Fire danger on the rise

Friday, January 17, 2014

It is fire season in Oklahoma. About this time every year, Oklahomans watch thousands upon thousands of acres of land burn up in wildfires.

A weather pattern change that began earlier this month and is expected to persist for several more weeks is leading to increased fire danger across Oklahoma, said J.D. Carlson, fire meteorologist in the department of biosystems and agricultural engineering at Oklahoma State University, and program manager of OK-FIRE (http://okfire.mesonet.org).

“Many locations in the state have higher fuel loads due to the wetter conditions during the spring and summer of last year,” he said “This year’s wildfire season stands in contrast to the winter wildfire seasons of 2012 and 2013, where the intense heat and drought of 2011 and 2012 severely limited vegetative growth.”

According to Carlson, Oklahoma has entered an extended period where the jet stream is coming in from the northwest, leading to periodic cold fronts coming through the state. With such a pattern, moisture from the Gulf of Mexico is cut off and the windy conditions ahead and behind such fronts, coupled with the low relative humidity in these dry air masses, cause fire danger to peak.

Aside from the amount and dryness of native fuels, Carlson said low relative humidity is the number one factor in fire danger, followed by wind speed. Thus, the period we are now in is a dangerous one, especially given the higher fuel loads of this year over much of the state.

“Most Oklahoma fuels this time of year consist of dead grasses and respond quickly to hour-to-hour weather changes,” said Carlson. “It’s not the amount of rain that we’ve had that matters, it’s the hour-to-hour relative humidity and wind speeds.”

There are two types of fuels available to wildfires – dead and live. Examples of dead fuels are dead leaves, dead grasses and dead wood on the ground surface. Live fuels include any vegetation that has any degree of greenness, which is essentially limited this time of year to evergreens, such as eastern redcedar.

The moisture content of dead fuels is controlled exclusively by hour-to-hour weather changes. Dead fuels are further categorized into four classes according to their diameters: 1-hour, 10-hour, 100-hour and 1,000-hour fuels.

The dead grasses and other dead fuels that now cover much of Oklahoma, fall in the 1-hour and 10-hour classes, and are the state’s primary fuel before spring greenup.

“The moisture content of 1-hour fuels, like dead grasses, is very responsive to any little moisture peak or dry period. So when it rains or the relative humidity rises, their moisture goes sky high, but also can quickly dry out,” Carlson said. “That’s because they are so thin and can respond to the changing weather conditions.”

The OK-FIRE system, a program of the Oklahoma Mesonet, is able to portray current fire danger across the state as well as forecast conditions up to three days in the future. Maps are available as are site-specific charts and tables. Fire managers are encouraged to consult this important tool during these days of high fire danger.