans, which is true of most eastern Washington grass fires.