Near Wallatowa, Photo by J. Brent Wachter

 

 

 

 

The two to three week period leading up to the moist onset of the Monsoon is one of the most critical periods for Southwest wildland managers in a calendar year. It is during this period that fuel conditions (grasses, tree canopy, shrubs, etc.) are usually the driest. It is also during this period that atmospheric moisture increases resulting in scattered thunderstorm activity.

 

Some of the thunderstorms can be considered wet (upper right), in the sense that it rains hard or long enough to moisten the fuel bed, but often times the thunderstorms during this period produce lighter amounts of precipitation or just virga (middle right).

 

Due to the susceptibility of the fuel bed and to a lesser extent the drier nature of the thunderstorms, lightning ignited fires usually result (bottom right). Gusty and variable winds also accompany these thunderstorms and can help promote rapid and unpredictable fire spread for short durations. This leads to dangerous situations not only to wildland managers and firefighters but also to the public. About half of all wildfires reported in the Southwest region in the past 10 years have been lightning caused.

 

   

Jim Tucker,  Chaves County Skywarn Coordinator,  captured this image of a large grass fire that was started by lightning from a severe storm westnorthwest of Roswell in early June of this year. Exceptionally dry conditions of 2011 will contribute to the threat of wild fires due to lightning.

 
 Keeping a situational awareness is an important safety aspect.

There are various products that can alert you to the potential of lightning fire outbreaks, and these are listed below. Keep in mind though that it can only take one thunderstorm and one cloud to ground lightning strike to produce a catastrophic fire.

Fire Weather Planning Forecast
Area Forecast Discussion
Southwest Area 7 Day Significant Fire Potential Outlook

There are various safety messages to adhere to during this critical fire period:

• Fires ignited by lightning can spread in multiple directions at varying rates of speed and intensity. Fires commonly demonstrate dramatic spread in “flashy” fuels such as grasses and shrubs, but can also spread quickly when surface wind and slope found within complex terrain align. Be aware of the fuel type you are within.

• Monitor thunderstorm movement and give yourself plenty of space between you and the thunderstorm. Cloud to ground lightning can occur on the backside of rain shafts and provide the best potential for fire ignition. These fires may look benign for a brief period but can flare up quickly due to outflow winds associated with the exiting thunderstorm. This can result in a fire being pushed back toward you even though the thunderstorm is moving away from you.

 

In the tables below, statistics for wild fires in New Mexico for the period 2000 through 2009 are shown (data for 2010 and 2011 are being compiled). The statistics are for the number of fire starts and the number of acres burned, and in each case the number have been partitioned into "lightning caused" or "human caused." In general, the lightning and humans start about the same number of fires each year, with 2008 an exception with humans responsible for nearly twice as many starts. Addiitonally, an unusually high number of lightning starts in 2001 skews the results for this most recent period with 73 percent of the fires due to lightning.  Note the relatively high number of starts in 2006 - a year that started out with an exceptionally dry winter and spring.

chart of the number of fires started in New Mexico

When considering the number of acres burned, lightning and humans can "share the blame." While lightning can start a fire that smolders for several days, humans often cause fires on dry and windy days leading to a rapid spread. For the ten years listed below, fires started by lightning were responsible for 49 percent of the area burned, while fires started by humans contributed to 51 percent of the total.

chart of the number of acres burned in New Mexico

 
 

In the fire agency world, large lightning based fire outbreaks are also known as “lightning busts”. These lightning busts can lead to multiple fire ignitions that can overwhelm local firefighting resources. Sometimes fires ignited from one of these thunderstorm patterns can lay undetected and smolder for several days. Often times this occurs in thick canopy forest such as found across the northern mountains. These fires are known as “sleepers” and can flare up under specific atmospheric conditions such as periods of low humidity, breezes, warm temperatures and an unstable atmosphere.

More information can be found in two new fire weather features below:

Dry Thunderstorms

What Fuels a Wildfire?

 

 


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